UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY 4, CALIFORNIA INVESTIGATIONS WITH DDT IN CALIFORNIA, 1944 A preliminary report prepared under the direction of the DIVISION OF ENTOMOLOGY AND PARASITOLOGY March, 1945 UNIVERSITY OF CALIFORNIA BERKELEY, CALIFORNIA UNIVERSITY OF CALIFORNIA LIBRARY COLLEGE OF AGRICULTURE DAVIS CONTENTS Page Introduction 1 Use against Household Insects or Insects of Medical Importance, by S. B. Freeborn 3 Control of Lygus Bugs on Alfalfa Seed Crop with DDT, by A. E. Michelbacher, R. F. Smith, and G. L. Smith 4 The Use of DDT on Almond Trees, by C. H. Swanson and A. E. Michelbacher . . . - 6 Tests with DDT on Larvae of Fruit Tree Leaf Roller and Tussock Moth, Attacking Apple, by A. D. Borden and L. R. Jeppson 7 Laboratory Tests with Two Berry Insects, by L. M. Smith 7 Control of Certain Cotton Insects with DDT, by G. L. Smith 7 Control of Flies in a Dairy Barn with DDT, by A. E. Michelbacher, R. F. Smith, and G. L. Smith 8 Experiments with DDT against the Grape Leafhopper, by N. W. Frazier and E. M. Stafford 9 Tests with DDT and Other Insecticides for Controlling Onion Thrips in the San Joaquin Delta, by L. R. Jeppson and A. D. Borden 11 Tests with DDT and Other Insecticides for Controlling Onion Thrips in the Salinas Valley, by W. H. Lange, Jr., and T. W. Thwaits 14 Field Tests with DDT to Control Codling Moth on Bartlett Pears, by A. D. Borden and L. R. Jeppson 17 DDT Test in the Control of Pear Bud Mite, by A. D. Borden and L. R. Jeppson 18 Test Comparing the Effectiveness of DDT and Rotenone against Pear Thrips Larvae, by A. D. Borden and L. R. Jeppson 18 Tests with DDT on Tussock Moth Attacking Pear, by A. D. Borden and L. R. Jeppson 18 Experiments with DDT in Oils Applied as Vapo-Sprays for Controlling Pea Insects, by W. H. Lange, Jr 18 Field Test with DDT and Sulfur against the Potato Leafhopper on Potatoes Grown for Seed, by L. R. Jeppson and A. D. Borden 22 Tests with DDT for the Control of Certain Scale Insects, by E. M. Stafford 22 Spraying with DDT in a Greenhouse to Control Thrips, the Vectors of Spotted Wilt in Tomatoes, by M. to". Gardner, A. E. Michelbacher, and R. F. Smith 24 Laboratory Tests with DDT on Truck-Crop Insects, by W. H. Lange, Jr 27 Controlling Codling Moth on Walnuts with DDT, by A. E. Michelbacher, C. H. Swanson, and G. L. Smith 29 Effect of DDT on the Walnut Aphid, by A. E. Michelbacher, C. H. Swanson, and G. L. Smith .... 30 Studies on the Possible Effects of DDT on the Growth of Rice and Barley, by T. C. Broyer .... 31 Analyses for Deposits and Residues of DDT in 1944, by W. M. Hoskins 31 INVESTIGATIONS WITH DDT IN CALIFORNIA, 1944' INTRODUCTION This preliminary report has been prepared be- cause of the current interest in DDT (dichloro- diphenyl-trichloroethane) as an insecticide. It will make available to other investigators the results of the past season's work. Though some of the studies here described have been rather extensive, none can be regarded as complete or final. At best they represent trends, and only further work can establish the desirability of using DDT to control the insects mentioned. In 1944, after a lengthy and detailed sym- posium on the uses of DDT, the following state- ment appeared: 2 New York, December 15. --The first official statement by the entomological profession on the new insecticide DDT was issued today by the American Association of Economic Entomologists, closing its 56th Annual Meeting. Adopted by 392 delegates representing 1,000 members, it was offered both to summarize re- sults of intensive research, revealed in the past day and a half of reports to the conven- tion, and to correct "misunderstanding, over- optimism and distorted impressions." The state- ment follows: "We feel that never in the history of ento- mology has a chemical been discovered that of- fers such promise to mankind for relief from his insect problems as DDT. There are limita- tions and qualifications, however. "Subject to these, this promise covers three chief fields: public health, household comfort, and agriculture. As public health we include control of the insects which carry diseases that have scourged humanity, such as malaria, typhus, and yellow fever. Household comfort is taken to cover such things as flies, fleas, bedbugs and mosquitoes. Agriculture includes not only farms, gardens and orchards, but for- ests, livestock, and poultry. "In the public health field DDT insecticides are so much more effective than previous weapons against malaria mosquitoes that for the first time there is a practical hope for eradicating that disease from this country. DDT proved in Italy that it is the first and only practical control for typhus. In the household field its amazing lasting effect promises relief for months from flies, mosquitoes and fleas. In the case of bedbugs, eradication from the Ameri- can home has become a possibility. "In agriculture, it is promising against a wide variety of destructive pests. These in- clude most potato insects, many orchard and vineyard pests, numerous vegetable insects, as Extensive work with DDT is being carried out at the Citrus Experiment Station at Riverside, but the results are not included in this report. 2 From National Pest Control Association Serv- ice Letter no. 351. Appendix 1, page 1 (Decem- ber, 1944). well as the chief insect enemies of vitally im- portant seed crops. It appears to be effective against the pink bollworm and outstanding against the Japanese beetle, two of our worst imported pests. It promises also a more prac- tical control of the pests which ravage thou- sands of square miles of forest, and against many of those which harass livestock. "DDT will not kill all the important insect pests. It will kill many beneficial insects which are allies of mankind against the destruc- tive species. Because of its toxicity to a wide variety of insects, its large-scale use might create problems which do not now exist. To illustrate, it is a superior insecticide for control of codling moths on apples, but in some sections at least will kill certain natural ene- mies and thus release other insects which may then become major problems. "The research reports emphasize that we have not had time to develop entirely satisfactory mixtures and dosages of DDT insecticides, nor the methods and timing of application for many possible uses. Modern agricultural pest con- trol often requires mixing several materials in combination treatments, and we know little of DDT's compatibility with many of these others. Researches thus far were made with material which was produced under pressure for military needs, and which is not necessarily the best form for agriculture. "We do not know enough about effects on plants, animals, and soils. While most plants were not harmed by DDT insecticides in the ex- periments, injury to squash, corn, tomatoes and possibly fruit trees was reported. DDT is toxic to animal life when large amounts are taken internally or absorbed through the skin from oil solutions, but reports indicate a reasonable margin of safety. In the light of our present knowledge, heavy deposits on edible parts of plants should be avoided. Reports show definite toxicity to cold-blooded animal life including fish and frogs. There has not been time to learn the possible cumulative ef- fects on soils. More and larger-scale experi- mentation is needed. Enough DDT for such re- search in 1945 should be provided." There have been conflicting reports on the efficacy of the material against specific in- sects. Doubtless many of the discrepancies were caused by variations in the stock material, for there are various isomers of DDT with varying insecticidal potencies. Any two batches might well contain varying proportions of these isom- ers and thus differ in toxicity. In some early reports diluted dusts were erroneously regarded by the investigators as technically pure DDT powder, with the result that further dilution was made and produced innocuous products. Fur- ther, the forms in which DDT has been supplied have given rise to conflicting reports. [1] There have also crept into the literature statements of "facts" that had originated as hy- pothetical suggestions, mere guesses to explain seemingly miraculous results. For instance, DDT dissolved in petroleum oil was said to produce a marked reduction in surface tension of the oil. This idea, which is without confirmation, was doubtless advanced as a tentative suggestion to explain the toxicity of small amounts of DDT in oil when applied to the water surface as a mos- quito larvicide. Already insecticide manufacturers are plan- ning to produce dusts and sprays in which DDT has been incorporated. This material, when ready, will probably be largely in the form of proprietary products such as impregnated dusts, powders for spray, and emulsive oils. With these products, even when the content of DDT is known, the complicating factors introduced by the additional processing details and by the diluents, solvents, or adjuvants involved will necessitate caution in interpreting the results. The material now being supplied to the armed forces is a micronized dust of technically pure DDT. This form is designed primarily for mixing with talc, pyrophyllite, or other diluent for use as a dry dust. The micronized powder can also be dissolved in diluents. Preferable for this purpose, however, is another product, much more granular and slightly moist, which is listed as "powder, dissolving." Apparently this product is only slightly toxic to human beings. It is harmless to handle in the powdered form. In oily solutions, how- ever, it may be absorbed through the skin; gross soiling should therefore be avoided. Absorption through the respiratory tract is apparently harmless; but, as a safety measure, men exposed to fogs or mists of the material should doubt- less wear respirators or moistened gauze strips. According to experimentation with dogs, a dose that is toxic when administered by mouth ranges between 0.1 and 0.2 gram per kilo of body weight. This is many times the amount that could ordinarily be ingested in sprayed or dusted food or drink; these amounts would become im- portant only in case of accidental ingestion through gross errors in food preparation or simi- lar accidents. DDT is apparently a true contact insecticide. There is some evidence that insects absorb it directly through the cuticula. It apparently has no repellent action whatever. An insect, after resting on it for a few moments, becomes highly irritable and makes every effort to leave. Soon a paresis of the limbs develops; then fol- lows a complete paresis, including the wings; and, finally, death ensues. Because of the en- ergy with which the insect at first attempts to get away, only a small percentage of the pests actually killed are found near by, provided means of escape are available. In treating a barn, one often finds more dead houseflies 25 to 30 yards away than inside the structure. In a house treated for flies or mosquitoes the great- est accumulation of dead pests often occurs near * the windows, toward which the doomed insects at- W tempt to fly. DDT is insoluble in water. The following tabulation indicates its range of solubilities at normal temperatures with some of the common solvents: Solvent Grams per 100 cc Cyclohexanone 100 to 120 Xylene 56 Ether 29 Diesel oil no. 2 10 Kerosene (crude) 5 to 8 Kerosene (purified) 2 to 4 Ethyl alcohol 1.5 DDT dissolves rather slowly in kerosene and diesel oil unless heated in a water or steam bath. The percentages mentioned above, however, will dissolve in a few hours at room tempera- tures, particularly if the material is agitated occasionally. The following reports are compiled summaries of actual experiments carried on during 1944, except the section entitled "Use against House- hold Insects or Insects of Medical Importance." Since enough DDT for field tests was not re- ceived until about May 1, early-season treatments against some insects were impossible. In the codling-moth investigation on pears, two lead arsenate calyx sprays had been applied before work with this chemical was initiated. In gen- eral, had it been known earlier that supplies of DDT would be secured, much of the investiga- tional work could have been better planned and organized. A further complicating factor was the lack of data on residue tolerance or residue removal; this limited the size of plots and the amount of work done on certain of the fruit crops. The DDT for these investigations was obtained from several sources. Because of the consequent variation in the product, an attempt has been made to distinguish the materials. Most of the experiments were conducted with DDT purchased from a single manufacturer. Of this firm's ma- terials the following were used: Description Code Commercial pure DDT GNB-A 3 per cent DDT dust A3 20 per cent DDT with wetting agent . . A20 20 per cent DDT without wetting agent AK20 5 per cent DDT in emulsive oil base . SH5 20 per cent DDT in emulsive oil base. SH20 20 per cent DDT in nonemulsive oil base SHN20 If no code number is noted in the text, the material was donated by the Shell Oil Company or the California Spray Chemical Corporation, to whom we are indebted. [2] USE AGAINST HOUSEHOLD INSECTS OR INSECTS OF MEDICAL IMPORTANCE 3 In order that this publication may serve a slightly wider field than that represented by the professional entomologists, a nontechnical sum- mary of the uses of DDT against household insects or others of medical importance will now be given, with no claim whatever to originality or finality. Interior Residual Spraying One chief advantage in the use of DDT lies in its residual effectiveness for periods of even months after the initial application. This makes it an ideal insecticide for household use; a single treatment will sometimes suffice for complete control throughout a season. DDT has been employed with some success as a space spray, that is, a substitute for pyrethrum fly sprays: small amounts dissolved in purified kerosene are atomized in order to kill any insects present at the moment. This procedure is fairly satisfac- tory and, if the applications are frequent, even- tually builds up a residue that will be effective against a reinfestation. For a real residual treatment, one must de- posit at least 100 milligrams per square foot. In most of the experimental work to date, 200 milligrams has been deposited per square foot; the recommendations below are made on this basis. There is evidence, however, that for practical purposes only half that amount is actually needed. Kerosene Sprays DDT may be dissolved in kerosene, up to 7 or 8 grams per 100 cc. For use with hand-sprayers a 5 per cent solution (about 2 pounds to 5 gal- lons) makes a very convenient mixture. In order to deposit 200 milligrams per square foot, which is considered an adequate residual application, the spraying should be so regulated that about 4 cc is applied per square foot. This is easily done on rough wood, papered surfaces, and compo- sition boards; but on painted surfaces the ma- terial tends to run off. Enough is deposited with a hand-power sprayer with this dilution if the surface is barely moistened. Water Emulsions A satisfactory emulsion can be made by dis- solving 3 pounds of DDT in 3 quarts of xylol to which has been added a satisfactory emulsifying agent. About 1 gallon of concentrate will be ob- tained. When this is diluted with 3 gallons of water, 2 cc of the emulsion per square foot will deposit approximately the required 200 milligrams. If power sprayers are used, it is probably ad- visable to add 6 gallons of water instead of 3 and to figure on depositing 4 cc of the emulsion per square foot instead of 2. Apparently the 3 Prepared by S. B. Freeborn, Professor of En- tomology, Assistant Dean of the College of Agri- culture, and Assistant Director of the Agricul- tural Experiment Station. water emulsion produces a more effective deposit of chemical than the pure oil sprays. Unless the varnished surfaces on highly polished furniture are covered during the spraying, spotting from the xylol may result. Spraying Suggestions The ordinary cylindrical sprayer of 3-gallon capacity is very effective, provided the cover can be screwed down tightly; preferably, the sprayer should have tight-fitting neoprene (oil- resistant) gaskets and tightening lugs that fit under the rim. Helpful accessories are 2 or 3 feet of additional hose length, and a stick that is easily attached to the spray wand to lift the nozzle to high places in the room. For general spraying, the ordinary circular disc nozzle with an aperture in the center is satisfactory. The aperture should be one that produces a medium spray, since either coarse or atomized sprays are very wasteful of material. With the nozzle properly adjusted, a spraying distance of 6 to 8 inches from the surface to be treated gives excellent results and keeps the spray well un- der control. Hand sprayers have proved more practical and economical than power sprayers except in build- ings with high roofs or ceilings. For ordinary rooms, whose surfaces can all be reached from the floor, the hand- sprayer is far more effec- tive: the material can be concentrated in a set spray without atomization, and no extra man is needed to manage the engine and hose line. Control of Mosquito Larvae For the control of mosquito larvae DDT as a dust is no more useful than paris green. At present, with a price differential of well over 50 cents a pound, it would seem inadvisable even to experiment with DDT as a dust larvicide. The chief. value of the spray is that extremely small quantities may be as effective as the application of large amounts of fuel oil. A 5 per cent dilu- tion in kerosene or diesel oil is excellent. The ideal treatment recommended for this dilution is 1 to 2 quarts per acre, as contrasted with 18 to 20 gallons of fuel-oil dispersal. These figures give some idea of the minute quantity that should be used. Contrary to reports from some of the earlier experiments, however, DDT has no magic power of spreading itself over water surfaces. In dis- persing the small amount of material required, one must make sure that it is introduced over the entire surface, particularly if rafts and booms of floatage will cut its access to the sheltered areas where breeding may be intense. In dispersing the 5 per cent kerosene mixture, one may well apply the material from the wind- ward side and allow it to drift over the breed- ing area. In order to increase the coverage and to cut down the care which is necessary in ap- plying minute amounts with kerosene as a carrier, considerable work has been done with emulsions. [3] By using a quick-breaking emulsion, such as is made with phthalic glyceryl alkyd resin, one can apply the material on the acre basis, figuring at the rate of about 1/10 pound of DDT per acre. If, however, a tight emulsion is produced, as with polyethylene emulsifier, the dosage should be figured roughly on a volumetric basis so that the DDT will not exceed 1 part in 10 million. Any greater concentration will certainly be det- rimental to fish. The addition of DDT to irri- gation water at its source has proved particular- ly disappointing. Control of Adult Mosquitoes A residual spray applied at the rate of 100 to 200 milligrams per square foot will effec- tively kill all adult mosquitoes that come to rest on the treated walls, for a period of at least 3 months and possibly longer. Control of Houseflies and Stable Flies Apply DDT exactly as for residual spraying for mosquitoes. The spraying of manure piles, the walls behind them, and the most obvious gathering places in the stables will destroy numerous flies before they leave the breeding grounds. For household control treat especially screens, door and window frames, shelves, the edges of pillars, and other areas that fly "specks" indicate as habitual resting places. Control of Fleas A 10 per cent DDT dust in talc or pyrophyl- lite rubbed into the fur of pets constitutes a very satisfactory primary source of control. A 5 per cent kerosene spray applied to floors, chicken houses and yards, dog kennels, and stable floors is extremely effective. Persons allergic to flea bites have gained considerable relief by treating the underclothes with a 10 per cent DDT dust in talc. Control of Roaches All the common household roaches except the German roaches are easily controlled. Either a 10 per cent DDT dust used along the areas gen- erally traversed by roaches or a kerosene spray containing 5 per cent of DDT heavily applied is the most effective means of control. A 25 per cent dust or repeated applications of the 5 per cent kerosene spray will eventually make some in- roads on the German roach. For practical pur- poses, however, there is little to be gained by substituting DDT preparations for the orthodox treatment with sodium fluoride, particularly for the German roach. Control of Bedbugs A kerosene spray containing 5 per cent of DDT is remarkably effective against bedbugs. Mat- tresses, pillows, springs, and bedframes should be lightly sprayed so that the surface is barely moistened. If the beds are placed upright against the wall, the spraying of this furniture incidentally treats the wall behind the bed. A cursory spraying along the baseboards facilitates^ the control; but even if only the beds and bed- ding are treated, the bugs in nocturnal search for food will encounter enough poison to kill them eventually. The advantage of this treat- ment as contrasted with fumigation is that re in- festations are eliminated for several months, whereas fumigation kills only the bugs present at the moment. Control of Body Lice A 10 per cent DDT dust in talc, applied to the inside of the clothing at the rate of about 1 1/2 ounces per individual, will adequately control body lice. Control of Argentine Ants A kerosene spray containing 5 per cent of DDT is extremely toxic to Argentine ants and has been successfully used around the casings of doors and windows of masonry buildings. The or- dinary frame house, however, provides many en- trances for this insect; under these conditions, standard Argentine ant poisons placed around the outside of the house are far more effective than an attempt to treat all of the interior. CONTROL OF LYGUS BUGS ON ALFALFA SEED CROP . WITH DDT 4 f Lygus bugs are among the most serious pests attacking alfalfa seed crops. In heavy infesta- tions they make crops unprofitable or cause com- plete seed failures. Thus far no satisfactory chemical control has been developed. According to observations in the northwestern portion of the San Joaquin Valley, the bugs are likely to reach a serious economic level when an average of 5 individuals are taken to a sweep of a standard insect net. A discussion of the rela- tion of lygus bug populations to alfalfa seed production in California is now being prepared. The experiment with DDT described here was conducted in a 38-acre alfalfa field about 2 miles east of Tracy. When this field came to our attention it had been producing blooms for a short period. The lygus population was so large that a satisfactory seed crop evidently could not be produced. The predominant species was Lygus hesperus Knight , and there were a very few specimens of L_. elisus Van D. A 2-acre plot was set out in a heavily infested portion of the field. One large plot rather than several smaller replicated plots was used in order to reduce to a minimum the hazard from migrating insects. In previous studies, small plots had proved undesir- able because the adult lygus fly about so freely. - 4 Prepared by A. E. Michelbacher , Assistant Entomologist in the Experiment Station; Ray F. Smith, Principal Laboratory Technician, Divi- sion of Entomology and Parasitology; and Gordon L. Smith, Associate in the Experiment Station. [4] In determining populations, collections were made at 18 to 30 stations within the plot, and from 30 stations surrounding the plot area. On July 25, the day before the first dust ap- plication, an average of 14.6 bugs were collected to a sweep of an insect net in the experimental area. This is considered to be the initial popu- lation. A 3 per cent DDT dust (A3) , with pyrophyllite as a carrier, was applied at the rate of approxi- mately 28 pounds per acre. The duster was a self-mixing machine with a 3 x 8 inch fan powered by an 8-horsepower Le Roi water-cooled engine. The normal fan speed was 3,000 to 4,000 r.p.m. , and the nozzle used directed the dust downward. A 25-foot strip could be dusted by rapidly mov- ing the single nozzle back and forth while the duster was being towed. The work was done under favorable conditions. The experimental area was first surveyed on July 28 (2 days after the dust was applied). The lygus population was greatly reduced in the dusted plot as compared with the surrounding area. Although the effect of the poison on other insects could not easily be measured, lepidop- terous caterpillars present appeared to have been killed. Bees were abundant and were not perceptibly injured. A heavy infestation of the differential grasshopper, Melanoplus differen- tials (Thomas), did not appear to be affected. Additional population surveys on August 2 and August 9 showed an increase in lygus within the plot, but only proportional to that which occurred in the undusted area. The ratio of adults to nymphs, however, was much greater in the dusted than in the nondusted check, perhaps because the nymphs are more easily killed by DDT than are the adults, or because the plot was reinfested by adults that flew in. Possibly both factors entered into the population changes. Judging from the following observations, however, the very young nymphs were more easily killed than the older forms. On August 2 only late- instar nymphs and adults were collected within the plot, whereas in the check, lygus bugs of all stages of development were taken. Not until the August 9 survey did any very young nymphs appear in parts of the dusted plot. At this time the prospects of obtaining a satisfactory seed crop looked better in the dusted than in the undusted area. The total bug population per sweep, how- ever, had risen to 10.46. Although this was small as compared with 48.7, the average number collected in the undusted area, it was consid- ered large enough to interfere greatly with seed production. The plot was therefore dusted again on August 12, the material, rate, and means of application being the same as in the July 26 treatment. Conditions were favorable for the work, and many adult lygus flew up as the dust was being applied. A survey 4 days later on August 16 showed a marked drop in the populations; again, nearly all the nymphs observed were large , a fact sug- gesting, as before, that the very immature forms are the ones more susceptible to DDT. The dusted area showed much more bloom than the check. Over most of the latter the buds were being blasted by lygus. As a result, the dusted area stood out from the rest of the field as a faint purple patch. Considerable seed was set- ting. On August 22 a survey revealed that the lygus population was still rather small in the treated plot, but that the population in the check area was falling off rather rapidly. Later surveys showed a slight rise in the population in the plot area; but the population outside continued to fall, so that the two tended to be- come equal. As the effect of DDT became less, lygus bugs apparently found conditions in the dusted plot more favorable than outside, perhaps because of the more abundant bloom; at least, in some of the later surveys the highest counts were associated with extra-heavy bloom. The information obtained in the surveys is summarized in table 1; and the lygus bug popula- TABLE 1 Lygus Bug Population Trends in Undusted Area, and in Area Dusted with DDT on July 26 and August 12, 1944 Average number of lygus bugs per sweep Date of survey Check area Dusted area Per cent reduction Adults Nymphs Adults and nymphs Adults Nymphs Adults and nymphs in total population July 25 4.1 10.5 14.6 4.10 10.50 14.60 0.0 July 28 4.3 16.3 20.6 0.73 1.32 2.05 90.0 August 2 5.5 19.3 24.9 2.37 1.43 3.80 84.7 August 9 11.2 37.5 48.7 4.78 5.68 10.46 79.3 August 16 16.6 22.2 38.8 1.25 3.22 4.47 88.5 August 22 16.1 12.9 29.0 3.40 0.35 3.75 87.2 August 29 11.4 4.6 16.0 5.13 0.21 5.34 66.7 September 8 6.0 7.2 13.2 4.08 2.76 6.85 49.2 September 21 2.6 8.0 10.6 2.11 4.83 6.95 34.4 [5] tion trends, both inside and outside the dusted area, are plotted in figure 1. For the non- dusted area this trend appears to be of the 26 31 JULY Fig. l.--Lygus-bug population trends for dusted and undusted plot in an alfalfa seed field. normal type that occurs in alfalfa fields during the growth of a seed crop. The rapid fall in the population may sometimes, though not always, result from the drying up of the seed field. There is also some possibility that parasites and predators may aid in reducing the population. Because of the high water table in the field where the plot was located, the alfalfa continued to put out new growth. There was, therefore, considerable vegetation that should have been favorable to the lygus bugs. Since there was, notwithstanding, a rapid decline in the popula- tion, the role of natural enemies may be regarded as a possible factor. The last survey was made on September 21. At this time much seed had been set in the dusted plot. The setting seed had caused the tips of the alfalfa to bend over and in some places had caused the alfalfa to lodge. As it became im- possible to make sweeps inside and outside the plot that would be comparable , further surveys were omitted. An analysis of the alfalfa, 25 days after the second application, showed a residue of 29 parts per million of DDT, as reported in the concluding section of this paper. The seed crop was not cut until October 20 to 27. Between September 15 and this time, some seed was set in the undusted area. In spots it was rather marked, and was undoubtedly associated with the decided falling off of the lygus popu- lation. The seed in the dusted plot was ready to harvest by October 1; but the farmer delayed harvest in order to obtain the maximum yield from the nondusted area. Shortly after October 27, however, a number of heavy rains caused the cut seed to germinate or rot in the field. The yield, therefore, could only be estimated. Ap- parently the nondusted area would not have pro- duced more than 150 pounds of clean seed, while the dusted area would have yielded at least 400 pounds per acre. Conclusions On the basis of the experiment, a 3 per cent DDT dust appears to be very effective in con- trolling lygus bugs on alfalfa seed. The re- sults are so promising that further and more ex- tensive work is strongly recommended. In this particular study a 2-acre plot was treated in a 38-acre field. For satisfactory control, two dustings appeared necessary. If, however, the entire field had been dusted, one treatment would probably have sufficed. This certainly is true if the second dusting in our investiga- tion was rendered necessary by large migra- tions of adult bugs from the undusted to the dusted area. The following points need further study: (l) methods of application, (2) timing of application, (3) rate of application, (4) con- centration of material to use, (5) effect of the DDT dust on other insects, and (6) natural fac- tors affecting the control of lygus populations. Although extremely promising, the use of DDT for controlling lygus bugs on alfalfa seed crops should not be recommended until further investi- gations substantiate the results of this season's study. THE USE OF DDT ON AIMOND TREES 6 At Escalon, three almond trees were sprayed on June 26 with the following mixture of DDT: 20 per cent DDT powder (AK20) ... 5 pounds Blood albumin 4 ounces Water 100 gallons The DDT and blood albumin were thoroughly mixed, then slurried, and added to a spray tank that was half full of water. The spray was applied to determine what ef- fect DDT has upon the shot -hole borer, Scolytus rugulosus (Ratz.). Although no definite infor- mation on this point was obtained, other obser- vations of interest were noted. A heavy infesta- tion of mites (the brown almond mite and the two- spotted mite) built up in the three trees that were sprayed, which soon evidenced severe injury. As the season progressed, the trees immediately 5 Prepared by Clark H. Swanson, Assistant in Agricultural Extension; and A. E. Michelbacher , Assistant Entomologist in the Experiment Sta- tion. [6] adjacent to the DDT trees showed an increase in the mite population — unquestionably because of a migration from the three sprayed trees. The sprayed trees finally suffered complete defolia- tion, while those in the main portion of the orchard retained their leaves. Although very limited, this experiment would indicate that DDT should be used on almonds with a great deal of caution. TESTS WITH DDT ON LARVAE OF FRUIT TREE LEAF ROLLER AND TUSSOCK MOTH, ATTACKING APPLE 6 Apple trees in Santa Clara County infested with nearly mature larvae of the fruit tree leaf roller, Archips argyrospila (Wlk.) and with im- mature larvae of the tussock moth, Hemerocampa vetusta Bdv. , were treated May 1, 1944, with a spray containing 2.5 quarts of 20 per cent DDT in oil, with emulsifier (SH20) , per 100 gallons of water. Within a short interval after spray- ing, both species of larvae left the foliage and were hanging by their silken threads. Many dropped to the ground; and none were found on the trees 48 hours after spraying, whereas un- sprayed trees remained infested. The spray caused a spotted leaf injury. LABORATORY TESTS WITH TWO BERRY INSECTS 7 DDT was tested in the laboratory as a spray and as a dust on the strawberry root worm, Paria cane 11a (Fab.), and on Fuller rose weevil, Pantomorus godmani (C rotch) . A water-suspension spray of 4 pounds of 20 per cent DDT powder with wetting agent (A20) per 100 gallons of water was poured on a glass plate. The plate was placed in a vertical position so that excess spray could run off. A white film was visible on the plate when dry. Then, 291 days later, the insects mentioned above were caged on this residue with clean strawberry leaves. After 4 days, all were on their backs; and after 5 days, all were dead. DDT was similarly tested as a 3 per cent dust (A3) by applying a thin deposit of dust on a glass plate, then immediately caging the insects on it with clean strawberry leaves. All were dead at the end of 5 days, and the speed of killing was no greater than that of the aged spray deposit described above. Prepared by A. D. Borden, Associate Entomolo- gist in the Experiment Station; and L. R. Jepp- son, Associate in the Experiment Station. 7 Prepared by Leslie M. Smith, Associate Ento- mologist in the Experiment Station. CONTROL OF CERTAIN COTTON INSECTS WITH DDT 8 Tests on cotton insects were made on four plots of cotton at the United States Department of Agriculture Field Station, Shafter, California. Each consisted of 4 rows 150 feet long, dusted with 3 per cent DDT dust (A3) at the rate of 25 to 30 pounds per acre, applied with a Root rotary hand-operated duster. The first applica- tion was made on May 18 at 7 p.m., and the sec- ond July 18 at 5 a.m. Cotton plants 5 to 6 inches tall that were injured by onion thrips, Thrips tabaci Lind. , and by western flower thrips, Frankliniella oc- cidentalis (Perg.), were selected for making a test with DDT to control these insects. There were also spotted infestations of cotton aphids and red spiders, Tetranychus atlanticus McG. An average of 2 adult thrips per plant was found by snapping the tops of the plants against a dark- colored card. There was also an average of 1 cotton fleahopper, Psallus seriatus (Reut.), nymph to each 5 plants. In 24 hours after the dust was applied, the 4 check plots of undusted plants showed no change in the number of thrips and fleahoppers present. On the dusted plants, these pests could not be found; but there was no apparent control of aphids and red spiders. The characteristic injury done to a cotton plant by the species of thrips involved is the killing of a terminal bud; the result is a bushy plant, which produces a crop later than a normal plant. If the growing season is long and favor- able, the quantity of fiber from these bushy plants may equal that for normal plants. In November, a count of the bushy and erect plants showed an average of 21.2 per cent bushy plants in the dusted and 20.2 per cent in the untreated plots. These figures are based on counts of about 1,800 plants from each plot. The damage had evidently been done before the dust was ap- plied, for shortly after the treatment on May 18, the growth of all plants was rapid, and no fur- ther injury of this type occurred. The same plots were dusted again on July 18 for control of the western plant bug or cotton dauber, Lygus hesperus Knight. Before applica- tion populations were determined by shaking tops into an insect net early in the morning. There were no nymphs; and the adult population was very low, averaging only 4 insects to each 25 tops. There were so few aphids, red spiders, and other insect pests present that none of these were con- sidered. After the treatment, some observations were made of insect activity on the dusted plants. Many insects alighted on these plants; there was no evidence of the dusts' being repellent. Honey- bees usually flew directly to freshly opened flowers and therefore usually did not alight on Prepared by Gordon L. Smith, Associate in the Experiment Station. [7] dusted surfaces; the few bees that did crawl over the bracts and corolla into the flower showed little or no apparent effect of the in- secticide. Lygus hesperus would alight on the dusted surfaces, but moved about and did not feed; after a few seconds the action of the hind legs became abnormal but the insect was able to fly 40 or 50 feet. Two predaceous insects, Geocoris pallens Stal and Nabis ferus (Linn.), behaved in much the same manner. In 24 hours, samples were taken again as on the preceding day, at an hour when it was too cool for much flight of plant bugs to occur. There were no Lygus hesperus collected on the dusted plants, and no Nabis ferus ; but there were a few Geocoris pallens . The seed cotton from these plots was harvested the last of November. The total yield from the four treated plots was 492 pounds, 'whereas that from the four check plots was 487. The 1 per cent increase in the total yield from the dusted plots over the checks is too small to be significant. The infestation of lygus bugs was very slight, and no other injuri- ous species were of importance on cotton in the San Joaquin Valley during 1944. Insects often found on cotton but swept from alfalfa were lightly dusted with 3 per cent DDT dust and held in the insect net with alfalfa cuttings over night. By this method complete kills were obtained of all lygus bugs, onion thrips, western flower thrips, and a leafhopper of the genus Empoasca . There were only 4 speci- mens of Diabrotica 11-punctata Mann., and they were killed. There were both living and dead bean thrips, Hercothrips fasciatus (Perg.) in the nets. About 50 per cent of the yellow- striped army worms (Prodenia praefica Grote) were shriveled; when these were placed on fresh alfalfa cuttings and observed for 24 hours, none of them fed. The 3 per cent dust gave no indi- cation of being toxic to the Pentatomids, Chloro- 1 and Thyanta custator (Fab.), and the differential grasshopper, swept from alfalfa. CONTROL OF FLIES IN A DAIRY BARN WITH DDT 9 The dairy barn used in this experiment was located at Patterson. Flies long had been troublesome, and the commercial sprays being used were not giving satisfactory control. The barn was of cement construction with a corru- gated iron roof. The ceilings of the storeroom, washroom, milk room, and passageway were fin- ished off with cement. The barn was built to accommodate 24 cows at a time, and its entire floor space was 1,826 square feet. The barn was first examined on July 7 just Prepared by A. E. Michelbacher, Assistant Entomologist in the Experiment Station; Ray F. Smith, Principal Laboratory Technician, Division of Entomology and Parasitology; and Gordon L. Smith, Associate in the Experiment Station. before the afternoon milking. Flies were swarm- ing throughout the structure. Even in the milk- % cooling room they were abundant. At this time the milk-cooling room and the washroom were treated with a 4 per cent DDT spray in light oil. This mixture was composed of 1 part by volume of 20 per cent DDT in nonemulsible oil base (SHN20) to 4 parts of solvent. All movable equipment was first taken away, and the milk cooler and the sinks were covered with heavy paper to pro- tect them. The chemical was then applied with a hand atomizer from which the nozzle had been removed. The entire surfaces of the walls and ceiling were wet with a coarse spray. About 3/4 gallon was used. The control obtained was good; but, apparently , better results could be expected if the entire structure were treated. All flies that gained entrance into the washroom and the milk room were killed; but since there was no control in the rest of the barn, more flies than desirable continued to enter these rooms. On August 2 the control in the washroom and the milk room was still very good, but on this date the entire barn was sprayed. All foodstuffs were removed from the milk barn; the floor and the feed troughs were hosed clean. The milk room and the washroom were then sprayed with the same material and in the same manner as on July 7. The same precautions were followed. About 1 gallon of the 4 per cent spray was used in cover- % ing the walls and ceiling. The following spray mixture was applied in the passageway and the milk barn: 20 per cent DDT powder (AK20) .... 10 pounds Blood albumin 6 ounces Water 100 gallons This mixture was applied with a Bean sprayer at a pressure of about 500 pounds through a Meyer spray gun. All inside surfaces of the milk barn and passageway were treated, as well as the en- tire outside surface of the structure except the corrugated roof. Just before the inside of the barn was sprayed, the feed troughs were thoroughly hosed off to wet them. Immediately after the spraying, hosing was repeated in order to wash off the spray residue that had dripped or other- wise drained into the feed troughs. About 85 gallons of the dilute spray was used to treat the entire structure. The control resulting was very good. In the mornings no live flies could be observed until 10 o'clock or later, and it was not necessary to strain the milk. During the afternoon milking some flies entered the barn, but by morning all were dead. The washroom and milk room remained almost free of flies, even in the afternoon. Not until August 25 was there any evidence of a re- duction in the control. At this time some few t live flies were observed on the walls of the milk barn in the mornings. As late as September 15 most of the flies that entered were dead by morn- ing, and only a few dozen live ones could be [8] found in isolated spots on the walls. In the afternoons many flies were present. By Septem- ber 21 the spray was still somewhat effective, but in the mornings many of the flies that dropped during the night were still partly alive. The loss in effectiveness, slow at first, was becoming much more noticeable; by the end of September some supplemental spraying had to be done on warm days. This was not necessary when the weatner was cool. Control was no longer ef- fective, although flies were still being killed as late as October 16, ten weeks after spraying. In the washroom and milk room excellent control was obtained; even at this time there was no ap- parent tapering off in the effectiveness of the treatment. On October 16, however, the entire structure was again sprayed, with the same precautions as on August 2. The spray used on the outside of the barn, interior of the milk barn, and passage- way was as follows: 20 per cent DDT wettable powder. . . 10 pounds Light miscible oil l/2 gallon Water 100 gallons The DDT was slowly added when the spray tank was 1/2 full, and the oil was added when the tank was 2/3 full. From 70 to 75 gallons of the spray mixture was applied to the barn. The washroom and the milk-cooling room were sprayed with a 13 per cent solution of DDT in oil, di- luted with water at the rate of 1 to 50 (6 level tablespoonfuls of the 13 per cent DDT solution to a gallon of water) . About 2 gallons of the dilute solution was used, applied with a 2-1/2 gallon compressed-air pressure sprayer. The experiment is still in progress, and ex- cellent control has continued; the barn was last examined on November 14 at 3 p.m. In the entire structure only 2 or 3 flies were found. Conclusions As used in this experiment, DDT resulted in excellent control of flies. With careful appli- cation, there appears to be little hazard to man and animals. The material should find wide use in dairies. Further investigations are necessary, however, before the practice can be recommended. In the present study a water suspension has been largely used; but further work with DDT in kero- sene or a light spray type of oil is desirable. Experiments are now being planned to compare water suspensions of DDT with DDT-kerosene mix- tures. The problem of determining the most satisfactory concentration of DDT to use in sprays will also be investigated, and the best means of application will be sought. EXPERIMENTS WITH DDT AGAINST THE GRAPE LEAFHOPPER 10 The grape leaf hopper, Erythroneura elegantula Osb., 11 is an important pest of grapes in the San Joaquin Valley. An investigation on the use of DDT as a possible method of control is here re- ported. Methods Applications made on May 5, 1944, in a vine- yard near Woodlake, Tulare County, were made on 3 varieties of table grapes ( Yitis vinifera) — namely Ribier, Red Malaga, and Emperor. The treatments were as follows : Vapo-sprays: DDT in a vapo-spray base oil, 12 applied at the rate of 1.75 gallons per acre. The application was made to one side of every row, with a machine constructed by the ranch owner and operated by experienced ranch employees. Applications were as follows: 1.2 per cent DDT vapo-spray was applied to 15 rows of Emperor grapes late in the morning; a 0.6 per cent vapo- spray was applied to similar numbers and at the same time of day, to Red Malaga grapes; and a 0.6 per cent vapo-spray was applied in the early afternoon to 13 rows of Ribier grapes. Dry-mix dusts: 50 per cent commercial dusting sulfur with DDT 13 and Frianite diluent to make 100 per cent by weight. These dusts were used at three concentrations, namely 1, 3, and 5 per cent DDT. The sulfur duster used was owned and operated by an experienced commercial worker. Treatment was made in the same manner as a normal application of sulfur for mildew control. The duster traversed every other middle. The rate of application was 20 pounds per acre and the three dust mixtures were applied each to 13 rows of Ribier grapes in the early afternoon. The grape shoots were 2 to 3 feet long; the foliage somewhat heavier on Emperor and Red Malaga than on Ribier. Pre-treatment and post-treatment counts were made on May 5 and May 9 respectively. Since the hatch of first-brood nymphs had not started in this vineyard, only the overwintering adults were counted. Samples were taken by treating indi- vidual vines with a hand-operated fly-spray atomizer containing pyrethrum in oil, after Prepared by Norman W. Erazier, Junior Ento- mologist in the Experiment Station; and E. M. Stafford, Junior Entomologist in the Experiment Station, i a Determined by R. H. Beamer, University of Kansas, Lawrence, Kansas. This is the predomin- ant and most damaging species on grape in the San Joaquin Valley. In literature concerning this species in this region it has erroneously been referred to as Erythroneura comes (Say) , a species that occurs east of the Rocky Moun- tains. s Ten per cent of a light summer oil of 60 per cent viscosity (92 U.R.) and 90 per cent kero- sene . 1 3 DDT stock for making these dusts was 20 per cent DDT without wetting agent (AK20). [9] spreading a canvas under the vine. The vines were shaken, and the fallen leaf hoppers were counted on the canvas. Counts were made under 6 vines for each treatment on Emperor and Red Malaga, and under 4 vines for each treatment on Ribier. On May 15, counts were made of live and dead nymphs in 25-leaf samples for each treatment ; on May 23, in 10-leaf samples. Samples were taken by picking the requisite number of basal leaves (where nymphal populations were highest) at ran- dom, 1 leaf from a vine, all vines in a single row. The leaves from each row were collected in a paper bag, and the nymphs counted under a binocular microscope. The live nymphs remained in an undisturbed state until exposed for a time to the light near the microscope. Population samples of nymphs and adults were taken on June 13 by picking 100 leaves per treat- ment into paper bags and killing the insects with HON. Basal leaves (where populations were high- est) were selected at random, 1 leaf per vine from vines in all the rows where previous samples had been taken. The leaves were removed from bags, and the dead leafhoppers brushed to a pane of glass and counted. A final population sample of nymphs and adults was taken on September 1, 2, and 3. The same method was used as on June 13 except that the samples had to be taken during a short period before sunrise, when the leaves could be gathered without too greatly disturbing the adults. A total of 50 leaves were picked per treatment, 1 leaf per vine, in the same rows from which pre- vious samples had been taken. Checks consisted of adjoining vines in the same blocks of Emperor and Red Malaga that had been treated on May 1 with a proprietary vapo- dust oil containing pyrethrins, applied at the rate of 1.75 gallons per acre. Because of this treatment no samples were taken from these vines on May 5. Results Table 2 gives the results. Evidently the greatest reduction of adults up to the fourth day after treatment occurred in the vapo-spray appli- cations of 1.2 per cent DDT on Emperor and 0.6 per cent DDT on Ribier. The good but lower re- TABLE 2 Experiments with DDT for Controlling the Grape Leafhopper at Woodlake. on Plots of About Three Acres Each Grape variety and treatment Average number of adults per vine Pre- treat- ment counts. May 5 Post- treat- ment counts , May 9 Per cent reduc- tion from pre- treatment counts Per cent dead nymphs Per 25 leaves , May 15 Per 10 leaves, May 23 Average number of leafhoppers (nymphs and adults) per leaf June 13 Septem- ber 2* Relative leafhopper damage to vines October 15 Vapo -sprays Emperor 1.2 per cent DDT Oil-pyrethrumt 69.2 1.1 12.8 98.3 4.5 0.0 55.0 3.8 0.07 1.32 12.84 19.66 Moderate Moderately severe Red Malaga 0.6 per cent DDT Oil-pyrethrumt 75.5 8.2 23.8 89.1 2.3 0.0 9.5 0.0 1.07 2.01 1.62 * Slight Slight Ribier 0.6 per cent DDT 64.7 0.66 98.9 59.1 67.3 0.84 10.16 Moderate Dusts Ribier 5 per cent DDT 50.0 6.3 87.5 83.7 93.8 0.66 10.56 Moderate 3 per cent DDT 50.7 18.3 64.0 94.8 73.9 0.76 27.66 Severe 1 per cent DDT 35.0 32.5 7.1 18.5 17.3 3.39 § Very severe *The approximate ratios of nymphs to adults were 1 to 5.5 and 1 to 3 , respectively, for the DDT and oil-pyrethrum vapo-spray treatments on Emperor as compared with a 1 to 1 ratio in the 0.6 per cent DDT vapo-spray, the 5 per cent, and the 3 per cent DDT dust treatments on Ribier. tA proprietary vapo-spray oil containing 3 per cent of a 20 to 1 pyrethrum extract (2 per cent pyrethrins) . iThis block treated with insecticide June 17 and rendered valueless for further use as a check. §Many leafhoppers killed by drift of gas from an adjoining vineyard treated with Cyanogas the night before the sample was to be taken; population estimated to be more than 3 times greater than present in the Ribier block that received the 3 per cent DDT dust treatment. [10] duction secured with the 0.6 per cent DDT vapo- spray on Red Malaga might possibly be explained by the denser foliage as compared with Ribier. Then, too, the position of Red Malaga leaves tends to be vertical, thus partially preventing effective coverage of the under surface, whereas Ribier leaves tend to be horizontal and allow somewhat better coverage. Treating vines from one side only and using a low dosage may have in- creased this natural difference in coverage. The dry-dust mixtures on Ribier were less effec- tive against the adults than the 0.6 per cent DDT vapo-spray. On May 15 the persistent toxicity of both the 5 per cent and 3 per cent DDT dry-dust mix- tures to nymphs appeared more effective than any of the 3 vapo-spray treatments. On May 23, how- ever, the 3 per cent and 1 per cent dry-dust treatments showed decreasing effectiveness as compared with the May 15 samp las, whereas all the remaining treatments indicated a continuing or increasing effectiveness. On June 13 the population consisted of very few remaining overwintering adults, an appreci- able number of newly matured first-brood adults, and a much larger number of first -brood nymphs. Very few adults had yet moved between blocks or into treated plots. Apparently the population within a given treatment area was composed largely of first-brood nymphs and adults that had hatched from eggs within that area. Judging from the population sample on that date, the 1.2 per cent DDT vapo-spray had more effectively re- duced and held the population to a low level than had any other treatment. The 5 per cent and 3 per cent dry-mix dusts and the 0.6 per cent vapo-spray treatments on Ribier grapes showed moderate control, with relatively small differences between the treatments. The rela- tive effectiveness was in the order mentioned. The effectiveness of the 1 per cent dry dust ap- parently was very low, and the population in this treatment might well serve as a check for the other 3 treatments on Ribier. By September 2, when the final population samples were taken, the DDT had lost much, if not all, of its persistent toxicity to the grape leafhoppers. This fact is indicated by the greatly increased populations recorded in table 2. Populations on this date were composed of nymphs and adults hatched within the treatment area, as well as adults that had migrated in. Differences were still apparent among the treat- ments, and relative effectiveness was much the same as evidenced on June 13. There were still fewer nymphs in the Emperor block treated with 1.2 per cent DDT vapo-spray than in the Emperor block treated with oil-pyrethrum or in the Ribier treatments, as is indicated by a footnote to table 2. The high ratio of adults to nymphs in the Emperor block, as compared with the Ribier treatments, might be explained by the fact that in years past there has always been a heavy movement of adults into the Emperor block during late summer. The Red Malaga vines in September had been harvested and were matured, relatively nonsucculent, and apparently less favorable to the leafhoppers than the other varieties that were irrigated later and kept in a more succu- lent condition of growth. This fact probably ex- plains the relatively low leafhopper population present on September 2. The October 15 survey of the relative accumu- lated leafhopper damage in the various treat- ments is contained in the last column of table 2. Conclusion In these trials, DDT applied either as vapo- spray or as a dry dust mixed with sulfur appeared to be effective against the grape leafhopper adults and the nymphs. Toxicity to nymphs per- sisted at least 22 days. The effect of treatment was noticed 4 months after application. A more refined technique of application, together with adjustment of dosages and concentration, would undoubtedly yield better results than those ob- tained. DDT appears to hold considerable promise as means of controlling the grape leafhopper. More experiments are necessary, however, before recommendations can be made for commercial use. TESTS WITH DDT AND OTHER INSECTICIDES FOR CON- TROLLING ONION THRIPS IN THE SAN JOAQUIN DELTA 14 Spray and dust applications with several in- secticides for the control of the onion thrips, Thrips tabaci Lind. , were made May 24 on Mc- Donald Island near Stockton on a field of green onions, using knapsack sprayer and rotary hand- duster. Plots were 5 feet wide and 30 feet long. A strip 5 feet wide was left untreated between the plots. The thrips population was sampled by counting the adults and nymphs from 10 plants in each plot 2 and 8 days after treatment. Table 3 gives the results. In these tests DDT, applied either as a spray or dust, was apparently more effective in reduc- ing and preventing the increase of the onion thrips population than was a rotenone or dinitro spray. In an adjoining young-onion field on June 1, dusts containing DDT were applied at the rate of about 50 pounds per acre by means of a rotary hand-duster to plots 25 by 50 feet. Population counts of adults were made 1 and 14 days after treatment, and of nymphs 1, 14, and 21 days af- ter treatment. For each survey the thrips were counted on 20 plants selected at random in each plot. Table 4 gives the results. According to these data, the 3 per cent DDT dust checked the increase of thrips for a longer period than the 1 per cent DDT dust, and the 1 per cent dust prepared with methyl naphthalenes was more persistent in its effects on the thrips 4 Prepared by L. R. Jeppson, Associate in the Experiment Station; and A. D. Borden, Associate Entomologist in the Experiment Station. [11] TABLE 3 Comparative Tests of Several Insecticides against the Onion Thrips Composition of insecticides Per cent reduction 2 days after treatment Adults Nymphs Per cent reduction 8 days after treatment Adul ts Nymphs 2.5 quarts of 20 per cent DDT in oil (SH20) , 4 ounces sodium oleyl sulfate spreader per 100 gallons water. 5 pounds 20 per cent DDT wettable powder (A20) per 100 gallons water 3 per cent DDT dust (A3) 2 pounds rotenone spray,* 4 ounces sodium oleyl sulfate spreader per 100 gallons water 1 pound dinitro powder, t 4 ounces blood albumin per 100 gallons water 99 86 98 87 38 91 93 87 89 61 96 85 96 65 22 increase 98 99 99 76 *A proprietary product containing 4 per cent rotenone, 8 per cent other cube resins, 17 per cent petroleum oils. 'A proprietary product containing 20 per cent amine salt of dinitro-o-cyclohexylphenol . TABLE 4 Thrips Populations on Onions after the Application of Dusts Containing DDT Average number adults per plant Average number nymphs per plant Dust used 1 day after treatment 14 days after treatment 1 day after treatment 14 days after treatment 21 days after treatment 3 per cent DDT (A3). . . 1 per cent DDT* 1 per cent DDTt Check 0.25 0.6 2.2 4.0 7.7 4.4 3.3 3.0 6.7 4.0 15.0 3.1 17.3 6.6 16.0 8.4 18.0 *DDT (GNB-A) dissolved in acetone and atomized on pyrophyllite , tDDT (GNB-A) dissolved in methyl naphthalenes and atomized on pyrophyllite , population than the 1 per cent dust prepared with acetone. Six spray combinations were tested on a sec- tion of a large block of heavily infested late onions. The materials were applied at the rate of 200 gallons per acre , by means of a power sprayer at 350 pounds' pressure, 3 nozzles being used for each double row. Two replicates were made of each treatment, and two samples were taken from each replicate. Table 5 shows the mean number of nymphs found on 20 onion plants taken at random from the north and south half of the test plots. Two applications were made; and the nymph population was determined 3 and 12 days after the first treatment and 18 days after the second. In one case, a count was also made after 28 days. The DDT powder (AK20) at a concentration of 1 pound of actual DDT in 100 gallons of water gave the most promising results. When the period be- tween applications was as long as 28 days, how- ever, the thrips population built up. There were no significant differences in the thrips population of plots sprayed with nicotine sulfate, dinitro with nicotine, or tartar emetic and sugar after the June 23 application. After the treatment on July 5, however, samples indi- cate better results from nicotine sulfate than from any of the three above combinations. The cool temperature on June 23 was unfavorable for nicotine sulfate treatment, whereas the high temperature prevailing on July 5 undoubtedly in- fluenced the effectiveness of control. By July, sufficient DDT was secured to treat 15 acres of onions. As the season was well ad- vanced, the only field of young onions available had a relatively poor stand. Of this, a sec- tion consisting of 15 acres was selected for the DDT treatment , and the remainder was sprayed with nicotine sulfate. The application was made with a power sprayer at 250 pounds' pressure, 3 nozzles being used to each double row. The rate of application was 220 gallons of diluted spray per acre. Thrips counts were made from 10 onion plants in each treatment. Table 6 records the treat- ments used, the dates of application and sampling, the average number of thrips per onion plant, and the yield per acre in each plot. In this test better control of thrips was ob- tained with the DDT treatments than with nicotine sulfate. A DDT dosage of 2 pounds of 20 per cent material with soap gave almost as good results as the 5-pound treatments. The omission of soap [12] TABLE 5 Field Tests with Several Insecticides for Controlling Onion Thrips Materials used in 100 gallons of spray 5 pounds of 20 per cent DDT powder (AK20) , 4 ounces sodium oleyl sulfate 5 pounds of 20 per cent DDT powder (AK20) , 4 ounces sodium oleyl sulfate 5 pounds of 20 per cent DDT powder (AK20) , 4 ounces sodium oleyl sulfate 2 pounds tartar emetic, 4 pounds sugar Date sprayed Days after treatment Average number nymphs per plant June 23 1 pint 40 per cent nicotine sulfate, 1 pound soap 1 pound dinitro powder,* 3 pounds nicotine sulfate,! 1 pint kerosene, 8 ounces blood albumin June 23 July 5 July 5 June 23 July 5 June 23 July 5 June 23 July 5 12 28 3 12 18 18 3 12 18 ( 3 112 18 { 3 112 18 59 467 7 75 53 82 43 192 254 44 212 141 30 221 273 Check Sampled June 26 Sampled July 5 73 364 *A proprietary product containing 20 per cent dicyclohexylamine salt of dinitro-o-cyclohexylphenol . tA proprietary product containing 2.8 per cent nicotine alkaloid and 13.5 per cent petroleum oil. TABLE 6 Comparison on Thrips Control and Onion Yields after Applications of DDT and Nicotine Sprays Materials used in 100 gallons of spray Spray dates Days after treatment Average num- ber nymphs per plant Yield in 100- pound sacks per acre 5 pounds of 20 per cent DDT powder (AK20) , 1 pound soap , 2 pounds of 20 per cent DDT powder (AK20), 1 pound soap 5 pounds of 20 per cent DDT powder (AK20) , no soap added 1 pint nicotine sulfate 40 per cent, 1 pound soap June 26 [ July 12 f June 26 1 July 12 Uuly 31 fJune 26 [july 12 fJune 24 July 7 Uuly 12 ( 4 1 16 ri9 128 ( 4 116 19 9 4 16 rl9 128 rl9 128 41 45 116 44 48 186 163 21 86 119 192 273 285 353 175* 260 270 253 183 'Onion tops were largely dried up, and conditions were unfavorable for thrips' development. [13] as a spreader to the 5-pound dosage did not re- duce the effectiveness of the spray. By July 31 the onion tops in the blocks sprayed with nicotine sulfate had begun to die due to the heavy thrips infestation. The result was a decline from 353 thrips per plant 19 days after the July 12 application to 175 thrips per plant on August 9. The onion tops on the plots sprayed with DDT remained green until the middle of August. This prolongation of growth in the area sprayed with DDT was undoubtedly the reason for the increased yield over that of the nico- tine sulfate plot. With flying insects such as onion thrips a migration from one plot to another might be ex- pected, especially when one section of a field having a high thrips population becomes dry and unsuitable for thrips. On August 9 (28 days after the July 12 treatment) 488 thrips per plant were found on the block treated with 5 pounds of DDT and soap. As this plot was ad- joining the nicotine-sprayed section of the field, the high thrips population no doubt indicated a migration from the drying onions in the area sprayed with nicotine sulfate. The increase in yield of the DDT plots over the nicotine-sprayed plots was between 70 and 87 (100-pound) sacks of onions per acre. These figures adequately illustrate the value of keep- ing the thrips population under control. TESTS WITH DDT AND OTHER INSECTICIDES FOR CON- TROLLING ONION THRIPS IN THE SALINAS VALLEY 15 16 Experiments testing DDT as a spray and as a 3 per cent dust (A3) were conducted at Salinas during 1944 on Ebenezer and on White Sweet Span- ish onions planted for dehydration. In the coastal areas of California, damage to onions by the onion thrips, Thrips t abaci Lind. , is not often serious enough to prevent harvest- ing a crop. Evidently, however, the yields are reduced by thrips feeding. According to previ- ous tests conducted during 1942 and also re- ported here, several nicotine preparations (in- cluding nicotine sulfate, nicotine alkaloid, and nicotine bentonite) give unsatisfactory kills of thrips. The use of tartar emetic and sugar on onions also proved unsatisfactory. In a series of sprays applied July 20, 1942, on Australian .brown onions at Salinas, only poor control was obtained by using single applications of several different spray combinations. Table 7 presents these data. The materials were used on approximately l/3-acre plots with check plots left on each side so that a treated plot could be compared with two adjacent checks. The materials 3 5 Prepared by W. H. Lange , Jr., Junior Ento- mologist in the Experiment Station; and T. W. Thwaits, Specialist in Agricultural Extension. 1 ^or help with the 1944 experiments the au- thors are indebted to C. Autsen, A. J. Cox, G. Eiscalini, P. A. Kantor, W. Lewis, V. E. Romney, A. Skow, A. A. Tavernetti, and others. were applied with a Bean sprayer at 400 pounds' pressure. The best control was obtained with • a combination spray consisting of cryolite-derris (rotenone extracted in pine oil) and pyrethrum, and with a nicotine sulfate oil spray. On July 21 and August 23, 1944, spray materi- als were applied at Salinas to test the effici- ency of DDT against onion thrips on onions in comparison with several other recommended treat- ments. The onions were approximately 60 per cent mature on the date of the first applica- tion. The materials used were nicotine sulfate- Vatsol-sugar as recommended by Campbell and Per- sing, 17 nicotine alkaloid (80 per cent alkaloid), tartar emetic and sugar, a dinitro-nicotine- kerosene mixture, DDT as a spray (GNB-A) , and DDT as a 3 per cent dust (A3) . The sprays were applied with a Bean sprayer at 400 pounds' pres- sure, using two 0.035-inch opening fan-type nozzles to a row (four to a bed of two rows) , at the rate of 190 gallons to the acre. Only a single application of dust was made by means of a Root rotary hand-duster at the rate of 35 pounds to the acre. The DDT stock emulsion was prepared as fol- lows : 1 B Material Parts by weight DDT 15 Acetone 30 Methyl cyclo-hexanol 15 Sulfonated vegetable oil 36 Ammonium hydroxide 4 In making the emulsion, the DDT is dissolved in acetone over a water bath. The hexanol is added, and then the vegetable oil is poured in very slowly while the temperature is maintained. Finally the ammonium hydroxide is added. The resultant mixture should be thoroughly shaken several times at intervals. Ordinarily it should be used within 10 days. This stock emulsion was used at the rate of one gallon per 100 gallons of water. The spray plots were of a modified randomized block type, 3 replications of each material be- ing made on l/3-acre plots. Dust was applied to a single l/3-acre -plot . A total of 32 plots, including checks, were considered in these ex- periments. Population counts were made on July 27 and 28, counting adults and nymphs on 25 onion plants from each plot (5 stations of 5 plants each), taken from the center two rows of each plot and in the same relative position in each plot. Table 8 shows the results of this first series of counts. Judging from these figures, DDT emulsion sur- passed any other spray treatment and also the 3 per cent DDT dust. The dinitro spray mixture 17 University of California Citrus Experiment Station experiments in 1943 for control of onion thrips in southern California. 1 page. (Mimeo.) 18 A modification (developed by V. E. Romney) of example no. 1, Soap and Sanitary Chemicals 19(7): 103. July, 1943- [14] TABLE 7 Results of Spray Treatments for Controlling Onion Thrips at Salinas, Sprayed July 20, 1942 Material (per 100 gallons of spray) Number of thrips per plant on July 28 Per cent reduction in population Adults Nymphs Adults Nymphs Check 5.3 13.7 5.0 16.6 9.4 14.4 5.8 8.2 3.8 10.4 1.8 614.0 44.7 72.3 42.8 89.2 45.2 82.4 86.8 56.2 78.8 91.0 64.6 105.6 65.1 66.9 39.4 48.7 59.1 78.6 44.7 Cryolite (98 per cent sodium fluoaluminate) , 3 pounds; rotenone extract (5 per cent rotenone) , 2 pints; pyre- Check 47.0 Nicotine sulfate, 1 pint; light medium oil emulsion, 1 gallon 47.3 Check Nicotine bentonite, 3 pounds; resin spreader, 2 pints . . . Check 0.0 Nicotine bentonite, 8 pounds; nicotine sulfate, 1 pint; oil spreader and sticker, 8 ounces 0.0 Check Check 34.3 *A proprietary mixture with declared active ingredients: 10.0 per cent (minimum (sulfonated residue, 90 per cent). pyrethrins, 1.2 per cent; petroleum oils, TABLE 8 Results of Onion Thrips Counts Made July 27 and 28, of Twenty-five Onion Plants in Each Plot at Salinas Material (per 100 gallons of spray except as noted) Thrips per treated plant Adults Nymphs Thrips per untreated plant Adults Nymphs Per cent reduc- tion in population* Adults Nymphs Nicotine sulfate, 1 quart; sugar, 4 pounds; wetting agent , t 450 cc Emulsion containing 15 per cent DDT (GNB-A) , 1 gallon Tartar emetic, 2 pounds; sugar, 4 pounds Nicotine alkaloid (80 per cent), 1 pint; wetting agent ,t 450 cc , DDT 3 per cent dust (A3) , 35 pounds per acre .... Dinitro,* 1 pound; nicotine powder (2.8 per cent nicotine alkaloid and 13.5 per cent petroleum oil), 3 pounds; kerosene, 1/2 pint; casein spreader, 1/3 pound Dinitro,* 1 pound; kerosene, 1/2 pint; casein spreader, 1/3 pound 2.9 0.6 2.3 1.9 0.9 1.6 2.4 8.4 0.1 12.1 12.9 1.7 3.0 2.7 3.1 3.5 3.4 3.3 3.1 2.5 2.9 6.9 7.7 8.3 9.7 10.2 7.5 7.9 6.4 82.8 32.3 42.4 71.0 36.0 17.2 0.0 98.7 0.0 0.0 83.3 60.0 65.8 Formula used in computing efficiency of materials is as follows: ^— ^x 100, where x = number of live thrips per onion on untreated check plots, and y = number of live thrips per onion on treated plots. TComposition: 1.5 pounds of a dioctyl ester of sodium sulfo succinate in 1 gallon of distilled water. ^Twenty per cent of dicyclohexylamine salt of dinitro-o-cyclohexylphenol . [15] also gave good reductions in thrips populations, with no perceptible injury to the onions. A dioctyl ester of sodium sulfo succinate was added to this mixture because with casein or blood- albumin spreaders it did not adequately wet the plants. The maximum temperature on July 21 was 58° F, a fact which may partly explain the poor results secured with the nicotine materials. On August 14 another series of population counts of 25 onion plants from each plot showed no difference between the treated and untreated blocks, except that the DDT spray plots gave a 68.6 per cent reduction in adults and a 61.8 per cent reduction in nymphs. After the second application of spray materi- als, August 23, counts on 15 onion plants from certain plots were made from September 12 to 14. The results appear in table 9. As in the previ- ous counts, the DDT emulsion was superior to any of the other treatments. The nicotine materials gave better reductions in numbers of thrips, though the maximum temperature for August 23 was, again, only 58° F. The higher temperatures after the second application may, however, have been a factor in determining the higher kills of thrips. TABLE 9 Results of Onion Thrips Counts Made from September 12 to 14, of Fifteen Onions in Each Plot at Salinas Discussion of Results | In the 1944 Salinas experiment, DDT as a spray in emulsion form apparently gave the greatest kill of onion thrips on Ebenezer and White Sweet Spanish onions, in comparison with several other recommended treatments. The onions in the DDT spray plots not only showed a reduction in thrips populations, but also exhibited much less silver- ing of the tops and remained greener several weeks longer than those in the other plots. The control of thrips was also shown by increased yields of onions on two of the DDT spray plots in com- parison with adjacent untreated control plots. The yield increases are shown in table 10. These yield figures are not conclusive, but only indi- cate what might be expected if adequate replica- tions and checks were possible. It would have to be assumed, however, that adjacent plots of this nature would have soils of comparable fer- tility. On October 5 a random sample of onion plants was collected from the DDT plots sprayed on July 21 and August 23 and sent to A. J. Cox of the State Department of Agriculture for DDT residue Material (per 100 gallons of spray] Nicotine sulfate, 1 quart; sugar, 4 pounds; wetting agent, t 150 cc Emulsion containing 15 per cent DDT (GNB-A) , 1 gallon Tartar emetic, 2 pounds; sugar, 4 pounds Nicotine alkaloid (80 per cent), 1 pint; wetting agent, t 450 cc Dinitro,! 1 pound; nicotine powder (2.8 per cent nicotine alkaloid and 13.5 per cent petroleum oil), 3 pounds; kerosene, 1/2 pint; casein spreader, 1/3 pound Thrips per treated plant Adults 2.4 1.3 2.4 0.9 1.5 Nymphs 26.8 13.2 39.2 31.1 28.3 Thrips per untreated plant Adults Nymphs 3.1 3.1 1.4 1.6 1.7 88.8 63.6 75.4 Per cent reduc- $ tion in population* Adults Nymphs 22.6 58.1 0.0 43.7 11.8 69.8 85.1 38.4 63.1 63.8 *Formula used in computing efficiency of materials is as follows: -^-^ x 100, where x= number of live thrips per onion on check plots, and y — number of live thrips per onion on treated plots. ■(•Composition: 1.5 pounds of a dioctyl ester of sodium sulfo succinate in 1 gallon of distilled water, ^Twenty per cent dicyclohexylamine salt of dinitro-o-cyclohexylphenol . TABLE 10 Yield of Ebenezer Onions on Certain Plots at Salinas, Harvested on October 23, 1944 Pounds of topped Increase in Total yield Treatment (DDT in 100 gallons of water) onions from 666 square feet (1/65 acre) pounds of onions per acre of onions in pounds per acre Emulsion containing 15 per cent DDT (GNB-A) , 1 gallon 271.25 1,072.5 17,631 Check 254.75 16,559 Emulsion containing 15 per cent DDT (GNB-A) , 1 gallon 325.75 4,875.0 21,174 Check 250.75 16,299 [16] analysis. The result of this examination is shown in the tabulation below: Weight grains parts in per per pounds pound millio: Onion tops . . . . 1.05 . . 0.081 . . 11.58 Topped onions. . . 2.90 . . 0.026 . . 3-72 Outer layer of topped onions. . 0.23 • . 0.328 . . 46.90 Peeled onions. . . 2.67 • . Test negative Tops and bulb. . • 3-95 • . 0.041 • • 5.86 Judging from this series of analyses, no resi- due problem is involved, provided the onions are peeled before use. To control thrips adequately throughout a growing season, spraying operations might well be started during the earlier stages of growth. This schedule might necessitate at least three applications. The fact that spraying onions with DDT would prolong their growth in the fall and retard their drying must be considered in spray operations involving this material. The use of dinitro may prove a practical con- trol under coastal conditions where burn does not occur. Since, however, no applications were made to small onions, this material must be tried under different circumstances and at various stages of growth before any recommendations can be made. Summary The use of single applications of sprays con- taining tartar emetic, pyrethrum and rotenone extracts, nicotine sulfate, and nicotine benton- ite in preliminary experiments conducted in the Salinas Valley during 1942 did not give adequate control of onion thrips on onions. During 1944 at Salinas, in a comparison of spray treatments, a DDT emulsion was apparently superior to tartar emetic and sugar, nicotine sulfate and sugar, nicotine alkaloid, a dinitro- nicotine-kerosene mixture, and 3 per cent DDT dust. Two applications of a DDT emulsion (l pound of DDT to 100 gallons of water), at the rate of 190 gallons to the acre, gave a 98.7 per cent reduction in nymphs on July 27, one week after the first application; a 61.8 per cent reduction 24 days after this spray treatment; and an 85.1 per cent reduction of nymphs 22 days after the second application (made on August 23) . An analysis of onions from plots sprayed twice with the DDT spray mixture indicated that DDT was present on the tops and bulbs. When the onions (bulbs) were peeled, no DDT was found. Yield records obtained on October 23 from two plots sprayed twice with DDT emulsion gave in- creased yields of onions when compared with un- treated check plots. Although DDT shows much promise for the con- trol of the onion thrips, further investigations are necessary before this material can be safely recommended for commercial use. FIELD TESTS WITH DDT TO CONTROL CODLING MOTH ON BARTLETT PEARS 19 The experimental plot was located in a large commercial Bartlett pear orchard in the Sacra- mento River area. In 1943 the orchard received 6 lead arsenate sprays; in 1944 it received 7. For the DDT experiment, 8 large trees were se- lected. When the investigation was started, 2 lead arsenate sprays had already been applied. Single-tree plots were used, and the experimental area was divided into two series. The composi- tion of the spray for one series was 2-1/2 quarts of a 20 per cent DDT in oil with emulsifier (SH20) per 100 gallons of water; for a second series, 5 pounds of 20 per cent DDT wettable powder (A20) per 100 gallons of water. The trees in each series received 2 to 4 applications. Single trees in each of the two series were sprayed on the following dates: May 5 and 22; May 5 and June 14; May 5 and July 6; May 5 and 22, June 14, and July 6. Close examination of 100 fruits per tree was made just before each application up to harvest time, as well as at harvest. No codling-moth injury was noted at any time. All dropped fruit was carefully inspected for worms, but no en- tries were found. In an adjoining area that re- ceived 7 applications of lead arsenate, applied by the grower, counts of picked and dropped fruit showed not over 2 per cent worms. The DDT suspension spray caused no discernible injury and gave excellent coverage. The 20 per cent DDT in oil showed poor wetting, and spotted- leaf injury occurred. The fruit drop on the oil- DDT plot at harvest time was nearly 500 fruits per tree, whereas the dry-wettable-sprayed trees showed a drop of 60 fruits apiece, a figure con- sidered normal where no hormone is applied. Red-spider injury appeared earlier and was more severe in the DDT plots than it was in the remainder of the orchard receiving the regular program of lead arsenate and dinitro. No 00c- cinelid or chrysopid eggs were observed on the fruit or foliage until a few weeks before har- vest . Analysis (see table in the section on spray residues) of the spray residue was obtained from samples of each treatment, taken at harvest. These analyses show a higher deposit of DDT re- maining on the fruit when the DDT was combined with oil than the deposit obtained by applying DDT in water-suspension form. Using the present tolerances for arsenic as a standard for compari- son, two applications of both materials weather- ing for 1.5 to 2 months were below the permissi- ble amount. Applications made 30 days before analysis came within the tolerance for the wet- table material and near the tolerance for the oil- DDT combination. Four applications, the last of which was made 30 days before analysis, gave Prepared by A. D. Borden, Associate Ento- mologist in the Experiment Station; and L. R. Jeppson, Associate in the Experiment Station. [IV] amounts higher than the tolerance with both types of materials. If compared with the tolerance for lead, all DDT deposits were below the permissible amounts. Though no definite tolerance has been prescribed for DDT, the data given above may be of interest in determining such an amount. DDT TEST IN THE CONTROL OF PEAR BUD MITE 20 This test was conducted in a Cornice pear or- chard heavily infested with pear bud mites. A spray consisting of 5 pounds of 20 per cent DDT powder (AK20), 4 ounces of sodium oleyl sulfate, and 100 gallons of water was applied to 6 large trees. On October 2, the date of application, there was a heavy infestation of bud mites un- der the first bud scales. A population count made a week later showed no apparent kill of the mites. In this test DDT in oil was as effective as rotenone or rotenone and oil in reducing the population of pear thrips larvae; a spotted in- jury appeared, however, on the leaves of the trees sprayed with DDT in oil. TESTS WITH DDT ON TUSSOCK MOTH ATTACKING PEAR 22 Bartlett pear trees in Santa Cruz County that were infested with tussock moth, Hemerocampa vetusta (Bdv.), were treated May 1, 1944, with a spray containing 2.5 quarts of 20 per cent DDT in oil with emulsifier (SH20) per 100 gallons of water. The control obtained was good, the same as noted on apple in another section. As on apple, the spray caused a spotted leaf injury. EXPERIMENTS WITH DDT IN OILS APPLIED AS VAP0- SPRAYS FOR CONTROLLING PEA INSECTS 23 24 TEST COMPARING THE EFFECTIVENESS OF DDT AND ROTENONE AGAINST PEAR THRIPS LARVAE 21 A Bartlett pear orchard at Sunnyvale, heavily infested with thrips larvae, was chosen for the test. A block of 12 trees was randomized to permit 4 treatments with 3 replicates. The plot was treated May 2 with a power sprayer. The in- secticides used were an oil solution of DDT, a rotenone spray, and a rotenone spray plus oil. The check plots were sprayed with water. Thrips larvae were counted on 25 fruit clusters from each of the trees before spraying and also 2 and 6 days after application. Table 11 summarizes the results of this test. TABLE 11 Comparison of DDT and Rotenone Spray Treatments for Controlling the Nymphs of Pear Thrips Spray materials per 100 gallons spray Average number nymphs per cluster Before spray- ing 2 days after spraying 6 days after spraying 20 per cent DDT in oil with emulsifier (SH20) , Rotenone spray,* 1.1 pounds, plus 4 ounces sodium oleyl sulfate . Rotenone spray with spreader, 8 ounces; and tank mix oil, 2 quarts t Check, sprayed with 42 42 33 54 3 4 8 24 3 2 5 19 *A proprietary spray containing rotenone 4 per cent, other cube resins 8 per cent, petroleum oils 17 per cent. t Unsulf onated residue 90 per cent, viscosity 60. During August and September several mixtures containing oil and DDT were applied to peas as vapo-sprays. The work was done at Gilroy, and observations were made on the population trends of the common pests and parasites present. The tests were conducted on single, unrepli- cated plots 18 rows wide (approximately 1 acre) extending through a 10-acre field. Including the check, a total of 9 treatments were compared. Two applications were made, the first on August 29, and the second on September 20 and 21. About 5 gallons of material was used per acre for each application. The equipment used was a vapo-spray machine. Drop curtains were used for the first application, but not for the sec- ond. The peas were planted about July 20 and received the usual dusting with lead arsenate and nicotine when about 3 to 4 inches high. They were 10 to 12 inches tall at the time of the first application, and pods were well formed at the time of the second. Population Counts Two methods of obtaining population counts were used in order to evaluate the relative ef- ficiency of the treatments. The first or "knock- down" method consisted of selecting plants at random, preferably in groups of 5, and shaking them vigorously over a large piece of black card- s0 Prepared by A. D. Borden, Associate Ento- mologist in the Experiment Station; and L. R- Jeppson, Associate in the Experiment Station. ' Same authorship as in footnote 20. ' 2 Same authorship as in footnote 20. 23 Prepared by W. H. Lange , Jr., Junior Ento- mologist in the Experiment Station. 24 A cooperative project with the Shell Oil Company. The writer acknowledges the help of Paul R. Jones, C. J. Boissonou, and W. Buchser of that company for assistance in securing field data. [18] board. The second method was to make 100 sweeps in each plot with an insect net 25 at 3- to 6-day intervals. According to knockdown counts made on August 30 and September 6 on populations of thrips (chiefly Frankliniella helianthi) , a significant decrease in numbers occurred in the plots treated with a spray having 1.2 per cent or higher DDT content (table 12) . TABLE 12 Number of Thrips Present on Five Plants in Each Plot, as Determined by the Knockdown Method Treatment (per cent by- Thrips Thrips on weight of material in a on Septem- vapo- spray oil)* August 30 ber 6 DDT, 0.3 152 83 DDT, 0.6 51 54 DDT, 1.2 29 28 DDT, 2.4 22 29 DDT, 1.2; cardolite, 7.5; derris resins, 0.65; ro- 31 43 DDT, 1.2; rotenone, 0.05; cresylic acid, 0.11 . . . 25 40 DDT, 1.2 (lighter oil) . . 22 50 Rotenone, 0.25; derris resins, 0.65; cardolite, 7.5 44 82 Check 90 99 Composition: 70 per cent mineral seal oil of 45 to 50 viscosity (90 U.R.); and 30 per cent kerosene. On September 30 the knockdown method was used to determine the populations of pea aphids in the different plots. Judging from preliminary counts, the examination of a few single plants did not adequately allow for the spotted nature of pea aphid populations in the field. For this reason, in each plot 5 plants were examined from 10 stations, for a total of 50 plants. Accord- ing to these counts, the treatments containing 2.4 per cent DDT or 1.2 per cent DDT and rotenone in combination gave the greatest reductions in aphid populations. DDT concentrations of less than 1.2 per cent were ineffective. Table 13 gives these results. In addition to these knockdown counts, 100 sweeps of the insect net were taken in each plot on the following dates: August 30; September 6, 18, 21, 23, 26, and 30. All of the common in- sects except thrips were counted. Table 14 shows the populations. The sweeps were made dur- ing the hours between 10:30 a.m. and 1:30 p.m. To summarize the results of these sweeping samples: it seems that next to thrips, which cannot be adequately counted by this method, the commonest inse ct was the pea aphid, Macrosiphum 2 5 The frame of the net is an ellipse, the major axis of which is 15 inches and the minor axis 13 inches. onobrachis (B. de Fonsc). Other insects com- monly taken in the sweepings included adults of the pea leaf miner, Liriomyza flaveola ( Fallen) ; TABLE 13 Number of Aphids on Fifty Plants in Each Plot on September 30, as Determined by the Knockdown Method Treatment* (per cent by weight of material in a vapo-spray oil) DDT, 0.3 DDT, 0.6 DDT, 1.2 DDT, 2.4 DDT, 1.2; cardolite, 7.5; derris resins, 0.65; rotenone, 0.25 . . . DDT, 1.2; rotenone, 0.05; cresylic acid, 0.11 DDT, 1.2 (lighter oil) Rotenone, 0.25; derris resins, 0.65; cardolite, 7.5 Check Composition: 70 per cent mineral of 45 to 50 viscosity (90 U.R. ) ; and cent kerosene. Number of aphids 1,460 1,044 568 511 376 250 347 748 1,638 seal oil 30 per adults of a predaceous fly, Thaumatomyia ( Chloropisca ) glabra (Meigen) ; adults of Dia- brotica 11-punctata (Mann.); various parasites of the pea leaf miner (small wasps of the genus Solenotus are the commonest); Lygus bugs; and several lepidopterous larvae. Besides these insects, a white fly was abund- ant on peas, although no counts were made. Ap- parently none of the treatments greatly reduced the numbers of adult flies. Only two specimens of the pea weevil were taken, one on September 18 and another on September 26. The false chinch bug migrated into the plots toward the end of September, apparently irrespective of any treat- ments. A fulgorid, Cixius cultus Ball, was taken in limited numbers. The lepidopterous larvae present included a small armyworm of unknown species, looper larvae of the genus Autographa , and occasionally a larva of the alfalfa butter- fly. Results From the standpoint of pea aphid control, DDT appears to be fairly effective in oil when ap- plied by the vapo-spray machine; the kill is in direct proportion to the amount of DDT present. The mixture containing 2.4 per cent DDT was the most effective and, as already noted, was com- parable with the 1.2 per cent DDT and rotenone combinations. In evaluating the degree of aphid control with the different concentrations of DDT, it was found that if curves were made depicting the total number of aphids collected by the sweeping method [19] TABIE 14 Populations of Insects Collected per 100 Sweeps of an Insect Net (Field was treated August 29 and September 20 and 21, 1944) Treatment (per cent by weight of materials in a vapo-spray oil) Aug. 30 Counts of insects on the survey dates given SeptS Sept 18 Sept. 21 Sept. 23 Sept. 26 Sept. 30 Total counted Adults of the pea aphid T Macrosipl- um onobrachis DDT, DDT, DDT, DDT, DDT, DDT, DDT, 0.3 0.6 1.2 2.4 1.2; cardolite, 7.5; derris resins, 0.65; rotenone, 0.25 1.2; rotenone, 0.05; cresylic acid, 0.11 1.2 (lighter oil) Rotenone, 0.25; derris resins, 0.65; cardolite, 7.5 Check 30 8 1 4 7 5 2 7 22 47 32 10 4 5 6 3 9 41 145 142 52 26 34 11 26 32 115 200 152 53 30 15 34 21 46 186 2S3 290 151 51 54 76 57 28 461 590 355 138 104 73 36 85 59 412 106 138 102 28 51 26 43 27 87 Adults of Lygus hesperus DDT, 0.3 DDT, 0.6 DDT, 1.2 DDT, 2.4 DDT, 1.2; DDT, 1.2; DDT, 1.2 (lighter oil] Rotenone , Check . cardolite, 7.5; derris resins, 0.65; rotenone, 0.25 rotenone, 0.05; cresylic acid, 0.11 0.25; derris resins, 0.65; cardolite, 7.5 3 9 1 1 5 9 1 5 5 2 5 2 2 2 2 1 1 1 1 1 1 2 1 2 1 3 1 2 2 1 1 1 1 1 2 1 Adults of Diabrotica 11-punctata DDT, 0.3 DDT, 0.6 DDT, DDT, DDT, DDT, DDT, 1.2 2.4 1.2; 1.2; cardolite, 7.5; derris resins, 0.65; rotenone, 0.25 rotenone, 0.05; cresylic acid, 0.11 1.2 (lighter oil) Rotenone, 0.25; derris resins, 0.65; cardolite, 7.5 Check 1 6 7 15 6 14 3 5 9 10 12 1 1 6 9 5 5 2 5 5 1 1 1 4 1 4 1 1 4 2 3 3 1 2 4 4 3 1 3 4 1 3 1 1 4 4 1 Adults of the pea leaf miner, Liriomyza flaveola DDT, DDT, DDT, DDT, DDT, DDT, DDT, 0.3 0.6 Rotenone , Check . . cardolite, 7.5; derris resins, 0.65; rotenone, 0.25 rotenone, 0.05; cresylic acid, 0.11 (lighter oil) 0.25; derris resins, 0.65; cardolite, 7.5 25 72 30 47 7 121 57- 8 67 50 72 41 116 103 11 183 64 24 47 106 6 4 228 61 54 75 72 1 67 178 63 16 76 120 q 3 209 110 38 46 44 1 1 18 126 8 58 24 5 12 66 6 4 12 33 259 45 21 128 48 58 214 Adults of a dipterous aphid predator, Thaumatomyia glabra DDT, DDT, DDT, DDT, DDT, DDT, DDT, 0.3 0.6 1.2 2.4 1.2; 1.2: cardolite, 7.5; derris resins, 0.65; rotenone. rotenone, 0.05; cresylic acid, 0.11 1.2 (lighter oil) . . . Rotenone, 0.25; derris resins. Check ' 0.25 0.65; cardolite, 7.5 20 15 1 260 58 26 2 5 213 129 19 1 5 10 4 124 87 134 62 12 17 17 13 33 156 157 120 59 30 29 16 10 46 55 52 49 63 24 41 7 11 54 31 [20] TABLE 14 (Continued) Treatment (per cent by weight of materials in a vapo-spray oil) Aug. 30 Counts of insects on the survey dates given Sept. 6 Sept. 18 Sept. 21 Sept 23 Sept, 26 Sept. 30 Total counted Lepidopterous larvae (figures followed by a represent green armyworms; by b, loopers LAutographa sp.J DDT, 0.3 DDT, 0.6 DDT, 1.2 DDT, 2.4 DDT, 1.2; DDT, 1.2; DDT, 1.2 cardolite, 7.5; derris resins, 0.65; rotenone, 0.25 rotenone, 0.05; cresylic acid, 0.11 1.2' (lighter oil) Rotenone, 0.25; derris resins, 0.65; cardolite, 7.5 Check la 10a 8b 2a 7b 5b la la 2a 20b Adults of larval parasites, Solenotus spp., of the pea leaf miner 27 1 8 2 68 216 45 5 17 5 46 408 4 17 2 8 49 234 1 5 17 1 1 21 113 1 5 7 1 2 25 146 1 21 1 7 12 88 4 3 7 4 17 78 5 11 1 6 121 5 23 10 52 67 68 261 DDT, DDT, DDT, DDT, DDT, DDT, DDT, 0.3 0.6 1.2 2.4 1.2; Rotenone , Check . , cardolite, 7.5; derris resins, 0.65; rotenone, 0.25 rotenone, 0.05; cresylic acid, 0.11 lighter oil) 0.25; derris resins, 0.65; cardolite, 7.5 322 526 314 159 187 130 113 144 486 Adults of pea leaf miner parasites of the family Chalcididae DDT, 0.3 DDT, 0.6 DDT, 1.2 DDT, 2.4 DDT, 1.2; cardolite, 7.5; derris resins, 0.65; rotenone, 0.25 DDT, 1.2; rotenone, 0.05; cresylic acid, 0.11 DDT, 1.2 (lighter oil) Rotenone, 0.25; derris resins, 0.65; cardolite, 7.5 Check 15 3 2 1 2 1 2 2 11 11 9 7 11 6 6 6 2 1 6 22 77 26 38 74 24 34 70 44 49 90 46 53 95 42 47 98 (a total of 700 sweeps of the insect net during the period August 30 to September 30), data from table 14 compared favorably with the total num- ber of aphids collected on September 30 by the knockdown method (from 50 pea plants, data from table 13) . This marked similarity in the control obtained as determined by the two sampling methods tends to substantiate the results, although the correlation in actual number of aphids is one of chance sampling. There was no sign of any correlation of DDT treatments with the cortrol of adults of the pea leaf miner. The fact that adults moved into the field from a nearby sugar-beet field might make the data misleading. There was a reduction in the degree of mining on the lower leaves of the plants in plots treated with the higher concen- trations of DDT and DDT plus rotenone. The larvae of Thaumatomyia glabra are common predators on the sugar beet root aphid. After harvest of the beets, adults of this fly moved into the pea fields in large numbers. At times they are more abundant than those of the pea leaf miner. Judging from the population trends of this insect, the higher DDT concentrations and DDT and rotenone mixtures may have affected the adults of this fly. The location of the plots, however, might give a similar population trend regardless of treatment. Parasites of the genus Solenotus were notice- ably reduced in the plots that contaioed the higher concentrations of DDT and DDT and rotenone. These larval parasites of the pea leaf miner emerged in large numbers toward the end of Sep- tember. There was, however, apparently no cor- relation with the treatments in relation to num- bers of chalcids, several of which emerged through the pupae of the pea leaf miner. All the treatments seemed effective on lepi- dopterous larvae, although the populations were not great enough to justify much emphasis on the results. [21] The populations of diabrotica beetles and lygus bugs were not considered adequate to show the effect of the treatments on these particular insects. Analyses for DDT residue on pods and kernels are given in the section on analysis of residues. These show that the residue deposit increased along with the amount of DDT in the oil. No evi- dence of residue could be detected in the kernels from the treatment that contained 2.4 per cent DDT. Summary Application of DDT in an oil as vapo-spray at different concentrations indicated that the kill of pea aphids was directly proportional to the amount of DDT present. The highest concen- tration used, 2.4 per cent by weight, gave the best reduction in populations of aphids. Combinations of 1.2 per cent DDT and 0.05 or 0.25 per cent rotenone were superior to 1.2 per cent DDT alone , and about the same as 2.4 per cent DDT. There was some indication that DDT in oil was effective at higher concentrations on flower thrips, white flies, and lepidopterous larvae. The higher concentrations of DDT seemed to have an effect in reducing populations of a predaceous fly, Thaumatomyia glabra , and para- sites of the genus Solenotus (parasites on larvae of the pea leaf miner) . No apparent effect was obtained on chalcid parasites of the pea leaf miner. No definite information concerning the effect of DDT on adult pea leaf miners was obtained. Apparently, however, the maggots did less damage to the lower leaves of the plants in the treated areas. The populations of diabrotica beetles and lygus bugs were not sufficient to justify any conclusions regarding the effect of DDT on these insects. Analyses of pea pods and kernels from plots treated with different percentages of DDT indi- cated the presence of DDT on pods, but its ab- sence on kernels. The amount of DDT present on the pods was proportional to the percentage of DDT used. FIELD TEST WITH DDT AND SULFUR AGAINST THE -POTATO LEAFHOPPER ON POTATOES GROWN FOR SEED 26 In the work with the potato leafhopper, Empoasca abrupta DeL. , the experiment was con- ducted with Early Rose seed potatoes growing on peat land. The plots, 25 by 30 feet, were repli- cated twice. DDT and sulfur dusts were tested alone and in combination with one another. They were applied with a rotary hand-duster on August 21, when the plants were 35 days from planting and about 10 inches high. Nymphs were counted 2, 11, and 19 days after treatment. The counts were made on the third and fourth leaves from the base of the plants because these were found to be the most heavily infested. Table 15 gives the results. TABLE 15 Results of Tests with DDT and Sulfur Dusts in Controlling Potato Leafhopper on Potatoes Dust materials 3 per cent DDT dust (H3) Check 20 per cent DDT powder (AK20), 5 pounds; dusting sulfur, 95 pounds Check Dusting sulfur 100 per cent Check Average number of leafhop- per nymphs on 20 leaves 2 days after dust- ing 12 157 26 165 No sample No sample 11 days after dust- ing 2 225 14 150 79 166 19 days after dust- ing 4 184 6 130 54 104 26, "Prepared by L. R. Jeppson, Associate in the Experiment Station; and A. D. Borden, Associate Entomologist in the Experiment Station. In this test the 3 per cent DDT dust alone, or the 1 per cent DDT dust with sulfur, reduced the leafhopper-nymph population and held it for at least 19 days. Dusting sulfur reduced the number of nymphs by about 50 per cent. TESTS WITH DDT FOR THE CONTROL OF CERTAIN SCALE INSECTS 27 The object of these experiments was to de- termine the toxicity of DDT residues to the sub- sequent settling of scale crawlers. Three types of experiments were conducted, as follows: (1) laboratory tests made; (.2) branches of host plants treated in the field with a hand- sprayer; (3) trees or bushes treated with a power-driven sprayer. Laboratory Tests with Olive Scale, Parlatoria oleae (Colvee) ~ In the laboratory two experiments were made. In the first test twelve different sprays (not listed here) containing DDT were applied to Japanese privet leaves. After the sprays dried, the leaves were infested with olive scale crawl- ers through contact with heavily infested un- sprayed leaves. Ten days later some live scales were present on all sprayed leaves. The higher the viscosity of the oil used with DDT, the greater the number of dead crawlers. 27 Prepared by E . M. Stafford, Junior Ento- mologist in the Experiment Station. [22] In the second test (of 7 spray treatments * used) the highest concentrations were as follows: 1. Five pounds of 20 per cent DDT wettable powder (A20) per 100 gallons of dilute spray. 2. Kerosene with 6 grams of DDT (GNB-A) per 100 milliliters, at the rate of 3 gallons per 100 gallons of dilute spray, plus emulsifier (phthalic glyceryl alkyd resin, 1:3200). 3. Mineral seal oil with 4 grams DDT (GNB-A) per 100 milliliters, at the rate of 3 gallons per 100 gallons of dilute spray, plus emulsifier (phthalic glyceryl alkyd resin, 1:3200). 4. Five per cent DDT in emulsible light oil (SH5) at 2 gallons per 100 gallons of dilute spray. These spray mixtures were applied to Italian jasmine twigs. After 1 week the twigs were in- fested as in the first experiment; and 10 days later, the order of increasing effectiveness of these treatments in controlling crawlers was 1, 2, 3, and 4. Some dead first-instar settled stages were found, but there were some live scales on all the sprayed leaves. Limited Field Experiments with Olive Scale In the field three experiments were made by using a compressed-air sprayer to treat branches heavily infested with scales. The experiments were as follows : In the first test several different sprays containing DDT were applied to Japanese privet on April 13,- before the scale eggs had started to hatch. None of these proved effective. The one showing the highest percentage (7.3 per cent) of dead second-instar scales 33 days after treat- ment, however, was a spray containing 5 per cent DDT in emulsible light oil base at 1/2 gallon per 100 gallons of water. The second test was made May 30 on olives when the peak of hatching had been passed. The following spray mixtures were used: 1. Kerosene with 4 grams of DDT (GNB-A) per 100 milliliters at the rate of 3 gallons per 100 gallons of dilute spray, with emulsifier (phthal- ic glyceryl alkyd resin, 1:3200). 2. Four pounds 20 per cent DDT wettable powder (A20) per 100 gallons of dilute spray. 3. Mineral seal oil with 4 grams of DDT (GNB-A) per 100 milliliters at the rate of 2 gallons per 100 gallons of dilute spray, with emulsifier (phthalic glyceryl alkyd resin, 1:3200). 4. Five per cent of DDT in emulsible light oil (SH5) at 2 gallons per 100 gallons of dilute spray . Forty-four days after treatment, live young scales could be found on the sprayed leaves. Many live recent adult females could be found in unprotected places on older leaves that had been sprayed with mixtures 1 and 2. Where mixture 3 had been applied, only a few live recent adult females could be found on the older leaves that were unprotected by the shells of old adult females. Leaves sprayed with mixture 4 showed almost no live recent adult females unprotected by old shells. On untreated leaves most of the live scales were recent adult females, though a few second-instar larvae were present. The third test was made on Italian jasmine on August 11, when hatching of the second brood of scale eggs was still incomplete. The following spray mixtures were compared: 1. Kerosene with 6 grams of DDT (GNB-A) per 100 milliliters at the rate of 2 gallons per 100 gallons of dilute spray, with emulsifier (phthalic glyceryl alkyd resin, 1:3200). 2. Kerosene with 6 grams of DDT (GNB-A) per 100 milliliters at the rate of 4 gallons per 100 gallons of dilute spray, with emulsifier (phthalic glyceryl alkyd resin, 1:3200). 3. Mineral seal oil with 4 grams of DDT (GNB-B) per 100 milliliters at the rate of 2 gallons per 100 gallons of dilute spray, with emulsifier (phthalic glyceryl alkyd resin, 1:3200). Forty-six days after the sprays were applied, young leaves that were sprayed showed fewer scales than untreated ones. On older leaves more live scales were found in unprotected places where kerosene-DDT sprays were applied than where mineral seal oil-DDT was used. When the mixture was kerosene-DDT, second-instar and recent adult females were observed among DDT crystals on the leaves. After 84 days, a survey was made to de- termine what proportion of recent adult female scales on the leaves were unprotected by old overlapping females. When 100 recent adult fe- males were counted from each of the 3 treatments and from unsprayed leaves, the per cent of those unprotected by o^d overlapping females was as follows: unsprayed leaves, 82; treatment 1, 54; treatment 2, 42; and treatment 3, 20. Field Experiments with Olive Scale Using a Power-Driven Sprayer In these tests the following spray mixtures were used: 1. Two pounds of 20 per cent DDT wettable powder (A20) per 100 gallons of water. 2. Five per cent of DDT in emulsible light oil (SH5) at 1/2 gallon per 100 gallons of water. 3. Same as mixture 2 but used at the rate of 1 gallon per 100 gallons of water. 4. Four pounds of a talc powder containing 20 per cent of DDT (GNB-A) and 1 quart of heavy medium soluble oil per 100 gallons of water. The mixtures were applied with a Friend sprayer at 500 pounds' pressure. Spray mixture 1 applied on April 22 to olive trees failed to prevent scale crawlers from settling in a normal manner. On April 24 one Italian jasmine plant was sprayed with mixture 2, and a second with mixture 3. Thirty-four days later most of the scales present were in the second instar, and very few dead young scales could be found on either of these trees. On July 17, before the hatching of the second brood of scale eggs, an olive tree was sprayed with mixture 4. Two days [23] later a count of 500 adult female scales showed 79.6 per cent to be dead. In early November, however, many live overwintering scales were found, as well as an abundance of scale-spotted olives. Information on DDT deposit residue on olive foliage and fruit is given in the section on residues. Conclusion : In none of the tests conducted with Parlatoria scale were the results satis- factory for commercial practice. Control of olive scale always increased with viscosity and concentration of oil regardless of concentration of DDT. Even fresh deposits of DDT did not pre- vent many crawlers from settling and reaching the second instar. The use of DDT seems, there- fore, to offer no solution to the problem of controlling olive scales that settle under their mothers' shells. Lepidosaphes ficifoliae Tests on Fig Scale. (Berlese ) Field tests of DDT residues for controlling fig-scale crawlers were unsuccessful in all but one doubtful case. In this case an Adriatic fig tree was treated with a power-driven tree sprayer, using a talc powder containing 20 per cent DDT (GNB-A) at 4 pounds per gallon plus 1/4 per cent heavy medium soluble oil. The spray was applied July 17, when most of the fig-scale population on the stems and upper surfaces of leaves was in the first instar. The scales on the under sur- faces of the leaves were mostly adult females. On November 2, very few adult overwintering females could be found on the 1S44 wood. This fact would indicate either a high reduction of the population in July or a residual toxicity of the DDT to crawlers. It is not known what con- trol a 1 per cent heavy medium oil spray would give if applied alone in July. Tests with Black Scale, Saissetia oleae Bern Two field experiments were conducted to de- termine the effectiveness of DDT in controlling black scale on olives. In the first experiment at Madera, California, sprays were applied on May 26 before the hatch- ing of the black-scale eggs. Two spray mixtures were tested: 1. Mineral seal oil containing 6 per cent DDT (GNB-A) at the rate of 1.25 gallons per 100 gal- lons of water, plus 4 ounces of blood albumin. 2. Two pounds of a talc powder containing 20 per cent DDT (GNB-A) , 1/4 per cent heavy summer oil, and 4 ounces of blood albumin, per 100 gal- lons of water. On July 31, live and dead young scales were counted. The per cent dead in untreated trees and treated trees was as follows: untreated, 36.8; treatment 1, 53.1; treatment 2, 52.6. The treatments compared in the second experi- ment are given in table 16. The sprays were ap- plied at Or land, California, on July 27, several weeks after the scale eggs had begun to hatch. Three weeks later, 100 heavily infested leaves from each treatment were searched for live young scales. Also, for each treatment, 50 adult females from the undersides of leaves were ex- amined for live eggs and crawlers. The table gives the results of these examinations. Although both treatments gave about equal control of adult females, apparently the crawlers were more efficiently controlled with the oil- DDT than with the oil-derris spray. A possible TABLE 16 Comparative Effectiveness of an Oil-DDT and an Oil-Derris Spray in Controlling Black Scale on Olives Per cent Per cent of of leaves' adult females Treatment with live with live eggs scales and crawlers 2 gallons of a light medium soluble oil containing 5 per cent DDT (GNB-A) per 100 gallons of water . . . 1.0 26.0 1 gallon heavy medium oil, 1/2 pound of derris root contain- ing 5 per cent rote- none per 100 gallons 10.0 28.0 explanation might be that crawlers from surviv- ing females were unable to settle on leaves sprayed with DDT. DDT shows promise as a control of black scale on olives. Before recommendations are made, further evidence of control should be obtained, and the presence or absence of harmful residues should be determined. SPRAYING WITH DDT IN A GREENHOUSE TO CONTROL THRIPS, THE VECTORS OF SPOTTED WILT IN TOMATOES 28 89 Spotted wilt, a virus disease transmitted by thrips, is an important problem' for many tomato growers in the San Francisco Bay region. It is particularly serious near the town of San Pablo; if the thrips are not controlled, greenhouse tomatoes are not profitable. In this area one grower who has specialized in producing tomatoes under glass obtains a fine quality of off-season Prepared by M. W. Gardner, Professor of Plant Pathology and Plant Pathologist in the Ex- periment Station; A. E. Michelbacher, Assistant Entomologist in the Experiment Station; and Ray E. Smith, Senior Laboratory Technician, Division of Entomology and Parasitology. 8 Acknowledgment is made of the wholehearted coSperation and assistance of Mr. and Mrs. Francis Aebi of the Aebi Nurseries, Richmond, California. [24] tomato by controlling thrips with frequent fumi- r gations with nicotine. He grows a late fall- winter and a spring crop. Spotted wilt is most troublesome in the late fall-winter crop. This crop is started in July and is transplanted from flats to the greenhouse beds in August. The thrips problem and therefore the spotted-wilt menace are most troublesome from this time on. The threat ends, however, with the advent of the first heavy fall rains, which greatly reduce the thrips population. In the late summer and fall, satisfactory control of thrips and spotted wilt is obtained only by fumigating with nicotine twice a week. Without this treatment it has proved impossible to grow a satisfactory greenhouse tomato crop. Even with the precau- tions taken, some spotted wilt develops. About once a week, therefore, the greenhouses are carefully examined, and the infected plants re- moved. In the early stages of growth, these culls are replaced with new plants or with lat- eral shoots from adjacent plants. After the first heavy rains, fumigation with nicotine is continued; but treatments are spaced much far- ther apart and finally stopped altogether. In the experiment here reported, spraying with DDT was compared with nicotine fumigation for con- trolling thrips and reducing spotted wilt. On August 3, 34 flats of newly transplanted tomato seedlings were treated with a 3 per cent DDT dust to determine whether damage would result. The dust was applied rather heavily but uniformly. Although some injury occurred to the margins and tips of the leaves, the plants outgrew this by the time they were ready to be transplanted into the greenhouse beds. The experiment was conducted with a battery of three similar adjacent greenhouses. Accord- ing to the grower's previous experience, the houses did not differ markedly as to infection by spotted wilt. In one of these, DDT was used; in the others, nicotine fumigation. The DDT was applied as a spray, and in all there were 4 treatments. The composition of the spray for the first 3 applications was as follows: 20 per cent DDT powder (AK20) ... 5 pounds Blood albumin 6 ounces Water 100 gallons The DDT and blood albumin were mixed dry, then slurried, and added to the spray tank when it was half full. For the fourth application 1 pound of pure commercial DDT (G-NB-A) was used per 100 gallons of water. The DDT was mixed dry with 6 ounces of blood albumin. Then a little water was added, and the whole slurried. There was diffi- culty in getting the DDT into suspension. That which went into suspension, however, was decanted off into a half -full spray tank. The residue was again slurried by means of the spray nozzle, and the suspension poured into the tank. This pro- cedure was repeated many times, until only an ounce or so of the residue remained. The deposit of DDT on the tomato foliage, as determined by analysis, was less with this mixture than with the spray mixture used for the first three appli- cations . Sprays were applied on August 21, September 5, 16, and 30. By September 30 the vines had set fruit the size of marbles, and further treatments would probably have created a residue problem. The first spray was applied at a pressure of 325 to 400 pounds. The plants and all inner surfaces of the greenhouse were sprayed. The coverage on the foliage, though somewhat spotty, was good. To avoid breaking off the plants by the force of the spray, the pressure for the second applica- tion was reduced to 125 to 150 pounds. This time only the plants and the supporting poles were treated. Good coverage was obtained. The third spray was applied at 250 to 270 pounds' pressure. The coverage, though fair, was somewhat spotty. An even coverage was obtained with the fourth spray, applied at 300 pounds. About September 16 some slight injury to the lower leaves of the DDT-treated plants was noted. This became apparent between the second and third spray applications. The injury became progres- sively worse; on September 26 it appeared as shown in figure 2, B and C. It first showed up as a yellowing between the main lateral veins and around the margin of the leaflet, followed promptly by a tan-colored papery necrosis of the tissue, first around the margin, then all over the leaflet. The plants in the DDT-sprayed V Fig. 2. --In jury to tomato leaflets caused by spraying with DDT. A, Normal leaflet. B and C, Sprayed leaflets showing interveinal and marginal yellowing. D, Sprayed leaflet in final stages of injury, showing tan-colored, papery drying out of the marginal tissues. house appeared to be lighter green and less stocky than those in the fumigated houses. The dwarfing effect as of September 26 was reflected in leaf length. For the sprayed plants the basal leaves averaged 9.3 inches, and those from the center, 10.5 inches, as compared with 11.6 and 15.2 in- ches, respectively, the average leaf lengths for the unsprayed plants. The sprayed plants were slightly taller: their average height was 38.7 inches, whereas the plants from the unsprayed houses measured 37.6 inches. This slight difference in height was offset by the more stocky and vigorous growth of the unsprayed plants. [25] The injured basal leaves showed no signs of recovery; by October 25 many of them had com- pletely dried up. By November 6, foliage injury had sometimes extended 2-1/2 feet up the plants. A leaflet near the final stage of injury is shown in figure 2, D. In the unsprayed houses the lower leaves remained a normal green. The sprayed plants set considerable fruit, but the first clusters did not size so well as on the unsprayed plants. Table 17 summarizes the information comparing the DDT-sprayed house with the nicotine-treated houses. The greenhouse sprayed with DDT showed less spotted wilt than either of the houses that were fumigated with nicotine. Apparently, then, this material controlled the thrips more effici- ently than did the nicotine. Since 2 to 3 weeks is required for the symptoms of spotted wilt to appear, the DDT sprays were presumably responsi- ble for the control up to and including the October 25 rogueing. Thereafter any control was probably the result of the nicotine fumigations which, starting October 9, were substituted for the DDT treatment. Towards the last of October, heavy rains ended any serious threat of spotted wilt. The DDT therefore proved very effective in controlling thrips and spotted wilt during the most critical period. The foregoing results were substantiated in a greenhouse of 418 plants that was located near by. Because of leaky conditions, this house could not be satisfactorily fumigated with nico- tine. It was therefore sprayed 3 times with DDT (September 5, 16, and 30). Only a very few plants became infested with spotted wilt. The spray caused some injury, although less severe than in the house where the principal experiment was conducted. The DDT residue on various portions of the tomato plants was analyzed. The results appear in the section on residues. There was a rela- tively high deposit of DDT on the basal leaves, the leaves which finally died. Conclusions Under greenhouse conditions, a DDT spray (1 pound of DDT per 100 gallons) , applied approxi- mately every 2 weeks , controlled thrips and re- duced spotted wilt of tomatoes more effectively than did frequent fumigation with nicotine. Since, however, the treatment seriously damaged the plants, it cannot be recommended unless further experiments show that such injury can be avoided. Summary In a region where spotted-wilt virus trans- mitted by thrips limits the production of a late- fall greenhouse crop of tomatoes, four applica- tions of a DDT spray were made, at fortnightly intervals (August 21, September 5, 16, and 30) during the critical infection period in 1 of 3 similar adjacent greenhouses. In the other 2 houses the standard fumigation with nicotine 2 times per week was applied. The infected plants were removed every week. By October 25 only 137 cases had occurred among the 1,421 plants in the sprayed house, as compared with 161 cases in one fumigated house containing at first 1,427 plants. There were 318 cases in the other fumigated house containing at first 1,456 plants. The DDT sprays injured the plants rather se- TABI£ 17 Summary of Information on a Comparison of DDT with Nicotine Fumigation for Controlling Thrips and Spotted Wilt on Tomatoes Grown in Greenhouses Green- Treatment of greenhouse Number of plants in green- house at start Number of moved spotted-wilt plants re- on the dates given* Total in- fested plants re- moved by October 25 house no. Sept. 7 Sept. 11 Sept. 18 Sept. 25 Oct. 5 Oct. 12 Oct. 25 1 Sprayed with DDT August 21, September 5 , 16 , and 30 ; t fumigated with nicotine on October 9, 11, and then twice a week until November 7, and once a week there- after until November 30. 1,421 22 25 22 14 23 20 9 135 2 Fumigated with nicotine twice a week up to Novem- ber 7 , then once a week until November 30. 1,427 12 37 32 19 14 24 22 160 3 Fumigated with nicotine twice a week up to Novem- ber 7 , then once a week until November 30. 1,456 11 25 76 32 73 27 71 315 *Later inspections resulted in the removal of additional spotted-wilt plants as follows: November 3, 11 from greenhouse 1, 25 from greenhouse 2, and 11 from greenhouse 3; December 4, 18 from no. 1, 28 from no. 2, and 26 from no. 3- t Amount of spray used August 21, 200 gallons; September 5, 185 gallons; September 16, 230 gallons: and September 30, 170 gallons. [26] verely, however, causing yellowing between the main veins and around the margins of the leaf- lets, followed by marginal drying of the leaf- lets and eventual death of the lower leaves up to a height of about 2-1/2 feet. There was also evidence of retarded growth and a markedly re- duced yield. LABORATORY TESTS WITH DDT ON TRUCK-CROP INSECTS 30 31 In the spring of 1944 a series of laboratory experiments were conducted with different con- centrations of DDT on several common insects affecting truck crops in the Salinas Valley. The mixtures used were as follows: vapo-sprays containing 0.3, 0.6, 1.2, 2.4, and 3.6 per cent by weight of DDT in 30 per cent kerosene and 70 per cent mineral seal oil; and dusts containing 1 and 3 per cent DDT (both in pyrophyllite) . The usual procedure was to run three series: first, an application of the material to potted broccoli plants on which insects were placed within 1 hour; second, direct application to the insects, and their transfer to unsprayed or undusted plants; third, application to broccoli or alfalfa plants, on which insects were placed at later intervals. The plants or insects were treated in the lathhouse and moved inside a greenhouse for observation. Circular cages of cloth were placed over the plants, and a strip of cotton sealed the space between the flower pots and the cage. The insects tested included adults of Dia- brotica 11-punctata (Mann.); adults of Lygus he s per us Knight; larvae of the vegetable weevil, Listroderes obliquus Gyllen. ; and adults and larvae of the potato tuber moth, Cnorimoschema operculella (Zeller). Insects to be treated were placed in small, fine-wire gauze cages. The vapor sprays were applied as three puffs from a Brown hand gun held 15 inches away. This would correspond to a treatment of about 5 gal- lons per acre. The plants sprayed were given two puffs from each of two sides at a distance of 16 inches, equal to about 6 gallons per acre. Dusts were applied in the same manner with a small hand-duster. The amount was estimated at 35 to 40 pounds per acre , although even dust distribution was difficult to obtain. The in- sects for the tests were largely swept from al- falfa, and were treated within 24 hours. For each test, 10 insects were used. The results are discussed under the headings of the various insects. Adults of Diabrotica 11-punctata Evidently, diabrotica beetles are very sen- sitive to DDT in any form. When they were placed 30 Prepared by W. H. Lange , Jr., Junior Ento- mologist in the Experiment Station. The help of Frank Herbert in conducting these experiments is gratefully acknowledged. on dusted or sprayed broccoli plants in cages, their immediate reaction to DDT was less notice- able than when they were sprayed or dusted di- rectly. In most cases, however, the time re- quired for a 100 per cent kill was the same whether the insects were directly treated or were placed on treated foliage. From the stand- point of eventual mortality there was no con- sistent difference between the different per- centages of DDT in the oils for the range 0.3 to 3.6 per cent. There was, however, a greater carryover effect where the higher percentages of DDT were applied to the plants. The 3 per cent dust was superior to the 1 per cent dust (table 18) TABLE 18 Summary of Laboratory Studies on the Effect of DDT on Diabrotica Beetles, Using Ten Insects for Each Test Days be- Days to Days to tween first 100 treatment dead per To what applied and cag- after cent ing of caging mortality insects 0.3 per cent DDT by weight in a vapo-spray Broccoli Insects(direct) 0.6 per cent DDT by weight in a vapo-spray Broccoli Insects(direct) 1.2 per cent DDT by weight in a vapo-spray Broccoli 2 7 Broccoli 2 2 6 Alfalfa 2 5 7 Alfalfa 35 3 ■ • ■ Insects(direct) ... 2 3 2.1+ per cent DDT by weight in a vapo-spray Broccoli 1 11 Broccoli 2 2 6 Broccoli 5 1 3 Broccoli 35 2 . • • Alfalfa 2 2 3 Alfalfa 6 1 3 Alfalfa 35 3 • • • Insects(direct) 1 5 3.6 per cent DDT by weight in a vapo-spray Broccoli 1 5 Broccoli 2 1 4 Alfalfa 2 2 3 Alfalfa 6 1 2 Alfalfa 40 1 5 Insects (direct) 1 7 1.0 per cent DDT dust Broccoli 2 4 Broccoli 2 5 7 Insects (direct) 2 10 3.0 per cent DDT dust Broccoli 1 5 Broccoli 2 1 7 Broccoli 5 1 2 Insects 1 2 [27] The beetles reacted to the DDT within an hour, first by nervousness, then by loss of equilibrium. After another hour, a few were on the bottom of the cages on their backs with their legs waving in the air. Several days were usually required for a final kill, although there was no further evidence of any feeding and although no beetles recovered. Spray on alfalfa plants treated on January 20 killed, within 5 days, 100 per cent of the pests introduced on March 11. A similar effect was noticed with the 2.4 and 1.2 per cent vapors, although the 1.2 per cent strength killed only 20 per cent of the beetles introduced March 6. Table 18 summarizes the results with dia- brotica beetles. Adult Lygus Bugs Vapo-sprays with 1.2, 2.4, and 3.6 per cent DDT were effective when applied to plants on which lygus bugs were subsequently placed or when applied directly on the insects. Little difference in the time required to kill all the bugs was found among the different percentages used. Alfalfa plants sprayed with the 3.6 per cent strength on January 20, and introduced January 31, killed 100 per cent of the bugs within 4 days. When the DDT materials were ap- plied directly to lygus bugs, the insects showed an immediate effect. At first they appeared to lose some locomotive power and would climb re- peatedly up the sides of the cages, only to fall down. Later they lost all power of locomotion and remained on their backs, kicking their feet, until death ensued. Table 19 presents the re- sults. TABLE 19 Comparative Effect of Various Concentrations of DDT on Lygus Bugs, Applied as Vapo-sprays Di- rectly on the Bugs, or Applied on Plants, the Insects Being Introduced Immediately after the Application* Per cent of DDT by weight in a vapo- spray 1.2 1.2 2.4 2.4 3.6 3.6 3.6 Plants or in- sects to which applied Broccoli Insects (direct) Broccoli Insects (direct) Broccoli Alfalfa Insects (direct) Days to first dead after caging Days to 100 per cent mor- tality *In each test 10 lygus bugs were used. Vegetable Weevil Larvae In these tests the materials were applied di- rectly to the insects or to l/4-inch carrot sec- tions on which larvae were later placed. The vapo- spray materials gave a more rapid kill than the dusts, although the 3 per cent dust was com- parable with the 1.2 per cent oil mixture. Wee- vil larvae did not die rapidly, but remained in a stupor for a week or more; the 1 per cent dust, for example, kept them in this state 6 to 9 days. They fed vigorously on the dusted or sprayed carrot sections at first, but then *f stopped feeding and remained inactive until death. When dead they were internally discolored. Table 20 shows the results. Potato. Tuber Moth Larvae of the potato tuber moth displayed a violent reaction to DDT materials applied di- rectly to them. Violent wriggling during the first 3 hours was followed later by spasmodic jerks and finally by death. Larvae placed on potato sections did not show much effect at first, but all died in a few days. The larvae reacted TABLE 20 Comparative Effect of DDT on Mature Vegetable Weevil Larvae Where the Materials Were Applied Either- Directly to the Insects or to Small Sec- tions of Carrots on which Larvae Were Placed Per cent of DDT Days Days to by To what to 100 weight , applied first per in dead cent treatment mortality Vapo-sprays 1.2 1.2 2.4 2.4 Larvae (direct Carrots Larvae (direct Carrots Dusts 1.0 1.0 3.0 3.0 Larvae (direct) Carrots Larvae (direct) Carrots faster to vapo-spray materials than to 3 per cent DDT dust. Adult moths when dusted or sprayed with the DDT combinations displayed an immediate loss of locomotion and flight, and died within 24 hours. Table 21 summarizes the results of these experiments. TABLE 21 Comparative Effect of DDT on the Potato Tuber Moth Where the Materials Were Applied Directly to the Adults or Larvae or to Sections of Potatoes Which Were Immediately Infested with Larvae Per cent of DDT by weight , in treatment To what applied Days to Days 100 to per first cent dead mortality Vapo-sprays 0.6 0.6 0.6 1.2 1.2 1.2 Larvae (direct Adults (direct Potato Larvae (direct Adults (direct Potato Dusts 3.0 3.0 3.0 Larvae (direct) Adults (direct) Potato [28] CONTROLLING CODLING MOTH ON WALNUTS WITH DDT 32 In the work with codling moth, Carpocapsa pomonella (L. ), the DDT plots were at Linden, in San Joaquin County,- and the study was conducted on the Payne variety of walnut. One and two spray applications were compared. The composition of the first spray was as fol- lows: 5 pounds of 20 per cent DDT wettable powder (A20) to 100 gallons of water. The second spray had the following composition: 5 pounds of 20 per cent DDT powder (AK20) and 4 ounces of blood al- bumin, to 100 gallons of water. For the first spray the DDT was slowly added to a tank half full of water. For the second, the DDT and blood albumin were thoroughly mixed, then slurried; and the mixture was slowly added to a half -full tank. The sprays were applied with a Bean spray rig through a Meyer gun at a pressure of about 500 pounds. From 30 to 35 gallons was used per tree for the first spray; 20 to 35 gallons for the second. The first spray was applied on May 9. No in- fested nuts were observed at this time, though an examination of 200 nuts from check trees re- vealed one small caterpillar on the outside of a nut. With the exception of the walnut aphid, nearly all insect life appeared to have been knocked from the trees shortly after the spray was applied. On the ground there were many in- sects, including ladybird and other beetles, parasites, flies, a few caterpillars, and adult codling moths. There were also spiders. Most of these organisms were nearly dead, or incapaci- tated and kicking. The adult codling moths seemed to be the least affected of the insects. On May 10, the day after the trees were sprayed, very few aphids were found alive on the trees, and there were many dead on the ground. The aphid population trends as probably affected by DDT are discussed in the section "Effect of DDT on the Walnut Aphid." In the growing season, 11 thorough tree ex- aminations were made to determine the degree of codling moth infestation. The surveys were gen- erally made at intervals of about 2 weeks. The DDT-sprayed trees remained practically free of infested nuts. The check trees became heavily infested; the infestation in them built up rapidly and by the end of May was about 30 per- cent. On May 29 the DDT-sprayed trees were near- ly free of insects. Three small codling moth larvae were observed on nuts. These moved slowly and did not appear normal. On June 7, numerous eggs were observed, and 3 newly hatched larvae. The latter, though apparently normal, showed no signs of trying to enter nuts. On June 26, 480 nuts were examined. Two of these were found in- 38 Prepared by A. E. Michelbacher , Assistant Entomologist in the Experiment Station; Clark H. Swanson, Assistant in Agricultural Extension; and Gordon L. Smith, Associate in the Experiment Sta- tion. fested. The larvae had not penetrated the normal husks, but gained entry through injuries. Many hatched eggs were observed, and there was some evidence of larval activity; but most of the lar- vae, at least, appeared to have died without be- ing able to penetrate the husks. Of 235 dropped nuts collected, 26 were infested. Possibly some of these nuts had been infested before the spray was applied on May 9. In 13 (or 5.5 per cent) of the nuts, entry was made through the blossom end; and in a like number, entry was through the side or the stem end. In the latter cases, all en- tries were through blighted or otherwise injured areas. Half of the treated trees were sprayed for a second time on June 26 because the first spray appeared to be losing its effectiveness. As with the first spray, most insects in the trees were dislodged. According to all the following surveys, trees that were sprayed twice remained nearly free of infested nuts. In trees sprayed only once, a fair infestation appeared likely to set in. On July 24, 2.08 per cent of the nuts (as determined by tree surveys) were found infested, and some of the larvae had penetrated through uninjured husks. Most of these larvae, however, apparently died before penetrating far, for the later surveys showed a marked drop in the percentage of nuts infested. In the harvested crop only 0.9 of 1 per cent of the nuts were infested where the trees were sprayed once , and none were found where the trees received two applications of DDT. Furthermore, two thirds of the nuts infested, where the one spray was applied, would have been classed as culls for other reasons. The degree of infestation of the harvested crop from the check trees was 18.63 per cent. Results of DDT residue determinations on wal- nuts, made late in August, are given in the sec- tion on residues. As would be expected, the nuts sprayed twice showed a much higher deposit than those that had only one spray, applied when they were very small. Conclusions According to the results of this experiment, DDT is highly effective against the codling moth. One spray applied May 9, when the walnuts were still extremely small, produced exceptionally good control. In July, when the second brood of moths was making its appearance, the spray seemed unlikely to give adequate protection. Although some larvae did penetrate the husks, most of these died without doing any damage. The second brood during the season was smaller than usual. Not until after further study will it be possible to conclude that a single early spray will effec- tively protect the walnut crop from the first and second broods of codling moths. Trees sprayed May 9 and again on June 26 gave as perfect con- trol as could be expected. Although the results are very promising, much more work is needed be- fore DDT can be safely recommended for commercial use. [29] EFFECT OF DDT ON THE WALNUT APHID 33 The effect of DDT on the walnut aphid, Chromaphis juglandicola (Kalt.), was observed during the use of this material against codling moth on walnuts at Linden, California. A single application of spray on May 9 was compared with two applications, one on May 9 and the second on June 26. The composition of the sprays and other data pertaining to treat- ment are given in the section "Controlling Cod- ling Moth on Walnuts with DDT." The walnut aphid was very abundant at the time of the May 9 spraying. Nearly all insect life except the walnut aphid was knocked out of the trees shortly after the spray was applied. On May 10, the day after treatment, many dead aphids were on the ground, and almost none could be seen on the trees. On May 18 very few aphids and no ladybird beetles appeared to be present. Twenty days after treatment no immature aphids could be found. The few winged forms noted are believed to have flown in from the surrounding heavily infested trees. One or two Hippodamia adults were observed, as was a green lacewing adult. Eggs of the latter species were also noted. By June 7 some aphid reproduction had occurred, and green lacewing. eggs were easy to find. At this time the aphid population became so great that the entire orchard where the plots were located was dusted with a nicotine dust. The resulting kill was exceptionally good. On June 26, very little aphid reproduction was evident on the DDT-sprayed trees. Half these trees were treated a second time. Some few ladybird beetles were knocked off; but since there were not many, conditions were apparently just becoming favorable for them to exist in the trees. By July 10 there was some increase in aphids where the trees had received but one spray; but the number present was far from being of serious proportions. Where two sprays were applied, practically no aphids were evident. On July 24, the trees sprayed once showed a marked build-up in aphid population, and numerous lady- bird beetle eggs were observed. Where two sprays had been applied, no aphids, or very few, were apparent . By August 8 the trees sprayed once with DDT showed considerable aphid injury in comparison with the trees sprayed twice or the surrounding unsprayed trees. The foliage was yellowish, and there were more drying leaves. On these trees there were many ladybird beetles as well as some syrphid fly larvae. Where two sprays had been applied, a very large aphid population had developed. No ladybird beetle larvae were noted , though a few adult Hippodamia and a few clusters of freshly laid ladybird-beetle eggs were seen. The aphid infestation in the orchard 3 3, Prepared by A. E. Michelbacher , Assistant Entomologist in the Experiment Station; Clark H. Swanson, Assistant in Agricultural Extension; and Gordon L. Smith, Associate in the Experiment Station. where the plots were located had reached a stage where much damage was being done. The aphid q population, though very large, was considerably less than on the trees sprayed with DDT. The number of aphids per leaflet was compared. The leaflet selected for examination was next to the terminal one; and leaflets were collected at ran- dom around the skirt of the trees. The number of leaflets examined and the average number of aphids found per leaflet were as follows: Treatment Number of leaflets examined Unsprayed 78 Sprayed once with DDT . 36 Sprayed twice with DDT. 36 Average number of aphids per leaflet 81 129 239 Because of the severe aphid damage the or- chard was treated with a nicotine dust. The re- sult was a satisfactory kill throughout the or- chard, including the DDT-treated plots. Without the nicotine treatment, the DDT-treated trees would probably have been seriously 'defoliated. The nicotine dust was applied just in time to save the trees sprayed twice with DDT; if the extremely large aphid population had not been destroyed, marked injury would have occurred within a few days. As a result, the trees sprayed only once were more severely injured than those sprayed twice. In addition, they showed fewer aphids, probably because many preda- % tors were present whereas in the two-spray treat- ment the predators were just becoming established. The delayed build-up in the aphid population with the two sprays was undoubtedly the result of the aphids' being controlled for a longer period with this treatment. On August 24 there were very few aphids on all trees sprayed with DDT. The trees of both treatments appeared to be lighter green than the surrounding unsprayed trees. At harvest, the DDT-treated trees were found to have been more severely injured by aphids than the others. The extent of the damage was as follows: 1. Most severely injured: trees sprayed once with DDT. 2. Less severely injured: trees sprayed twice with DDT. 3. Least severely injured: unsprayed sur- rounding trees. Conclusions In the work on walnuts at Linden, DDT was effective in killing the walnut aphid. It also killed the predators and probably the parasites. Eventually, however, serious infestations of the pest developed. According to observations, this trouble could be explained by the fact that DDT remains effective against the predators and para- sites longer than against the aphid itself. Thus fr the aphid becomes established and builds up a large population before being subjected to its natural enemies. The results of this investiga- tion indicate that more work is needed before any recommendations can be made for the use of DDT as an insecticide on walnuts. [30] STUDIES ON THE POSSIBLE EFFECTS OF DDT ON THE f GROWTH OF RICE AND BARLEY 34 As a result of studies on mosquito control in irrigated agricultural areas, it became essential to determine the possible secondary effects of DDT on crop production in the districts affected. Controlled greenhouse experiments were performed to elucidate the possible effects of this chemi- cal — first, on the germination of seed; second, on the soil microflora; third, on the over-all growth of plants under conditions similar to those suggested by the United States Public Health Service for controlling the mosquitoes in areas where rice is normally grown. This is a brief summary of the studies; a more complete report is being prepared for publication else- where . Rice seed was germinated in aerated suspen- sions of DDT in water. The percentage germina- tion was not materially affected over a wide range of quantities dispersed in suspensions of 0.1 to 100.0 p. p.m. of DDT. The viability of the seed was not markedly impaired by concentra- tions soluble in aqueous media, nor by excess of the chemical in suspension in intimate contact with the germinating organism. In another experiment, 35 water suspensions containing 10 and 100 p. p.m. of DDT were incor- porated with appropriate media; and the subse- quent development of a mixture of microorganisms common to soils was observed therein. Growth was not significantly affected by the presence of this insecticide. The third phase of this investigation em- bodied the possible effects of soil treatment on the growth of rice and barley. Two situations were of interest: first, a pretreatment of the soil equivalent to an annual DDT irrigation flooding of rice lands, followed by the growth of either rice or barley in 18 kilos (air-dry) of Yolo silty loam contained in metal tanks, the rice soils being flooded with water at seed- ing time or later; second, a concurrent DDT irrigation of soil by flooding (to a depth of 4 inches) and by growth of rice. Soil, pretreated with the DDT-oil emulsion, as recommended by the Malaria Control Division of the Public Health Service, 36 supported subsequent growth of either rice or barley under the experimental conditions, comparable with that produced on similar untreated soil. Under the limited conditions of these con- trolled experiments, germination and vegetative development of rice were not significantly im- paired when the concurrently applied irrigation 34 Prepared by T. C. Broyer, Assistant Plant Physiologist in the Experiment Station. 3 This study was carried on by H. A. Barker, I Assistant Professor of Soil Microbiology and Assistant Soil Microbiologist in the Experiment Station. Eive per cent DDT and 0.5 per cent of ap- propriate emulsifier in no. 2 Diesel fuel oil, applied at the rate of 1 part to 20 million parts of water. water contained the insecticide-oil emulsion. Plant growth, including grain production and maturation, was satisfactory when soil was either previously or simultaneously treated with the mixture lethal to mosquito larvae. In one series, grain production was reduced, but the difference from the control was not highly sig- nificant. Flooding of the soil with water, alone or in conjunction with the insecticide mixture, at or before the time of seeding, as compared with later flooding, may reduce the percentage germination. Experimental differences, however, were statistically not highly significant. From the standpoint of agricultural soil economy, the recommendations for destroying the mosquito larvae are probably not objectionable, at least for a limited number of applications. ANALYSES FOR DEPOSITS AND RESIDUES OF DDT IN 1944 37 A quantitative analysis for DDT may be based upon a certain fact --namely, that when the com- pound is warmed with alcoholic alkali, one mole- cule of hydrogen chloride is split out. This liberated chloride may then be determined by any quantitative method such as titration with standard silver nitrate solution, chromate being used as indicator. To exclude inorganic chloride, the sample suspected of containing DDT is ex- tracted with benzene, in which DDT is readily soluble but inorganic chloride is not. In the present work, the procedure was as follows. The sample was placed in a jar with benzene and gently shaken for about 1/2 minute. The benzene was then poured through a cotton filter in a funnel to remove dirt and other for- eign matter, and another portion of benzene was added to the sample. A third portion was used for rinsing the sample and funnel; and the total volume of benzene was brought to a definite vol- ume, usually 250 or 500 cc. An aliquot portion, usually 100 cc, was placed in a vacuum distill- ing flask resting on an asbestos screen, and the benzene removed by very gentle heating. Without removing the residue from the flask, 10 cc of 1 normal alcoholic sodium hydroxide was added; and the contents were refluxed gently for 20 minutes. The solution was then cooled, and the condenser was rinsed by introducing 10 cc of distilled water from the top. A drop of phenolphthalein solution was then added to the flask, and just enough 1 normal nitric acid was put in to dis- charge the red color. Next, 1 cc of 1 per cent sodium chromate solution was added, and the total volume brought to about 30 cc. The solution was then titrated with standard 0.01 normal silver nitrate solution to the first change of color due to formation of a minute amount of the reddish silver chromate. This end point is definite ex- 37 Prepared by W. M. Hoskins, Professor of Ento- mology and Entomologist in the Experiment Station, [31] cept when the solution is contaminated with dark or otherwise strongly colored substances from the sample. Such difficulty was seldom encoun- tered, and filtering after the digestion with alkali was an effective remedy for it. The accuracy of the procedure was checked at intervals by adding known amounts of pure DDT to field samples not previously exposed to DDT. Recovery was satisfactory: for example, the ad- dition of 8.8 and 11.8 mg of DDT to pear peel- ings resulted in recovery of 8.7 and 11.7 mg; and from 7.0, 9.5, and 10.2 mg added to pear meat, 7.4, 9.3, and 10.]. mg, respectively, were recovered. Allowance was made, of course, for chloride in the reagents, which amounted to 0.16 mg, chiefly in the alkali. When samples of the various plant materials from untreated plots were analyzed, no material was found that yielded chloride and hence might be mistaken for DDT. Samples of plant materials treated with DDT sprays or dusts were obtained from the field and analyzed by the procedures described above. Residues on Alfalfa An alfalfa field was treated with 3 per cent DDT dust (A3) at the rate of 28 pounds per acre on July 26 and again on August 12. A sample was taken on September 8 and found to contain 29 p. p.m. DDT, calculated on fresh weight of the alfalfa. Residues on Bartlett Pears Analysis was made for DDT deposit residues on Bartlett pears that had been sprayed with 5 pounds of 20 per cent wet table DDT powder (A20) per 100 gallons of water. The fruit was picked August 7, and analyzed August 22 to 26, inclu- sive, with the following results: Dates sprayed DDT, p. p.m. May 5 and 22 0.5 May 5 and June 14 2.7 May 5 and July 6 2.1 May 5 and 22, June 14, and July 6. . 3.7 Likewise, analysis was made of deposit resi- dues from spraying with 2-1/2 quarts of 20 per cent DDT in emulsible oil base (SH20) per 100 gallons of water. Results were as follows: Dates sprayed DDT, p. p.m. May 5 and 22 1.7 May 5 and June 14 2.3 May 5 and July 6 3.6 May 5 and 22, June 14, and July 6. . 6.1 Residues on Walnuts Walnuts of the Payne variety were analyzed for DDT deposit residue. A spray containing 5 pounds of 20 per cent DDT wettable powder (A20) was applied on May 9. Samples were picked July 10 and analyzed August 27. The residue was 13 micrograms DDT per square inch of walnut . surface . On a portion of the plot a second spray was applied June 26. This consisted of 5 pounds of 20 per cent DDT powder (AK20) , plus 4 ounces of blood albumin, per 100 gallons of water. Samples taken at the same time as described above gave a residue of 43 micrograms of DDT per square inch of walnut surface. The results of analysis of other materials are shown in tables 22, 23, and 24. TABLE 22 Deposit Residues of DDT on Peas, Resulting from Two Applications of Vapo-spray at the Rate of 5 Gallons per Acre, on August 29 and September 2 Composition of spray* Portion of plant analyzed t DDT in parts per million 1.2 per cent DDT by weight in oil Pods Pods Pods Pods Kernels 39 54 76 93 Not over 0.5 *The oil-base carrier used was 30 per cent kerosene and 70 per cent mineral seal oil (92 U.R. ) . IA11 samples were picked on September 26 and analyzed October 2. [32] TABLE 23 Deposit Residues on Olive Foliage and Fruit Composition of spray per 100 gallons Date sprayed Portion of plant analyzed Date sample picked Date ana- lyzed DDT in micro- grams per square inch DDT in parts per million Mission variety 1 per cent by volume of light medi urn oil plus 1/2 per cent by volume of 20 per cent DDT in oil ISH20) 1-2/3 per cent by volume of light medium oil containing 40 grams DDT (GNB-A) per liter 2 per cent by volume of mineral seal oil containing 40 grams DDT (GNB-A) per liter 1/4 per cent by volume of heavy medium oil, plus 4 pounds of 20 per cent DDT powder (AK20) . . . July 27 July 18 July 18 July 15 July 18 July 18 July 18 Leaves Leaves Olives Olives Leaves Olives Olives July 17 Olives Aug. 9 Aug. 15 Sept. 7 Sept. 5 Sept. 5 Sept. 7 Sept. 7 Sept. 7 Aug. 29 Sept. 22 Sept. 23 Sept. 25 Sept. 28 Sept. 26 Sept. 30 Sept. 27 47 21 40 30 26 23 27 33 Manzanillo variety 3 per cent by volume of kerosene containing 60 grams DDT (GNB-A) per liter, plus 1 ounce of dioctyl ester of sodium sulfo succinate emulsifier Sept. 11 Sept. 11 Leaves Olives Sept. 12 Sepf. 12 Sept. 23 Sept. 20 97 71 TABLE 24 DDT Deposits and Residues on Foliage, on Small Green Fruit, and on Mature Fruit of Tomatoes* Date sprays were applied Date samples were taken Portion of plant analyzed DDT in micrograms per square inch DDT in parts per million Aug. 21 and Sept. 5 Aug. 21 and Sept. 5 and 16 Aug. 21 and Sept. 5 and 16 Aug. 21 and Sept. 5, 16, and 30 Aug. 21 and Sept. 5, 16, and 30 Aug. 21 and Sept. 5, 16, and 30 Sept. 10 Sept. 16 Sept. 30 Sept. 30 Sept. 30 Nov. 20 Leaves from Leaves from Leaves from Leaves from Leaves from Leaves from over whole plants, top of plants . . bottom of plants . over whole plants top of plants . . bottom of plants . Leaves Leaves Leaves Leaves Leaves Leaves Small i inch from top third of plants from center third of plants from bottom third of plants from top third of plants from center third of plants from bottom third of plants ?reen tomatoes, about 1/2 in diameter Ripe tomatoes, average weight 50 grams 86 58 53 240 260 157 83 113 138 69 103 210 Not over 0.5 ^Composition of spray per 100 gallons used August 21 and on September 5 and 16 contained 5 pounds of 20 per cent DDT powder (AK20) and 6 ounces of blood albumin. Spray used September 30 contained 1 pound DDT (GNB-A) and 6 ounces of blood albumin. -Feb. ,'45(3446; [33]