UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA PARADICHLOROBENZENE AS A SOIL FUMIGANT E. O. ESSIG BULLETIN 411 October, 1926 UNIVERSITY OF CALIFORNIA PRINTING OFFICE BERKELEY, CALIFORNIA 1926 Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://www.archive.org/details/paradichlorobenz411essi PARADICHLOROBENZENE AS A SOIL FUMIGANT E. O. ESSIGi INTRODUCTION Paradichlorobenzene was first demonstrated as a commercial insecticide in 1915 by A. B. Duckett, 2 Scientific Assistant in the Bureau of Entomology of the United States Department of Agricul- ture, who showed that it was an excellent fumigant for stored-product insects, case-bearing clothes moths, roaches, ants, museum pests, and miscellaneous household insects. In 1919, E. B. Blakeslee, 3 also of the Bureau of Entomology, issued a bulletin on the control of the peach tree borer by the use of toxic gases and thus paved the way for a wide field of experimental work throughout the United States and the world. A number of important bulletins from various experiment stations followed. 4 Practically all the work done in the East and South has been on the peach tree borer, Aegeria exitiosa Say, an insect which is a pest chiefly on peach trees grafted on peach roots in those sections. In the experi- ments thus conducted excellent results have been obtained in the control of the peach tree borer, the average being about 94 per cent, but all eastern and southern writers caution against the use of this soil fumigant on peach trees less than three years old and on apple and pear trees of any age. Experiments were started with paradichlorobenzene for the con- trol of the Pacific or California peach tree borer, Aegeria opaleseens Hy. Edw., in the summer and fall of 1921 and have been continued to the present time. Conditions in California are very different from 1 Associate Professor of Entomology and Associate Entomologist in Experi- ment Station. 2 Duckett, A. B. Para-dichlorobenzene as an insect fumigant. U. S. Dept. Agr. Bur. Ent. Bui. 167:1-7, pi. 2. 1915. s Blakeslee, E. B. Use of toxic gases as a possible means of control of the peach-tree borer. U.S. Dept. Agr. Bur. Ent. Bui. 796:1-23, pi. 2. 1919. 4 Peterson, Alvah. Some soil fumigation experiments with para-dichloro- benzene for the control of the peach tree borer, Sanninoidea exitiosa Say. Soil Science 11:305-318, pi. 1. 1921. Paradichlorobenzene (p-c-benzene) for controlling the peach-tree borer. New Jersey Agr. Exp. Sta. Circ. 126:1-12, pi. 2. 1921; Circ. 156 (revision of above) 1923. Snapp, O. I., and Alden, C. H. Further studies with paradichlorobenzene for peach borer control. U.S. Dept. Agr Bui. 1169:1-18, pi. 3. 1923. Guyton, T. L., and Stear, J. R. PDB (paradichlorobenzene) and other con- trols for the peach tree borer. Pennsylvania Dept. Agr. Gen. Bui. 383:1-10. 1924. 4 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION those in the eastern and southern states and the results obtained have proved to be of interest and value. Experiments have been conducted not only for the control of the Pacific peach tree borer, but also for woolly apple aphis, Eriosoma lanigera (Hausmann) ; the pear root aphis, Eriosoma languinosa Hartig ; the grape phylloxera, Phylloxera vitifoliae Fitch; wireworms; and the garden centipede, Scutigerella immaculata (Newport). A preliminary report was made in 1922 5 and a short review appeared in 1923 and again in 1925. 6 Since the pre- liminary investigations, tests and demonstrations have been conducted by different farm advisors, horticultural commissioners, and farmers in many parts of California. It is the aim of this bulletin to present the data available from all these sources in as clear and brief a manner as possible. The wide use being made of this fumigant is well illustrated from the fact that during the year 1924, 39,695 pounds, and in 1925, 59,469 pounds, or a total of 99,164 pounds for the two years, were used in California, according to figures furnished by one of the lead- ing wholesale distributors. One of the important manufacturers estimates the amounts used in California as about 50,000 pounds in 1924 and 120,000 pounds in 1925. These figures, while not accurate, serve to give an idea of the large amounts of this material now being used in California. The counties which have shown the greatest interest in paradichlorobenzene used approximately the following amounts in 1924 and 1925: Santa Clara, 62,500 pounds; Alameda, 41,250 pounds; San Benito, 15,500 pounds; Sonoma, 3250 pounds. The reports given in table 1 on the use of paradichlorobenzene in various parts of California during the past three years (1923, 1924, 1925) will give an idea of the extensive trials and uses of the material within the state. CHEMICAL AND PHYSICAL PROPERTIES OF PARADICHLOROBENZENE Paradichlorobenzene, commonly called P D B, is a white crystalline material and appears much like rock salt (fig. 1). It is rather volatile and the vapor is a little more than five times as heavy as air and slighly more than twice as heavy as carbon disulfid. The vapor diffuses readily through the air, downward, outward, and eventually upward, and has a strong ether-like odor which is practically non-poisonous s Essig, E. O. Para-Dichlorobenzene, a soil fumigant. Monthly Bui. Cali- fornia State Dept. Agr. 11:28-30. 1922. e Home, W. T., Essig, E. O., and Herms, W. B. California Agr. Exp. Sta. Circ. 265:80-81. 1923, reprint 96-97. 1925. BUL. 411] PARADICHLOROBENZENE AS A SOIL FUMIGANT TABLE 1 General Summary of Work in California with Paradichlorobenzene* County Person conducting or reporting work done Kinds of trees treated Number of trees treated Results Trees killed Contra Costa A. M. Burton, Farm Ad- visor. Pear 50,000 Good 1 Alameda J. B. Hammon, Assistant Farm Advisor. Apricot Prune Cherry 330,000 Excellent None Peach, 1 year 200 Only 2 borers None from seed. found when trees dug. San Benito W. J. Tocher, Farm Ad- visor. Apricot Prune 15,500 Good None Santa Cruz Paul Williamson, Assistant Farm Advisor. Apricot Apple 7,500 Good 1 Sonoma H. P. Everett, Assistant Farm Advisor. Apple 25,900 Good None Santa Clara L. R. Cody, Horticultural Commissioner. Apricot Prune 550,000 Good None Mendocino C. S. Myszka, Farm Ad- visor. Pear Poor 18 trees badly in- jured or killed! Napa H. J. Baade, Farm Advisor Apple 4,000 Good None Pear Lake L. C. Barnard, Farm Ad- visor. Pear 2,000 Good None * A number of other counties used considerable amounts of paradichlorobenzene but accurate data could not be obtained as to the work done and results secured. t These trees were given an extra heavy dose and the surface of the soil was wet after treatment. (See full statement, p. 14). Fig. 1. — Crystals of paradichlorobenzene and two types of measures. The measures hold one and two ounces of crystals respectively. 6 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION to human beings but is very toxic to insects, affecting chiefly the nervous system. The crystals are soluble in hot alcohol or in ether, benzene, and carbon disulfid, but are only slightly soluble in water. Volatilization in or on the soil is very slow at 55° F and more rapid at 75° F or above. For general purposes the best results are obtained when the soil temperature is from 75° to 85° F. In the soil at a temperature of 75° F, from three to six weeks are required for com- plete volatilization, while at low temperatures paradichlorobenzene may remain intact throughout the winter and spring under most conditions. Because of its volatility paradichlorobenzene is packed in air-tight tin or other containers and should be kept sealed in such when not in use. METHODS OF APPLICATION The application of paradichlorobenzene is very simple and is based upon the fact that the evolving vapor is heavier than air and will gradually settle downward if protected against upward and outward diffusion. It is placed either on or above the object to be fumigated in treating grain, carpets and clothing, and on the surface of the soil or in a trench, for the control of soil pests. For soil treatments the chemical is covered with a layer of earth to prevent an upward and outward extension of the vapor, and to confine it so that it will gradually work downward to the haunts of the pest. Various recommendations have been made relative to the time allowed for the paradichlorobenzene to remain about the tree or plant. Some have recommended the removal of the residue after three weeks from the date of application. In most of the work done in California, no attempt has been made to remove the residue at anytime and it does not appear that any serious injuries have resulted from this practice. In a few instances, the material which has been applied in late summer or fall and allowed to remain throughout the winter and spring, gave satisfactory control, whereas material applied at the same time but removed in the fall, has not given the desired results. It is recommended that the paradichlorobenzene be left until entirely evaporated or until farm practices require its removal, excepting in the cases of nursery stock and young trees as herein designated. Only those insects which live at or below the surface of the ground can be successfully treated. Flat headed borers which work at or below the surface of the soil may be controlled but those working on the trunks of the trees above the ground cannot be killed by this treatment. BUL. 411] PARADICHLOROBENZENE AS A SOIL FUMIGANT Fig. 2 Fig. 3 Fig. 4 Fig. 5 Fig. 2. — The ring method, first step: leveling the surface of the ground for a space of two or three feet in diameter about the tree. Fig. 3. — The ring method, second step: the paradichlorobenzene applied in a ring two or three inches wide, the inside about three inches- from the bark of the tree. Fig. 4. — The ring method: ring of paradichlorobenzene applied too close to the tree. Injuries may result from such a treatment. Fig. 5. — The ring method: ring of paradichlorobenzene applied too far from the bark of the tree for the control of the Pacific peach tree borer, but satis- factorily for the woolly apple aphis and pear root aphis. 8. UNIVERSITY OF CALIFORNIA EXPERIMENT STATION APPLICATION TO TREES In making applications to trees the surface of the soil about the trunk is first smoothed with a shovel (fig. 2), then a ring of para- dichlorobenzene is sprinkled about the base in a band two or three inches wide and with the inner circumference two or three inches from the bark of the tree for Pacific peach tree borer (fig. 3), and from four to six inches for pear root aphis or woolly apple aphis (fig. 5). The ring of paradichlorobenzene is then covered with a cone-like mound of earth (figs. 6 and 7). The whole process requires less than five minutes to a tree. In treating the woolly apple aphis on large trees it is often advisable to make a series of six or eight furrows four inches deep radiating from the trunk in order to treat a much larger root area (fig. 8). The material is sprinkled in the bottoms of the furrows and then covered so as to level the ground. USE WITH NURSERY STOCK Nursery stock including apples, apricots, cherries, peaches, pears, plums, and prunes may be treated by means of a single furrow two or three inches from the row, or in severe infestations, by a furrow on each side of the row from four to six inches from the trees. In the eastern and southern states nursery stock and young trees up to six years of age are often severely injured by this treatment. In California our experiments, so far, have shown no injury, but as a precautionary measure it is recommended that the treatment of all nursery stock and young trees be preceded by small scale tests to ascertain exact effects of the gas in every locality and that all residue paradichlorobenzene be removed three weeks after treatment. DOSAGES One of the most difficult problems experienced is to convince the farmers that it requires but a very small amount of paradichloro- benzene to control most soil insects. For an average size ten-year-old prune, apricot, or apple tree from iy 2 to 2 ounces is all that should be used for a single treatment. In severe cases as much as 3 or 4 ounces may be permissible, but more than that amount may result in the permanent injury or death of the tree. For young trees less than six years old, % ounce is sufficient. For the nursery trees mentioned, 1 ounce distributed along one yard of the furrow gave satisfactory results where the single furrow was used. Any residue of para- dichlorobenzene remaining after three weeks should be removed. Bul. 411] PARADICHLOROBENZENE AS A SOIL FUMIGANT 9 It is very difficult to lay down hard and fast rules for dosages, because of the great variation in the degree and extent of insect infestation, especially in the cases of the woolly apple aphis and the pear root aphis which often occur on the roots at considerable distances from the trunk. Fig. 6 Fig. 8 Fig. 7 Fig. 9 Fig. 6. — The ring method, third step: covering the ring of paradichloro- benzene with earth. Fig. 7. — The ring method completed with the paradichlorobenzene completely covered. Fig. 8. — The radiating furrow method of treatment for woolly apple aphis, before covering the furrows with earth. Fig. 9. — The two-ring method of treatment for woolly apple aphis. Excessive doses must be avoided as they may kill the trees or plants outright, or may cause a lingering death, and accomplish no more in killing the pests, than the amounts recommended. 10 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION CONDITIONS FOR APPLICATION Two conditions of the soil are essential to successful applications of paradichlorobenzene as a soil fumigant. They are proper tempera- ture and moisture. As previously stated, this chemical vaporizes very slowly at 55° F, and is more effective when the soil temperature is between 75° and 85° F. The vapor cannot readily penetrate a moisture-laden soil ; and, on the other hand, if the soil is too dry and porous the gas soon dissipates. Therefore, to secure best results one must select a time when the soil is neither soaked nor entirely dry. It is not necessary to allow the trees actually to suffer for want of water in order to make successful treatments with the chemical, but it is ordinarily advisable to make the applications two or three weeks prior to irrigating rather than just after. Treatments made from one to three weeks after irrigation have all been successful. In non-irrigated soils treatments have all been successful except in very porous, peaty, or coarse gravelly types which were not sufficiently compact to retain the gas. It is practically useless to treat a cold wet soil, first because of the very slow formation of vapor, and secondly because the vapor cannot diffuse in the water-logged soil. TIME OF APPLICATION The conditions of application which require a warm, somewhat dry soil determine the time of application, which in California covers a very extended period because of the long dry summers and autumns. While treatments may be made as early as May, June, and July, they are usually most effective in late August and in September and October before the winter rains commence. Any time that the soil temperature to a depth of eight to twelve inches is over 75° F, treat- ments may be made if the soil is not actually wet. Paradichloro- benzene applied in late November and December and followed by cold wet weather may remain almost intact until the following spring or summer when the rising temperature causes it to diffuse and become effective. This practice of late applications, however, is not economical or practical because of the necessity of plowing or cultivating before the paradichlorobenzene has had proper conditions to do its work effectively. BUL. 411] PARADICHLOROBENZENE AS A SOIL FUMIGANT 11 USE IN CONTROLLING THE PACIFIC PEACH TREE BORER Inasmuch as a great deal of work with paradichlorobenzene was done for the control of the peach tree borer, Aegeria exitiosa Say, in the eastern states and in Georgia, it was natural that attention in California should first be given to testing this fumigant against the Pacific peach tree borer, Aegeria opalescens Hy. Edwards, a pest of considerable proportions in some parts of the San Francisco Bay region, particularly in San Mateo, Santa Clara, San Benito, and Alameda counties. Here the laborious methods of worming or digging out, as well as the less satisfactory asphaltum treatment, were not only difficult and expensive, but often resulted in as much injury to the tree as that done by the borers. Preliminary tests demonstrated at once the efficacy of the paradichlorobenzene treatment and immedi- ately presented an unexplored field in the control of agricultural pests, and particularly those difficult and hitherto almost uncontrollable forms which inhabit the soil. KINDS OF TREES TREATED In California, the Pacific peach tree borer attacks a variety of trees on different rootstocks, so that the problem here was much more varied than in the East, where only the peach on its own roots was treated. Among the hosts are peaches, cherries, prunes, and apricots. Cherries. Cherry trees are only occasionally infested by the peach tree borer. Peaches. The acreage of peaches grown in the region infested by the peach tree borer is small as compared with that of prunes and apricots and the number of trees treated was relatively small ; however, a large amount of peach nursery stock is grown therein and it was often seriously infested. A number of experiments in treating one- year-old budded and grafted trees were made in three different nurseries without any noticeable injury. The material was applied in a small furrow from two to four inches deep and from two to three inches from the young trees at the rate of 2 ounces to a yard. The furrow was then covered. Prunes. Approximately 80,000 acres of French prunes, chiefly on Myrobalan and peach rootstocks, were treated. The ages of the trees varied from four to twenty-five years, the very large majority being over ten years. Control of the borers averaged from 80 per cent to 12 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 95 per cent in most cases, and in a few instances approached extermi- nation for the season. During the past four years thousands of acres have been treated without the loss of a single tree or evident injury to any. Apricots. Apricot trees make up the largest acreage treated. A few orchards are on apricot roots and these are very nearly immune to the attacks of the peach tree borer. The majority of the older apricot trees are on Myrobalan rootstocks and these are very susceptible to borer attacks. As many as one hundred and twenty borers have been taken by the old worming process from a single large fifteen-year-old tree, and from sixty to eighty from many other trees of the same age or older. All ages of trees, from those just set out to those twenty-five or thirty years old, were treated. The only injury noted was to large roots which were almost exposed at the surface ; some of these were either injured or killed by the gas and the trees themselves greatly weakened. Practically all of the 330,000 trees treated in Alameda County in 1924 were apricot trees infested with the Pacific peach tree borer. During the past three years no serious injury to any of these trees has been noted. Table 2, giving results of demonstrations conducted in Alameda County by Assistant Farm Advisor J. B. Hammon, will give a fair idea of results obtained there. The dosage of parabichlorobenzene for control of the Pacific peach tree borer is 1 ounce for an average tree. On very large trees it may be increased to 1% or 2 ounces. Nursery stock treated by the furrow system should receive about 1 ounce to a linear yard of furrow. The material is applied in a concentric ring about three inches from the bark of the tree trunk. (See Methods of Application, page 6, and figs. 2-9.) Usually but one application a year is necessary. In severely infested districts where all orchards are not treated or where a com- plete clean-up has not been secured from a single treatment it is advisable to make a follow-up treatment the succeeding year. The material may be applied with fair success from May to November but best results have been secured from applications made in September and October. (See Time of Application, page 10.) BUL. 411] PARADICIILOROBENZENE AS A SOIL FUMIGANT 13 TABLE 2 Report on Some Trial Plots for the Use of Paradichlorobenzene to Control Pacific Peach Tree Borer on Apricots, Cherries, Peaches, and Prunes in Alameda County Name Variety of fruit Size* of plot Age of trees, Years Spring treated Time dug Per cent con- trol Fall treated Time of exam- ination Per cent con- trol Condition of trees 6 months after treatment J. J. Silveria Silver prune 10 A 30 Oct. 3 wk. 6 mo. 30 97 Good Chas. Long Apricot 5A 8 Oct. 6 mo. 95 Good F. Cober Peach 200 T Kin nursery) Sept. 3 wk. 99 Good J. A. Borgas Apricot 10 A 28 May 3 wk. 35 Sept. 8 wk. 98 Good J. H. Langdon Apricot 10 T 6 Sept. 6 mo. 98 Good Ed. Chadburne... Apricot 140 A 35 May 6 wk. 75 Sept. Not dug (100) Good H. H. Jameson Apricot 10T 12 Oct. 3 wk. 10 No im- provement Driscol Apricot 80 A 12 Oct. 8 mo. 97 Good M. V. Perry Prune 75 A 12 May 6 wk. 37 Sept. Not dug No report Good H. L'Ecuyer Apricot 10 A 6 June 6 wk. 50 Oct. 3 wk. 6 mo. 80 96 Good F. A. Stevens Cherry 10 T 15 Sept. 8wk. 95 Good P. H. Moore Apricot 10 T 35 June 6 wk. 35 No im- provement A=Acres, T = Trees. FOR USE IN CONTROLLING THE PEAR ROOT APHIS Two years of experimental and demonstrational work with para- dichlorobenzene on the pear root aphis, Eriosoma languinosa Hartig, were conducted in Contra Costa County in cooperation with Mr. A. M. Burton, Farm Advisor. These experiments proved the treatment to be efficacious on all ages of pear trees grafted to both French and Japanese rootstocks. According to Mr. Burton, 237 acres of pear trees were treated in 1924, and more than 230 acres in 1925, in Contra Costa County. During the two years only one tree was killed and there was no appreciable injury to others. 14 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION In Mendocino County, Farm Advisor C. S. Myszka conducted an experiment in which doses of 2, 4, and 8 ounces of paradichloro- benzene were used on four-year-old trees. The material was applied around the trees in circular trenches of two-foot radius. Water was afterwards applied to wet the surface of the soil around eighteen of the trees which had received the heavier dosages with the result that these trees were either badly damaged or killed. This can likely be explained by the combination of the extra heavy dosages and the wet surface soil which confined the evolving gas about the roots of the tree. No injury was noted even with the heavier dosages where water was not thus applied. Irrigation immediately after the application of paradichloro- benzene has not injured any of the thousands of trees treated. This is undoubtedly due to the fact that the soil when thoroughly wet does not permit of a ready penetration of the gas while the lower tempera- ture delays its rapid evolution and beneficial results in insect control will not follow until the soil becomes warm and dry enough to permit of the proper evolution and penetration of the gas. Other orchard treatments have been made in Lake and Sonoma counties with very satisfactory results. The proper dosage of paradichlorobenzene for the control of pear root aphis is from 1 to 1% ounces to a tree. The material is applied in a concentric ring four to six inches from the tree. (See Methods of Application, page 6, and figs. 2-9.) Usually but one application is necessary. It is made preferably in early summer, in June or July, but may be made any time from June until November. USE IN CONTROLLING THE WOOLLY APPLE APHIS Experiments and demonstrations with paradichlorobenzene for the control of the woolly apple aphis, Eriosoma lanigera (Hausmann), were conducted in Alameda, Sonoma, and Santa Cruz counties. In all, several thousand trees were treated, including nursery trees, three- and four-year-old trees, and mature trees ten years of age and over. In Sonoma County, 6400 apple trees were treated in 1924, and 18,000 in 1925. The injury reported to apple trees in the East is not at all apparent from the work done to date in California. So far, the only injury noted has been to trees having one or more of the main roots near the BUL. 411] PARADICHLOROBENZENE AS A SOIL FUMIGANT 15 surface of the soil. Several trees have been greatly weakened, and a few have actually been killed. It has not been satisfactorily estab- lished that the paradichlorobenzene caused the death of these few trees. Very satisfactory results have been obtained, on the whole. Infes- tations far out on the roots are difficult to kill, but are much less serious than those near the crown. Constant reinfestation from the tops by flying migrants make yearly treatments necessary. Paradichloro- benzene treatment should be combined with the application of oil sprays, the former made in late summer and in fall and the latter during the dormant winter period, to reduce the infestations of the trunks and tops of the trees. The dosage of paradichlorobenzene for the control of woolly apple aphis is from 1 to 1% ounces to a tree. In Sonoma County, Assistant Farm Advisor H. P. Everett recommends as a normal dose, 1 ounce to a tree applied in a single ring six inches from the tree, but in severe infestations 1% ounces are used in two rings (fig. 9). Excessive doses are believed by some growers in certain localities to have caused injury. The material is applied in one or two concentric rings or in radiat- ing furrows. (See Methods of Application, page 6, and figs. 2-9.) From one to three applications may be made, depending upon the seriousness of the infestation. They may be made any time during the summer and fall when the soil has a temperature of 55° F or over. Normally, one application a year is sufficient. Because of the constant migrations of the winged females from tree to tree, it is necessary to keep constant watch of the orchards and repeat the treatment each year that the aphis is present. USE IN CONTROLLING THE RASPBERRY CANE BORER Mr. H. J. Ryan, County Horticultural Commissioner of Los Angeles County, conducted some extensive tests with paradichloro- benzene in the control of the raspberry cane borer, Bembecia mar- ginata (Harris). He has very kindly furnished a copy of the report on this work, from which the following information was secured. The work was conducted in more than a hundred separate patches of raspberry and blackberry plantings on light and heavy soils in the San Fernando Valley. All treatments were made between June 7 and July 17. The method of treatment is similar to that already described on pages 6 to 10, and consists in first leveling the soil about 16 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION the bases of the infested plants, then applying the material in a circle or ring and covering it with three or four inches of soil. The recom- mended dosage is % ounce for the smaller plants and % ounce for the larger ones. Based on twenty cents a pound for the paradichloro- benzene and 1200 berry plants to the acre, the cost is, including labor, from forty-three to forty-five dollars an acre. This experience in California is contrary to that of New Jersey, where paradichlorobenzene applied at the rate of from % to 2 ounces to a blackberry bush and left for three weeks "killed most of the borers and injured and in most cases killed the bushes." 7 USE IN CONTROLLNG THE GRAPE PHYLLOXERA Experiments with paradichlorobenzene for the control of grape phylloxera, Phylloxera vitifoliae Fitch, have been conducted in only two counties and on a very limited scale. Mr. C. S. Myszka conducted two tests in Mendocino County on grapevines which were very seriously infested with this insect. One test involving one hundred vines was made in June, 1922. From 8 to 16 ounces to a vine was used, applying in furrows radiating from the vines. Some of the vines showed injury soon after treatment. Subsequent examination of the roots of the treated vines showed no evidences of phylloxera, while roots from the check vines showed great numbers present. The next spring insects were found on the roots of the treated vines in limited numbers, but the injured vines had made a noticeable advance in growth over the infested and untreated vines in the same vineyard. Another experiment conducted in July, 1923, involved two plots, one on gravelly loam soil and another on clay loam soil. One-half ounce of paradichlorobenzene to a vine was used, applied as in the first test. In November of that year, examination showed practically no control on the light soil, while no insects could be found on the vines in the heavier soil. The experiment on the gravelly soil bears out the observation that very porous soils do not confine the gas sufficiently to secure control with ordinary doses. From the results of these limited preliminary tests it will be seen that as yet no specific recommendations can be made for the control of phylloxera on grapevines. 7 Peterson, Alvah. Paradichlorobenzene (p-c-benzene) for controlling the peach-tree borer. New Jersey Agr. Exp. Sta. Circ. 156:12. 1923. BUL. 411] PARADICHLOROBENZENE AS A SOIL FUMIGANT 17 USE IN CONTROLLING WIREWORMS IN DAHLIAS A large number of experiments were performed during the spring of 1924 in cooperation with F. H. Wymore 8 to determine the efficacy of paradichlorobenzene for combating wireworms in planted dahlia tubers. The species of wireworms was not determined. These experi- ments were carried out in sandy soil in Alameda in a patch where a great number of tubers were already so badly infested that the owner was ready to abandon the entire lot of several hundred fine varieties. Surface oj_Sol2__^_^ ggJ^BBJjmparadichloro benzene fc,*^ ■■"■;■•■■■ ^r Dahlia Root ^A o> Fig. 10. — Method of treating dahlia with paradichlorobenzene to control wireworms. Tubers containing from twenty-five to fifty worms were returned to their former position and covered with about two inches of soil. Various amounts of paradichlorobenzene were then added and the top soil replaced. These infested tubers were taken up and examined in seven days and in no case were any wireworms located in the tubers. A few dead ones could be found in the soil, but the great majority could not be accounted for. So promising were these preliminary results that practically the entire patch was treated. The retarded plants at once took on new life and an excellent crop of flowers and tubers was produced in the face of what seemed sure ruin. The dosage Research Assistant in the Division of Entomology and Parasitology. 18 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION used was approximately one teaspoonful placed about two inches above the planted tuber as illustrated in figure 10. From these experiments we concluded that the chemical acted chiefly as a repellent, but it was so effective that not a single living wireworm was to be found in any of the several hundred tubers treated. USE IN CONTROLLING THE GARDEN CENTIPEDE Mr. Wymore has conducted a large number of experiments with paradichlorobenzene in the control of the garden centipede, Scuti- gerella immaculata (Newport), on asparagus which may be sum- marized as follows : 1. The tests were made largely on loose peaty soils of the delta asparagus growing section. 2. Preliminary tests in which one ounce of paradichlorobenzene was used to three linear feet of furrow gave good results in killing the centipede when applied close to and on both sides of the row. These tests were made in May when the soil was noticeably moist and its temperature 58° F. 9 3. Applications of large amounts of paradichlorobenzene were made later as follows: (a) On August 9, 1923, 175 pounds was applied at the rate of 1 ounce to every three linear feet in the row between the asparagus rows, with a beet drill. The rows of paradichlorobenzene were twenty inches apart and four or five inches deep. (6) On the same date 125 pounds of the material was applied in a furrow on each side of the rows of asparagus by first making a furrow with a small hand wheel plow and applying the material by hand. The amount used and depth applied was the same as in the preceding test. (c) In July, 1924, one-eighth acre of asparagus was treated at the rate of 460 pounds to an acre. The material was applied at a depth of five or six inches in six or seven furrows between every two rows of asparagus. 4. In all of the above three tests ideal moisture and temperature conditions for the dissemination of the gas prevailed in the soil for at least a week after the work was finished. 9 Wymore, F. H. The garden centipede, Scutigerella immaculata (Newport), a pest of economic importance in the West. Pomona Col. Jour. Ent. and Zool. 16:73-88, fig. 5. 1924. BlTL. 411] PARADICHLOROBENZENE AS A SOIL FUMIGANT 19 5. The results obtained were not at all satisfactory since large numbers of live centipedes could be found about the crowns of the asparagus a week after the material was applied. 6. The unsatisfactory results obtained are believed to be due to the rapid escape of the vapor because of the very porous condition of the peaty soils where the tests were made. OTHER USES Because of its persistent odor and deadly effects to insects gener- ally and its comparatively harmless effects to human beings, para- dichlorobenzene may also be used for many other purposes. 1. Fumigating cereals and other food products in mills, granaries, warehouses, storerooms, residences, etc. 2. Fumigating furs and textiles in stores and residences. 3. Fumigating botanical and insect museum specimens. In all the above cases the material is placed in shallow open con- tainers under as warm, dry and confining conditions as possible and the vapor allowed to evolve as rapidly as it will. For clothing, furs, and museum specimens it is used the same as napthalene or moth balls, but is much more effective in destroying insects pests than are these agents. WARNING Although paradichlorobenzene has been extensively used over a period of five years in California there is still much to be learned regarding it and a great deal of experimental work remains yet to be done. The action of the vapor on the roots of the plants is so slow that the after effects may not be noticed for several years, and trees treated in the preliminary tests of 1921 are still under observation. While a single treatment may produce no ill effects we are not now able to determine what results may develop from successive treat- ments over a period of years. Therefore, the growers should take all precautionary measures possible and at least observe the following : 1. Avoid using excessive dosages. 2. Do not place the crystals in contact with the bark of the trunks, stems, or the roots of the plants. 3. Do not apply the material immediately before or after irri- gation and do not wet the surface of the soil following applications of the crystals. 20 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION 4. In treating nursery stock and young trees remove the residue after three weeks. 5. Do not apply paradichlorobenzene during the winter and early spring. Late summer and fall are the best times for such applications. 6. Only one application a year is advisable. Several treatments in one season may be fatal to the plants. 7. Paradichlorobenzene is recommended in the orchard and garden only for insects which attack the plants at or below the surface of the soil and cannot be used for borers which infest the trunks and limbs above the ground. COSTS OF MATERIAL AND APPLICATIONS The price of paradichlorobenzene is variable but ranges around thirty -five cents a pound retail in small lots, and twenty-five cents a pound in larger lots of a hundred pounds or over. The total cost of chemicals and labor for treating a vine or a tree should not exceed five cents because of the small dosage and ease of application. In most cases the average cost in orchards was four cents a tree. Paradichlorobenzene is available in small lots through many local insecticide dealers and florists under various trade names such as "Crystal Glass," "Paracide," "P.B.D." "Paradi," and so forth. For all practical purposes these materials appear to be identical. lOm-10,'26