UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA CIRCULAR 337 June, 1935 LIFE HISTORY AND CONTROL OF THE GLADIOLUS THRIPS IN CALIFORNIA 1 HOWARD L. McKENZIE 2 The gladiolus thrips has caused considerable damage to gladiolus plantings in Ventura, Los Angeles, Orange, and San Diego counties since its first appearance in California in 1932. Beginning in May, 1934, the writer was stationed at Encinitas, California, to conduct life-history and control experiments. ORIGIN AND DISTRIBUTION The gladiolus thrips, Taeniothrips gladioli M. and S., was described by Dudley Moulton and John B. Steinweden in 1931, from specimens taken on gladiolus plants August 1, 1930, at Vineland Station, Ontario, Canada. The first appearance of the gladiolus thrips in the United States was noted in 1931 in the Rochester and Buffalo, New York, areas. It is now known to occur in parts of California, Colorado, Connecticut, Delaware, District of Columbia, Florida, Georgia, Illinois, Indiana, Kentucky, Maine, Maryland, Massachusetts, Michigan, Minnesota, Missouri, New Hampshire, New Jersey, New York, North Dakota, Ohio, Oregon, Penn- sylvania, Rhode Island, South Carolina, Tennessee, Vermont, Virginia, Washington, West Virginia, and Wisconsin. It has also been found in Australia, Africa, Canada, the Hawaiian Islands, and New Zealand. The gladiolus thrips was unknown in the United States prior to 1931, yet during the last three years it has rapidly spread throughout the country and has caused considerable damage. In view of these facts it is reasonable to believe that it is an introduced pest and not a native of the United States. 1 Paper No. 316, University of California Citrus Experiment Station and Graduate School of Tropical Agriculture, Riverside, California. 2 Laboratory Assistant in Entomology. 2 University of California — Experiment Station The first specimens of gladiolus thrips to be taken in California were those collected on April 30, 1932, by S. N. La Follette, County Agricul- tural Inspector, and C. A. Weigel, of the Federal Bureau of Entomology, on the property of W. A. Ritto, La Habra Heights, Los Angeles County. Since this first record the thrips has been collected in about fifty locali- ties in Los Angeles County and has also been found in Ventura, San Diego, and Orange counties. S. F. Bailey also collected it at Davis, Yolo County, on July 16, 1932, On June 4, 1934, the writer collected this thrips on gladiolus flowers at Riverside, Riverside County. HOSTS The literature to date indicates that the gladiolus thrips is primarily a pest of gladiolus. "Weigel and Smith 3 state that collections made on gar- den and wild plants growing adjacent to heavily infested gladiolus plantings showed that the thrips was breeding only on the poker plant (Tritoma or Kniphofia) , montbretia (Tritonia) , and tiger flower (Tigri- dium). Collections of adults of the gladiolus thrips have been made in flowers of buddleia, calendula, dianthus, and Lathyrus latifolius, and on the foliage and in the leaf sheaths of Japanese and German types of iris which were growing near heavily infested gladiolus plantings. Weigel further states that under greenhouse conditions the gladiolus thrips has been successfully reared on a rather extended list of plants. Among the most severely attacked are amaryllis, narcissus, bulbous iris, and freesia. Other hosts include aster, clover (white), goldenrod, mul- lein, coreopsis, cucumber, dahlia, dandelion, delphinium, hollyhock, Spanish iris, Kniphofia aloides and K. hybrida, plantain, tomato, and watsonia. NATURE OF INJURY Perhaps the first thing that the observer will note about the foliage of thrips-infested plants is the silvered or bleached areas. The injury is caused by their rasping, sucking mouth parts, which puncture the plant tissues. Once the cell contents are sucked out, the cell no longer remains green but actually becomes transparent or bleached. Bailey 4 states : In the feeding act the mouth cone is applied to the leaf surface and the initial incision made with the mandibles and their stylets, which are protruded from the concave side of the labrum. The long maxillary stylets are then brought into play and used to puncture the deeper cell layers. A rooting motion of the head is em- ployed to enlarge the opening and cause the plant juices to flow more rapidly. s Weigel, C. A., and F. F. Smith. The present status of the gladiolus thrips in the United States. Jour. Econ. Ent. 26:523-528. 1933. * Bailey, S. F. The biology of the bean thrips. Hilgardia 7(12) :475. 1933. Cm. 337 History and Control of Gladiolus Thrips The question has been asked many times as to the relative amount of injury done by the adults as compared with the larvae. On the basis of careful observations in the field and laboratory, it appears that the larvae actually feed more than the adults. This is especially true when the larvae attain the spike. A spike possessing only adult thrips shows decidedly less injury than one containing the larvae. This is to be ex- pected since the larvae are present in much larger numbers and confine their feeding to smaller areas because of their lack of activity. It is also necessary for them to store up a sufficient amount of food to carry them through the pupal stages. Fig. 1. — Left, normal corm. Eight, conn showing ultimate result of gladiolus thrips attack. It is very unlikely that a corm with this de- gree of injury will even germinate. The actual damage done by the thrips is a weakening of the plant, causing the prevention of normal flower opening and a rapid drying-out as a result of the injured areas on the leaves. The foliage injury naturally inhibits normal growth, vigor, and the ability to develop a new corm. The flowers are bleached and streaked in appearance owing to the ex- traction of the coloring matter in the feeding areas. Of course the type of injury is more conspicuous on the red and purple varieties than on the white or pale-colored ones. The injury to corms (fig. 1) is sometimes serious and, in cases of severe infestations, the corms may fail to germinate. The thrips gain entrance to the corm surface through the cracks and crevices in the scales and between the scales when the top is cut off. A bleached appearance is noted on the surface layer of cells for a few days; then the injured areas become brown or russeted and also roughened. Young roots may be 4 University of California — Experiment Station killed by the voracious feedings of the thrips. The cormels are, however, protected by a capsule-like covering, and thus escape attack by this insect. LIFE HISTORY AND DESCRIPTION OF STAGES In order to follow the individual thrips through its different stages, small leaf cages (lxl inch) were used, which were cut from thick card- board. A %-inch hole was then cut in the center of the inch square card- board. These leaf cages were then clamped securely to young, clean, and healthy gladiolus leaves and the hole of each cage was covered with a small cover glass. A pair of adult gladiolus thrips were placed in each cage and after 24 hours were removed, during which time eggs would be deposited. Eggs are deposited in the tissue of the leaf or corm of the gladiolus plant. The eggs (fig. 2) are delicate, smooth, white, and bean-shaped. As the embryo develops the egg swells and the outlines of the larva may be seen. The eye spots are bright red and are very conspicuous. The incu- bation period ranged from 5 to 6 days during the month of June, 1934, at Encinitas. After hatching from the egg, each larva was placed in a separate cage and observed from day to day. The first-stage larva (fig. 2) is trans- lucent white with red eyes. It is very small and is difficult to see with the unaided eye. The duration of this stage varied from 2 to 3 days. The second-stage larva (fig. 2) is pale yellow. It possesses a fusiform abdomen and the posterior segment is tubular. This stage lasted from 2 to 4 days. The prepupa (fig. 2) is orange in color, and is similar in shape to that of the second-stage larva. It possesses short translucent white wing pads, the f orewing pad extending to the first abdominal segment and the hind- wing pad extending to the second abdominal segment. The females are usually somewhat larger and more plump than the males. The legs are white. The duration of this stage varied from 2 to 3 days. The newly formed pupa (fig. 2) is orange, but within a few hours changes to a light yellow. The entire body is, as a general rule, much shorter and stouter than the prepupa. The eyes are large and red. The antennae, wing pads, and legs are a translucent white. The wing pads extend to the seventh or eighth abdominal segments. The duration of this instar was 3 days. The adult of the gladiolus thrips (fig. 2) is dark-brown to black with the head and thorax usually darker than the abdomen. The wings have a light-colored area at the base where they join with the thorax, and Cir. 337] History and Control of Gladiolus Thrips when folded give the appearance of a band across the insect. The adults measure about %g inch in length. G> EGG FIRST-INSTAR LARVA SECOND- I NSTAR LARVA PREPUPA PUPA ADULT Fig. 2. — Stages of development of the gladiolus thrips, Taeniothrips gladioli M. and S. Instead of the small leaf cages previously used in conducting life- history studies, cormels (with the outer covering removed) were later used. A fine copper wire was thrust through one end of the cormel while the other end of the wire was forced into and held securely in a cotton 6 University of California — Experiment Station plug. The cormel, wire, and cotton plug were then placed in a small vial. A camel's hair brush was used to transfer the larvae and adult thrips. The length of the life cycle of the gladiolus thrips reared under condi- tions as existed in the month of June, 1934, at Encinitas, is given in table 1. The data are based on 25 specimens. The maximum temperature range for this month was from 68° to 80° F. TABLE 1 Life Cycle of Gladiolus Thrips Stages Minimum, days Maximum, days Average, days Egg 5 2 2 2 3 14 6 3 4 3 3 19 5 5 2 5 3 2 5 3 Total 16 5 TABLE 2 Life-History Records of the Gladiolus Thrips as Given by Weigel* Temperature, degrees Fahr. Incubation, days Larval devel- opment, days Pupal stage, days Total, days 80 3-4 6-7 10-12 3-4 6-7 10-12 3-4 6 10 10-11 70 : 19 60 30 50f . * Weigel, C. A. The gladiolus thrips. Florists' Exch. and Hort. Trade World 81: 9, 23, 24, 39. 1933. t Although adults lived for a period of five months and laid eggs at 50° F, none of the eggs hatched. Experiments with small lots of corms have shown that storage at controlled temperatures between 85° and 40° F for three months or longer kills all stages, including eggs. Table 2 is adapted from the life-history records for the gladiolus thrips given by Weigel, 5 for comparison. Weigel also states that from 100 to 200 eggs may be laid by a single female. The average length of adult life is about one month, but it has been found to extend over two and one-half months for certain indi- viduals. The gladiolus thrips normally reproduces bisexually and males are found throughout the year. The females, however, are able to reproduce parthenogenetically, the resulting progeny being males. 5 Weigel, C. A. The gladiolus thrips. Florists' Exch. and Hort. Trade World 81:9, 23, 24, 39. 1933. Cir. 337] History and Control of Gladiolus Thrips SEASONAL LIFE HISTORY The gladiolus thrips is taken into storage on the corms and there lives and reproduces. At planting time the thrips are taken to the fields and planted with the corms; thus a new infestation is started. Both adults and larvae then follow the developing shoot to the surface of the soil, encased within the leaf sheaths. In the morning around 8 or 9 o'clock, and in late afternoon around 4 or 5 o'clock, or on cloudy days, the thrips, Fig. 3. — Gladiolus thrips trap 14 inches wide and 10 feet high ; tanglefoot paper tacked on a support. especially the adults, come out on the exposed leaf surfaces. This may be correlated with the lack of available food in the sheaths. The thrips appear to reach their greatest abundance at flowering time. OVERWINTERING IN THE FIELD In California the gladiolus thrips overwinters between crops on volun- teer gladiolus plants. This is in accordance with the observations of J. R. Watson (see Weigel 6 ) , who believes that it also overwinters on volunteer gladiolus plants in Florida. In such regions of moderate climate, other hosts on which the thrips can survive become of importance. DISPERSAL To determine how high or how far the gladiolus thrips will fly, two traps, 10 feet high and 14 inches wide (fig. 3), were constructed. On each sup- port tanglefoot paper was tacked from the ground to the top. The traps were placed in an infested planting, one facing west and the other facing north. After 6 days the tanglefoot paper was removed from the « Weigel, C. A. The gladiolus thrips. Florists' Exch. and Hort. Trade World 81:9, 23, 24, 39. 1933. 8 University of California — Experiment Station supports and the number of adult thrips counted. The experiment was performed twice. The results of this experiment are given in table 3. About 20 feet beyond this infested planting, tanglefoot paper was tacked 10 to 15 feet high on a telephone pole. A week later a few thrips TABLE 3 Number of Adult Thrips Caught on Tanglefoot Trap Height from ground, Facing west Facing north feet First test Second test First test Second test 1-4 243 26 11 286 36 33 19 161 100 47 11 280 4-6 43 6-8 29 8-10 16 were found in the tanglefoot paper. Three hundred feet beyond this same planting, tanglefoot paper was tacked on a lath fence from the ground up to a height of 12 feet. A week later the tanglefoot paper was examined and no thrips w r ere found. These preliminary data indicate that the gladiolus thrips may fly some distance of its own accord or with the aid of wind. PARASITES AND PREDATORS In California a predacious sucking insect, Orius (Triphleps) tristicolor (White), is occasionally observed feeding on the gladiolus thrips. Bailey 7 states that the parasite Thripoctenus russelli Crawf. was taken on gladiolus thrips at Sacramento. TREATMENT OF CORMS Four simple treatments may be employed to insure an effective kill of the gladiolus thrips infesting corms. They are as follows : 1. Naphthalene flakes. 2. Calcium cyanide. 3. Corrosive sublimate or mercuric chloride (used also as a fungicidal dip). 4. Hot water. Naphthalene Flakes. — Naphthalene flakes are recommended espe- cially for the treatment of small lots of corms. They are effective, readily available, cheap, and safe to the user and to the corms, even when an overdose is used. The treatment may also be used at any time 7 Letter of S. F. Bailey to H. S. Smith. Cm, 337] History and Control of Gladiolus Thrips 9 during the storage season. It should not be used, however, on sprouting corms, for it injures the young shoots and roots. Naphthalene flakes are used at the rate of 1 ounce (a good handful) for every 100 medium- sized corms, or 1 pound for 2,000 corms. When the corms are in trays, the flakes should be sprinkled over and among them, after which the trays should be covered with a light canvas or wrapping paper. If the corms have been placed in paper bags the fumes may be confined by folding over the top of each bag. At temperatures between 60° and 70° Fig 4. — Tent covers of muslin used to prevent thrips from reinfesting corms fumigated with calcium cyanide (side view). F the flakes should remain with the corms at least three weeks; at lower temperatures they should remain four or five weeks. An exposure of two or three months is harmless to dormant corms. It is necessary to re- plenish the supply of naphthalene flakes, usually every two weeks. The excess flakes should be shaken out before planting the corms. A covered tin can or other equally tight container should not be used, for the corms may be induced to sprout prematurely. Calcium Cyanide. — Calcium cyanide is effective in controlling the thrips on gladiolus corms. It reacts with the moisture of the air to pro- duce hydrocyanic acid gas. In order that effective fumigation with calcium cyanide may be carried on, it is necessary to use an airtight con- tainer. Duck boards 8 to 12 inches from the floor should be placed suffi- ciently far apart to insure good ventilation. All trays used in the treat- ing process should have fairly coarse wire-mesh bottoms. The trays should not be filled with more than two layers of corms, and the corms should be fumigated with a dosage of 2 ounces for each 100 cubic feet, with an exposure of 4 hours. Provision must be made for applying the calcium cyanide in a thin layer over the entire floor space of the fumatorium. 10 University op California — Experiment Station Since the eggs are not killed with this dosage in the first fumigation, a second fumigation after a 10-day interval has elapsed should insure a complete kill. After each fumigation the corms should be thoroughly aired; also after the corms have been fumigated, as well as during the period between fumigations, the greatest precaution should be exercised to prevent reinfestation. If the corms are taken out of the fumigation chamber after the first fumigation, naphthalene flakes should be sprin- kled over them, at the rate already suggested, and the trays should be enclosed with coarse paper tacked to the trays, or canvas thrown over them, to prevent reinfestation. Since the corms must be held for 10 days according to the above pro- cedure, it was deemed advisable to see whether extremely heavy dosages of calcium cyanide would kill the eggs at the first fumigation and what the effect might be on the corms. After fumigation the corms were planted under a thrips-tight cage (figs. 4 and 5) to prevent reinfestation. Results of this experiment are given in table 4. TABLE 4 Results of Fumigating Heavily Infested Gladiolus Corms* With Calcium Cyanide Dust: Four-Hour Exposure Test No. Number of corms Dosage, in 40-cubic foot chamber Date corms planted under thrips-tight cage Date plants examined Number of thrips found Injury to corms 1 6 6 6 6 Aug. 22, 1934 Aug. 22, 1934 Aug. 22, 1934 Aug. 22, 1934 Sept. 19, 1934 Sept. 19, 1934 Sept. 19, 1934 Sept. 19, 1934 None None None Abundant 2 3 4 * Dormant corms of variety Albania used. The results of this experiment indicate that a single dosage of 20 ounces per 40 cubic feet (50 ounces to 100 cubic feet), with an exposure of 4 hours, kills all stages, including the eggs. No injury to the dormant corms was observed. Corrosive Sublimate (Mercuric Chloride). — Corrosive sublimate may be used as an insecticidal dip, mixing 1 ounce to 7 gallons for an ex- posure of 17 hours. This kills all stages of the thrips and also seems to control any fungus or bacterial organisms infecting the corms. It is advisable first to dissolve the corrosive sublimate in a small quantity of hot water, since it dissolves very slowly in cold water, then dilute to the correct amount with cold water. On peeled corms, 1 ounce of corrosive Cm. 337] History and Control of Gladiolus Thrips 11 sublimate to 6 gallons of water for 3 hours will kill all stages. Since the corms absorb a considerable amount of the preparation, more of the stock solution must be added after each treatment. Hot Water. — Providing a constant temperature of 112° to 114° F is held from 20 to 30 minutes, and providing that the corms are held be- neath the surface of the heated water during the period, a complete kill of all stages of the gladiolus thrips is obtained. Hot water vats used in Fig. 5. — End view of tent cover shown in figure 4. the treatment of narcissus bulbs are satisfactory for treating large quan- tities of gladiolus corms. A thermometer of good quality should be sus- pended in the water and should be watched carefully during the process. EXPERIMENTAL TREATMENT OF PLANTS IN THE FIELD Most of the contact insecticides tested were effective in killing the thrips with which they came into direct contact, but daily 8 applications were necessary to bring the flowers through clean. Table 5 gives a list of the contact insecticides tested. In view of the results with contact insecticides, experiments were made with stomach poisons mixed in brown sugar for bait (table 6) . Re- sults of experiments with cryolite and potassium fluoaluminate (stom- ach poisons) indicate that these materials cannot be used with safety along the coast, owing probably to high humidity. Results with com- binations of paris green and zinc sulfate and of paris green and hy- drated lime appeared to be fairly satisfactory, except during the winter when they showed a considerable amount of plant injury. The manga- nese arsenate spray results were much more satisfactory and no injury was observed, providing the spikes were not sprayed, even during the winter months. s Some growers were treating twice a day. 12 University op California — Experiment Station TABLE 5 Contact Insecticides Tested to Control Gladiolus Thrips in the Field Material and concentration Kill of thrips Remarks Black Leaf 155, 5 pounds. Water, 100 gallons Contact No. 1, 1 to 2 pints. Water, 100 gallons Cubor, 2 quarts Water, 100 gallons. Cubor, dust Premek, 1 gallon Stabilizer (Premek), 1 pound. Water, 100 gallons Special naphthalene, dust Beta naphthol (150 mesh), 25 per cent. Diatomaceous earth, 75 per cent Pest Dust No. 8 Rotenone, 12.5 per cent Sulfur (gas house), 87.5 per cent Rotenone, 5 per cent Deguilin, 1.5 per cent Inert ingredient, 93.5 per cent. Rotenone, 0.5 per cent Pyrethrum, 0.325 per cent Inert ingredient, 99.175 per cent. Rotenone, 0.5 per cent Pyrethrum, 0.125 per cent Flotation sulfur, 15 per cent Zinc sulfide, 11 percent Inert ingredient, 73.375 per cent. Fair Good Good Fair Good Good Poor Fair Poor Poor Fair Fair Adheres well; plant uninjured Kills only thrips hit; several daily applications necessary; plant un- injured Adheres well; plant uninjured Kills only thrips hit; several daily applications necessary; plant un- injured Adheres well; plant uninjured Cm. 337 History and Control of Gladiolus Thrips 13 TABLE 6 Stomach Poison Insecticides Tested to Control Gladiolus Thrips in the Field Material and concentration Cryolite (synthetic), 1 to 4 pounds Brown sugar, 20 to 66 pounds Water, 100 gallons Cryolite (synthetic), 4 pounds Liquid blood albumin, 1 pint Water, 100 gallons Potassium fluoaluminate, H to 2 pounds Brown sugar, 66 pounds Water, 100 gallons Copper arsenite, 7.66 per cent Water-soluble arsenic, 0.33 per cent Copper as carbonate, 0.14 per cent Inert ingredient, 91.87 per cent Standard lead arsenate, 4 pounds Brown sugar, 66 pounds Water, 100 gallons Standard lead arsenate, 4 pounds Liquid blood albumin, 1 pint Water, 100 gallons Basic lead arsenate, 4 pounds Brown sugar, 66 pounds Water, 100 gallons Basic lead arsenate, 4 pounds Liquid blood albumin, 1 pint Water, 100 gallons Paris green, 2 pounds Hydrated lime, 1 pound Brown sugar, 66 pounds Water, 100 gallons Paris green, 2 pounds Hydrated lime, 2 to 6 pounds Brown sugar, 66 pounds Water, 100 gallons Paris green, 2 pounds Zinc sulfate, 2 to 6 pounds Brown sugar, 66 pounds Water, 100 gallons Paris green, 2 pounds Iron oxide, 2 to 4 pounds Brown sugar, 66 pounds Water, 100 gallons Kill of thrips Good Poor Fair Poor Fair Poor Poor Poor Good Good Good Fair Remarks Severe plant burn; unsafe along coast Adheres well; plant uninjured Slight plant burn; kills slowly No plant burn; kills slowly. Severe plant burn ; unsafe along coast Slight plant burn with 2 to 3 pounds hydrated lime ; no burn (in summer) with 4 to 6 pounds hydrated lime; best results with 4 pounds hydrated lime. Has tendency to bead up and roll off. Some burn in winter Using 4 pounds zinc sulfate gave no plant burn in summer; some burn in winter Using 4 pounds iron oxide gave no plant burn in summer; some burn in winter 14 University of California — Experiment Station TABLE 6— (Concluded) Stomach Poison Insecticides Tested to Control Gladiolus Thrips in the Field Material and concentration Paris green, 1 rounded tablespoon Zinc sulfate, 2 rounded tablespoons Brown sugar, 1 pound Lemon juice, 30 drops Yeast, ? cake Water, 1£ gallons Paris green, 1 rounded tablespoon Hydrated lime, 2 rounded tablepsoons Brown sugar, 1 pound Lemon juice, 30 drops Yeast, \ cake Water, \\ gallons Paris green, 1 rounded tablespoon Iron oxide, 2 rounded tablespoons Brown sugar, 1 pound Lemon juice, 30 drops Yeast, 1 cake Water, \\ gallons Manganese arsenate, 4 pounds Brown sugar, 66 pounds Water, 100 gallons Manganese arsenate, 1 rounded tablespoon Brown sugar, 1 pound Lemon juice, 30 drops Yeast, i cake Water, \\ gallons Kill of thrips > Good Good > Fair Excel- lent Excel- lent Remarks No plant burn during summer; some burn during winter No plant burn; applies and adheres well ; some injury to spike in winter time RECOMMENDATIONS FOR CONTROL Since the gladiolus thrips is so generally distributed in most of the areas where it occurs, it is advisable to apply a spray in addition to the treat- ment of the corms. The final and most serious attack of this species of thrips is on the spike and flower. In order to insure clean flowers and to prevent rapid increase in population of the thrips, it is necessary to be- gin spraying when the plants are about 2 inches high. The application should be repeated once a week, or after each overhead irrigation. Spray- ing should terminate when flower spikes appear. If spraying is delayed until flower spikes appear, nothing can be done to save the flower. Some growers have found it practical to pull out infested young plants when the infestation first appears in the field. Cm. 337] History and Control of Gladiolus Thrips 15 Along the coast where high humidity prevails, as in the Carlsbad and Encinitas districts, the most satisfactory spray that has been used in the summer months is as follows : Small Quantity Large Quantity Manganese arsenate 9 .... % pound 4 pounds Brown sugar 4% pounds 66 pounds Water 6^ gallons 100 gallons This spray is also effective in controlling caterpillars that occasionally attack the gladiolus. When this formula is used during the winter months, it is advisable to spray less often (at two-week intervals) , since the thrips are less active and are not reproducing so rapidly as in the summer. In the event of ex- tremely humid conditions, such as usually prevail in the winter, it might be well to reduce the concentration of the manganese arsenate. However, during the 1934-35 winter season, no apparent injury to the gladiolus plants was caused by this spray. The spray formula given below cannot be used with safety in the coas- tal areas because of danger of burning the plants. This burning is over- come to a considerable extent by the addition of hydrated lime or zinc sulfate, but these preparations counteract to some extent the effects of the poison on the insect. This is particularly true for caterpillars. How- ever, in dryer sections, such as Vista, Pomona, and Riverside, this for- mula, recommended by A. G. Dust an of the Canadian Department of Agriculture, may be used. It is as follows : Small Quantity Large Quantity Paris green % pound 2 pounds Brown sugar 4% pounds 66 pounds Water 6^ gallons 100 gallons In using the paris green formula it is necessary to keep the spray mix- ture well agitated during the application because the paris green tends to settle out very rapidly. To obtain the maximum results from these sprays, it is advisable to wet thoroughly all leaf surfaces of the plant. In cases where the infestation has become too great for ordinary means of control, it is advisable to : (1) cut and burn the infested flower spikes; (2) make one or two applications of spray; (3) irrigate heavily. This will reduce the infestation of thrips and lessen injury to the foliage of the plant, and thus induce better corm growth. 9 The manganese arsenate used was a proprietary product which met the fol- lowing specifications: total arsenic oxide, 40 per cent; arsenic as metallic arsenic, 26 per cent; water-soluble arsenic as metallic arsenic, not over 0.5 per cent; man- ganese arsenate, 68 per cent. 16 University of California — Experiment Station SUMMARY The gladiolus thrips is now generally distributed throughout the United States. It has also been found in Australia, Africa, Canada, Hawaiian Islands, and New Zealand. Although the host range of the gladiolus thrips is relatively large, it is primarily a pest of the gladiolus. All parts of the gladiolus plant are subject to the attacks of this in- sect except the cormel or bulblet, which is protected by a capsule-like covering. The life stages of this insect consist of the egg, first and second larval stages, prepupa, pupa, and adult. The length of the life cycle (from egg to adult) of this thrips in June, 1934, at Encinitas, California, required a minimum of 14 days, a maximum of 19 days, and 16.5 days on an average. The females of the gladiolus thrips are able to reproduce partheno- genetically, the resulting progeny being males. This thrips overwinters between crops on volunteer gladiolus plants in California. Dispersal experiments indicate that the gladiolus thrips are capable of flying 15 feet in the air. The wind is probably an aiding factor. The predacious sucking insect, Orius (Triphleps) tristicolor (White) and the parasite Thripoctenus russelli Crawf . are found attacking the gladiolus thrips in California. Thorough treatment of the corms with any one of the following prep- arations will insure clean corms : naphthalene flakes, calcium cyanide, corrosive sublimate (mercuric chloride), or hot water. In addition to the treatment of corms, the most effective field control is a spray consisting of: manganese arsenate, brown sugar, and water; or paris green, brown sugar, and water. ACKNOWLEDGMENTS The writer wishes to express his appreciation to H. J. Quayle, Ento- mologist at the Citrus Experiment Station, for his suggestions during the investigation. Through the courtesy of R. R. McLean, Agricultural Commissioner of San Diego County, the San Diego County bulb plant was placed at our disposal in conducting the experiments. 12m-7,'35