May 1946 3-692 k United States Department of Agriculture Agricultural Research Administration Bureau of Entomology and Plant Quarantine SUMMARY OF RESULTS WITH DDT AGAINST TRUCK CHOP, TOBACCO, AND SUGAR BEET INSECTS DURING I9U5 Compiled by W. F. White, Division of Truck Crop and Garden Insect Investigations This statement has been prepared from reports received from the various field laboratories of the Division of Truck Crop and Garden Insect Investigations and is issued for the information and guidance of persons interested in experimental v/ork with DDT as a crop insec- ticide. This supplements circular E-6H2, issued in March 19^5 » on the results of the work with DDT conducted by this Division during the season of igUh# Recommendations on the use of DDT dust mixtures in the control of Lygus bugs on sugar beets grown for seed and cab- bage caterpillars were included in a statement issued by the Depart- ment of Agriculture on August 22* I9H5, under the title "Suggestions Regarding the Use of DDT by Civilians." Acknowledgments are made to the Division of Insecticide Investi- gations for DDT determinations, and to the Bureau of Plant Industry, Soils, and Agricultural Engineering, the State experiment stations, and growers for valuable cooperation. Although the results have indicated that DDT in proper formula- tions is more effective against a number of common insect pests of truck crops than are other insecticides, several important factors limit its usefulness at this time. DDT is a poison which makes a health hazard of its application to any portions of a plant that are to be marketed as food or consumed by livestock. Under some condi- tions DDT causes injury to squash, cucumbers, pumpkin, and rhubarb. Some of the more toxic formulations of DDT insecticides contain oils or emulsifying agents that are injurious to plants. The storage of DDT in the fats and tissues of animals feeding on treated plants and the excretion of it in milk requires further investigation before this insecticide can be recommended for some crops, such as pea, when the vines are used as stock food. The technical grade of DDT was used in all the DDT preparations discussed in this report except in the aerosols, in which the more highly refined aerosol grade was used. Dust mixtures, suspensions (wettable powders), emulsions, aerosols, and solutions were tested, but the majority of the tests were with dust mixtures. cabbage plants along ditch rows but are generally of minor importance. In preliminary experiments spray suspensions were as effective as dust mixtures when the same quantities of DDT were applied per acre. At Baton Rouge, La., a 3-percent DDT dust did not control the fall armyworm ( Laphygma frugiperda (A. & S,)), which is an important pest of the young cabbage plants of the fall crop in Louisiana, Very poor control of the cabbage looper (Trie hop lusia ni (Hon.)) was obtained in the fall experiments, but undiluted cryolite was even less satisfactory. Excellent control of cabbage looper was obtained in the spring experi- ments, Lygus bugs,— A single application of the 5-percent DDT dust applied at 30 pounds per acre (1,5 pounds of DDT) continued to give satisfactory control of the several species of Lygus bugs on sugar beets grown for seed in Arizona, New Mexico, Oregon, Utah, and California, The dust mix- ture was applied at the time the plants were in full bloom. It had some effect in keeping down populations of the larger plant bugs, such as the Say stinkbug ( Chlorochroa sayi Stal,), A ^-percent DDT- sulfur dust was effective in controlling Lygus bugs and the two-spotted mite (Tetranychus bimaculatus Harvey) on lima beans in a 10-acre field trial at Ventura, Calif, , but seemed to be ineffective against the lima bean pod borer (fltiella zinckenella (Treit,)), A 5-P©rcent DDT dust controlled Lygus apical is Fieber on heavily infested carrots at Charleston, S, C, Tomato fruitworm, — Field experiments in southern California on the control of the tomato fruitworm (Heliothis armigera (Hbn,)) yielded re- sults similar to those reported for 19^T~ A dust mixture containing 10 percent of DDT, 25 percent of sulfur, and the remainder pyrophyllite gave a higher degree of control of the fruitworm than did the recommended cryolite dust mixtures containing 67 to 70 percent of sodium fluoalumlnate and 22 to 25 percent of sulfur. This DDT-sulfur insecticide also controlled the tobacco hornworm^Protoparce sexta (Johan,)) on tomato in California and the beet armyworm (Laphygma exlgua (Ebn,)), A 5-percent DDT dust mixture was as effective as cryolite against the fruitworm, but was not satisfactory , for controlling the other caterpillars. The DDT was applied at the time and rates recommended for cryolite - namely, in three applications at 2-week intervals beginning when the first fruits of the main crop began to set and at the rate of 20, 30, and UO pounds per acre, respectively. By this treat- ment 9 pounds of technical DDT was applied per acre for the season. As discussed elsewhere in this circular (p. ), DDT dust mixtures ere effective against the tomato fruitworm, or corn earworm, on crbbage and beans. Aphidb affecting potatoes. — In experiments conducted at Houlton and Presque Isle, Maine, fungicide dustt. and sprays to which technical DDT had been added appeared to have no aaverse effect upon the potato plants and substantially reduced infestations of the green peach aphid (Myzus persicae ( Sulz. ) ) , the potato aphid ( Macros iphum solanifolii (Ashm.)) , the buckthorn aphid (Aphis abbrevlata Patch), and the foxglove aphid (Myaus convolvull (Kalt*))* The efficiency of the fungicides to which DDT was added was apparently not impaired, since late blight control was good in all treated plots. The yield of potato tubers was increased as ouch as 25 barrels per acre as compared to that in check plots treated with the fungicide only* The most efficient and practical control of the aphids was ob- tained by using the emulsion at the rate of 75 *° 100 gallons per acre- application. This emulsion contained 0.5 pound of DDT, 0.9 pint of either xylene or a highly refined methylated naphthalene, and U tea- spoonfuls of emulsifler per 100 gallons. A dust mixture containing 5 percent of DDT in pyrophyllite applied at 35 pounds per acre seemed to be as effective as the emulsion, but this involves about 1*75 pounds of DDT per aore as compared with 0*5 pound in the emulsion* Similar control with DDT suspensions was obtained with 0*8 pound to 2*0 pounds of DDT per 100 gallons derived from wet table DDT powder* A ^percent DDT aerosol was about as effective as the DDT emulsion, but about twice as much DDT was applied per aore* In observations incidental to experiments on other insects, a 5- percent DDT dust mixture gave satisfactory control of the green peach aphid on potatoes at Union Gap, Wash* , and on bell peppers grown for seed in southern California* Colorado potato beetle* -—Results of field experiments at Columbus, Ohio, corroborated those of previous years in demonstrating that Ho pounds of 1-percent DDT dust mixture is effective against the Colorado potato beetle ( Leptlnotarsa decemlineata (Say)). In these experiments the DDT dust was more effective than one containing 0*5 percent of rotenone* One pound of DDT per acre was effective when applied in spray suspensions and emulsions. At Houlton, Maine, 0*5 pound of DDT per acre applied in methylated naphthalene and xylene emulsions for control of aphids satisfactorily controlled the Colorado potato beetle. Potato leafhopper. — In field experiments at Columbus, Ohio, DDT was compared with pyrethrum, sabadilla, and other insecticides for the control of the potato leafhopper (Empoasca fabae (Harr*)) on potatoes and beans. Best results were obtained with DDT* The results were not conclusive with regard to the dosage of DDT required per acre* It appears, however, thnt a total of about 1*5 pounds of DDT will be re- quired in three applications using either 1 percent of DDT in a dust mixture or 0.0U percent In a kerosene emulsion* The emulsion was more effective than the dust mixture* -6- Flea beetle on potatoes. —-Experiment s with DDT dust mixtures against the tuber flea beetle (EpjL^rlx tuberls Gentn. ) were conducted in the Yakima Vail ley $ Wash. The main objective was to reduce the in- jury to potato tubers caused by the larvae* The western potato flea beetle (1. subcrinita ( Lee ) ) was also present, but this species does not appear to cause appreciable injury to the tubers. In one experi- ment five applications were made of a 2- and a S~percent DDT dust mix- ture and undiluted cryolite applying totals of 1.7 SJX^ 2*8 pounds of DDT and 35 pounds of cryolite in the three dust mixtures, respectively. The green peach aphid threatened to destroy the cryolite-dusted plots; therefore, 0*9 pound of DDT per acre was applied in the form of a U- percent dust as a substitute for the third application of cryolite* The percentages of tubers injured by the flea beetle at harvest were 58, U9, and 39 for the 2-percent DDT, the 5-percent DDT, and the cryo- lite-DDT plots, respectively. The yields were 16,799, 17,225, lU,6U3 pounds, respectively* In another experiment applications of 2, 5, and 10 percent of DDT resulted in 60„ 59 1 and 39 percent of the tubers being injured by the flea beetlr larvae. The yields were 26,U07'» 27»265* and 27»5^ pounds per acre* Although in these experiments about one-half of the tubers showed some flea beetle injury, very few were damaged sufficiently to be unmarketable; therefore the control was considered to be good. In field experiments near Boulton, Maine, the potato flea beetle (Epitrix cucumerls (Harr. )) was killed by DDT dusts and sprays applied for the control of aphids, but no estimates were made of the degree of control. Pea aphid. — In experiments at Madison, Wis., 5-percent DDT dusts were more effective against the pea aphid ( Macro siphum pi si (Kltb.),) than 0.75-P«rcent rotenone dusts* Single applications were made at the rate of about 35 pounds per acre (1*75 pounds of DDT). Most satis- factory control was obtained with a ^percent DDT dust prepared by dissolving technical DDT in equal parts of warm acetone and spraying the solution into talc being mixed in a mechanical mixer. In exten- sive field experiments against the pea weevil by the Moscow. Idaho, laboratory, a 5**percent DDT dust mixture also gave better pea aphid control than the 0.75-Percent rotenone dust. DDT is not being recom- mended for pea aphid control until it can be determined whether residues remaining on pea vines at harvest will be detrimental to livestock* In experiments in Maryland in cooperation with the University of Maryland, lU aerosols containing 5 percent of DDT were tested against the pea aphid. Some of the solvents in the aerosols injured the pea vines* The most satisfactory aerosol contained 5 percent of the aerosol- grade DDT. 5 percent of cyclohexanone, 5 percent of a mixture of di- and tri- methyl naphthalenes, 35 percent of acetone, and 50 percent of -7- technical methyl chloride* This aerosol was very effective when ap- plied at the rate of about 20 pounds (1 pound of DDT) per acre with special equipment developed for the purpose. In commercial field trials with the aerosol, the aphid population was decreased 95 *° 99 percent and the yield of shelled peas increased from 10 to 67 percent* Emulsions were less effective than the aerosols, hut were more effec- tive than suspension or dust mixtures. Excess quantities of emulsi- fier in the emulsions caused plant injury. The most promising emulsion contained 0*5 pound of DDT, 0*0 pint of highly refined methylated naphthalene, and k teaspoonfuls of emulsifier per 100 gallons. Prom- ising results were obtained with a dust mixture prepared from a solu- tion of DDT in methylated naphthalene. Pea weevil. — In extensive field experiments by the Moscow, Idaho, laboratory, in which a 5-percent DDT dust mixture and a 0.75-Percen* rotenone dust mixture were applied by airplane and by ground machines, the DDT gave the more satisfactory control of the pea weevil ( Bruchus pisorun (!•))• The usual rate was 1 pound of DDT in 20 pounds of dust mixture per acre* This is in agreement with results obtained on a smaller scale in I9UU. DDT cannot be recommended for use on peas, however, until studies on the hazard of residues left on the pea vines intended for livestock feed have been completed. Onion thrlps. — Extensive experiments on the control of the onion thrlps (Thrips tabacl Lind.) on onions were conducted in Idaho and southern California, and limited tests were made in Maryland and, Utah. The results of this work corroborated those of I9H3 and I9HU in that DDT compared favorably in toxicity with other insecticides used against this thrips, such as nicotine sulfate and tartar emetic. On onions grown for bulbs or for table use satisfactory control was usually obtained with dust mixtures containing 5 sjicL 10 percent of DDT and with a toluene emulsion containing 1 pound of DDT per 100 gal- lons. The experiments were not conclusive with respect to the formu- lation, strength, and schedule of application. Experiments in south- ern California, however, indicate that in dust mixtures a total of 9 pounds of technical DDT may be required per acre of table onions, ap- plied in three weekly applications beginning when the plants are 8 to 12 inches high. Smaller quantities of DDT may be required in the toluene emulsions. In limited field tests suspensions and aerosols gave less promising results than the dust mixtures and emulsions. On onions grown for seed the application of DDT insecticides before the seed heads began to form did not protect the seed heads from migrating thrlps. Applications to the seed heads are not prac- tical with the available ground equipment. -8- After onion fields are harvested, in late summer or early fall in southern California, the onion thrips often migrate to seed "beds where cabbage plants and other cole crop plants are grown. The young plants are subject to severe damage unless the thrips are controlled* Later, the thrips attack young onion plants of the next crop as they appear above the ground, and sometimes destroy all the plants of a seedbed* The 5-Percent DDT dust was found to be very effective in controlling these infestations* On young onion seedlings in the fall two applica- tions of the dust mixture may be required, one as soon as the plants appear and the other when they are 2 to 3 inches high* Pepper weevil.-- Observations were made of tests conducted by pepper growers in San Juan Capistrano and San Clements, Calif,, on the control of the pepper weevil (Anthonomus eugenll Cano) with DDT dust mixtures* Approximately 125 acres of bell peppers grown for seed were dusted with mixtures containing 2 to 5 percent of DDT, with excellent control* In- cidentally it was observed that the green peach aphid (Myzus persicae (Sulz*)) was also controlled* This aphid often causes considerable damage in fields dusted with calcium arsenate or cryolite unless it is controlled with nicotine or rotenone* From seven to nine applications 8 to 10 days apart, at 15 to 30 pounds of dust mixture per acre, were made* Tobacco moth* — A 5-percent solution of DDT in oil was tested at Richmond, Va, , as a residual spray for the control of the tobacco moth (Ephestia elutella (Ebn. )) in tobacco warehouses* The technical DDT was dissolved in a highly refined volatile oil of the kerosene type and applied at the rate of 100 ml, of solution per 1,000 cubic feet of air space with a portable power sprayer especially designed for use in tobacco warehouses* A single application was made in one warehouse, and three were made in another, while a third was left untreated. The spray killed adults confined in cages within the treated warehouses and caused some reduction in the general infestation but did not give adequate control* Beet leafhopper. — At Twin Tails, Idaho, a DDT- toluene emulsion gave encouraging results against the beet leafhopper (Eutettix tenellus (Bak, )) on sugar beets when applied at approximately 1 pound of DDT per acre, A dust mixture, a spray suspension, and an aerosol were only slightly less promising at the 1-pound dosage* The sprays contained the equivalent of 1 pound of technical DDT per 100 gallons* The dust mixture and the aero- sol contained 5 percent of the technical and aerosol grades of DDT, re- spectively* At Logan, Utah, a 5-percent DDT dust was partially effective in re- ducing damage to tomatoes by curly top disease, transmitted by the beet leafhopper* The DDT treatment, however, was not so effective as the recom- mended practice of planting two or three plants in each hill. -9- In commercial trials in Arizona on sugar beets grown for seed, tetter control of the beet leafhopper was obtained with atomized oil containing 2.H percent of DDT than that containing 0.06 percent of pyrethrins. The rate of application was S to 9 gallons* or about 1*5 pounds of DDT, per acre. The solvent was one- third white oil and two- thirds highly refined kerosene. Sweetpotato weevil,— In experiments at Baton Rouge, La. , DDT has been effective in residual sprays and dusts for the control of the sweetpotato weevil (Cylas formicarius elegantulus (Summers)) in sweetpotato storages. Its use in this manner, however, cannot be ad- vocated at this time, because of the residue hazard as the treated sweetpotatoes may be fed to livestock. Although tests have been made with emulsions, solutions, and dusts, the best formulation has not been determined* It appears, however, that the most practical one would be a 10-percent dust mixture or a kerosene emulsion. The strength of the emulsion does not seem to be important as long as about 1 pound of DDT is applied per 3*000 cubic feet of storage space. In sweetpotato weevil baits DDT has not been so effective as paris green or sodium fluo silicate. Attempts to control the Insect in field plantings by use of dusts and sprays containing DDT, arsenical, and flu- orine insecticides have not been successful. Corn earworm on beans. — A 3-percent DDT dust mixture was more ef- fective than the recommended cryolite against the com earworm (Heliothis armigera (Hbn.)) on snap beans in a field experiment at Norfolk, Va. Squash bug. — Laboratory and field experiments at Beltsville, fid. , indicate that 10-percent DDT dust is effective against the squash bug (Anasa tristis (Deg.)), but under some conditions will cause severe in- jury to young plants of summer squash. Under field-plot conditions large numbers of adults that came into the plots were able to survive DDT residues on the plants and lay eggs, but the newly hatched nymphs were not able to live. Laboratory studies indicate that pyrethrum when added to the DDT hastens mortality, and also that it may be pos- sible to control squash bugs on large plants with low dosages of DDT. Farther studies are necessary, however, to determine effective dosages and formulations that will be tolerated by the plants. In southern California squash bugs were controlled on Banana squash with 10-percent DDT dust without injury to the plants. The young nymphs seemed to be affected most, as the infestations did not build up as rapidly on the dusted plants as on the undue ted plants, and very few dead adults were found. The 2.5- and 5-percent strengths were slightly less effective than the 10-percent. 10- Strlped cucumber beetle. — At Columbus, Ohio, 1 pound of DDT per acre applied in different emulsions was effective against the striped cucum- ber beetle (Diabrotica vittata (7.)) on cucumbers, but it caused injury to the plants. DDT in a dust mixture applied at about 0.6 pound per acre did not cause injury but was not so effective as the emulsion. This dust mixture compared favorably in effectiveness with dust mixtures con- taining 0.5 percent of rotenone or M*> percent of sodium fluoaluminate. Picklevora. — Incidental to tolerance experiments with DDT on squash at Beltsville, Md. , data were obtained indicating that a spray suspension containing k pounds each of 10-percent DDT dust and tribasic copper sul- fate vas more effective than a 1-percent rotenone dust against the pickle- worm (Diaphania nitidalis (Stoll)). This spray suspension did not injure the squash plants. Better control was obtained with a 2.5-percent DDT dust, but the yield was reduced considerably, indicating plant injury. The foliage of the plants was not visibly injured by the 2.5-percent DDT but were seriously injured by a 10-percent DDT dust mixture. At Charleston, S. C. , a 2-percent DDT dust applied at 10-day intervals con- trolled the picklevora, but stunted yellow summer and Cocozelle squash and reduced the yield. Serpentine leaf miner. — DDT dust mixtures were not effective against the serpentine leaf miner (Liriomyza pusilla (Meig.)) infesting cucumber foliage at Irvine, Calif. Three replicates were made of the 5- and 10- percent strengths of DDT. The leaf miners were more abundant In the DDT plots than in the check plots ?.h days after the first of three applica- tions. A severe infestation of this insect was also noted in a tomato field treated several times with DDT- sulfur dust. Hymenopterous para- sites were less abundant where DDT was applied. Hornworms on tobacco.— Experiments during I9U5 at Florence, S. C, Olarksville, Tenn., and Oxford, H. C, corroborate those of I9H1 and I9I& in showing that the tobacco homworm (Protoparce sexta (Johan.)) is less susceptible to DDT than is the tomato hornwormTPro toparce crulnquemaculata (Haw.)), and is not satisfactorily controlled with up to 30 pounds per acre of the 10-percent strength of dust mixture. Experiments at Florence, S. C. , however, demonstrated that the tobacco homworm could be controlled hy applying UO pounds per acre of 25-percent DDT dust or 70 to 80 gallons Ser acre of a spray containing 3 pounds of a wettable powder containing D percent of DDT. These DDT treatments are not so practical as spraying with basic copper arsenate owing to excessive cost. Mexican bean beetle. --Although the more common DDT dust and sprays and aerosols have not been effective against the Mexican bean beetle (Epilachna varlvestis Muls.)> some of the emulsions and suspensions tested during 1945 at Columbus, Ohio, were toxic to the insect but injured the plants. These sprays contained 3 pounds of DDT per 100 gallons of water so that approximately 3 pounds were applied per acre. In general, these preparations were not so effective as the recommended rotenone du-t mix- ture. -11- Tobacco flea "beetle, — la a field experiment at Oxford, N. C,, DDT was more effective than cryolite, "basic copper arsenate, or ro- tenone against the tobacco flea "beetle (Spltrix hirtipennis Melsh,), A dust mixture containing 3 percent of DDT was used at the rate of about 15 pounds per acre per application. Three, applications were made, so that a total of about 1,H pounds of DDT per acre was ap- plied for the season. The DDT did not injure the plants, and the yield of tobacco was not affected by the different treatments. The DDT dust was also more effective than the cryolite or basic copper arsenate when mixed with the fungicide ferric dimethyl dithio car- bamate and applied to tobacco-plant beds for the control of both the flea beetle and the blue mold disease, A 5-percent DDT dust applied to plants just before they were transplanted protected them from flea beetle injury for a week or 10 days. It appears that DDT will be a useful insecticide for the flea beetle in tobacco-plant beds and pos- sibly on newly set tobacco plants. It is not promising, however, for use on well-established plants in the field, because of ineffective- ness against the tobacco horaworm, which becomes more important than the flea beetle under field conditions* Six- spot ted leafhopper on lettuce,— During I9UU the six-spotted leafhopper ( Macrosteles divisus (Uhl,)), which transmits the destruc- tive aster yellows disease to many ornamentals and vegetable crops, was found to be susceptible to DDT aerosols. At Beltsville, Md,, during 19^5« a field experiment was conducted on the control of this leafhopper on lettuce with an aerosol containing 5 percent (by weight) of DDT, 5 percent of cyclohexanone, 5 percent of aromatic petroleum oil, 35 percent of acetone, and 50 percent of methyl chloride* Eight applications were made during May and June at Intervals of 7 to 10 days at the rate of about 10 pounds per acre-application* A total of about U pounds of DDT per acre was applied during the season. There were seven plots about one-quarter acre in size, four being treated and three left untreated as a check. Almost all the leafhoppers in the treated plots were killed, but after a few days others moved in from the untreated plots* Some of the incoming adults seemed to be killed by the insecticide residues but frequent applications were necessary* The control of the leafhopper populations, as judged by sweep-net counts made on the treated and check plots just prior to the second and subsequent applications, was over 90 percent. At harvest- time 12. k percent of the plants in the check plots and 1,1 percent in the treated plots were diseased. Miscellaneous insects on greenhouse plants, — A number of commercial greenhouses in Maryland were treated with an aerosol containing 5 per- cent (by weight) of DDT, 5 percent of cyclohexanone, 5 percent of me thy late naphthalene, and 85 percent of dichlorodiluoromethane. The aerosol was dispersed from a knapsack tank, through a hose, rod, and oil-burner nozzle of the 5-gallon-per-hour capacity. The aerosol was -12- directed into the air over the crops for 35 to 50 seconds per 1,000 square feet of greenhouse area. By this method the aerosol was released at 10 to 20 pounds per acre (about 0,5 to 1 pound of DOT), This aerosol treat- ment was effective for the azalea leaf miner ( Gracllaria azaleella Brants) on azalea, the greenhouse leaf tier (Phlyctaenla rubifialls (Guen.T), and the chrysanthemum thrips ( Thrips nigropilosus Uzel) on chrysanthemum, and the onion thrips (Thrips tabaci Lind«) on roses. In five commercial greenhouses near Hidgeville, Md», the aerosol treatment gave excellent control of the onion thrips on roses without injury to the plants or flow- ers. Pacific Coast wireworm. — Laboratory and small-plot experiments have been conducted at Walla Walla, Wash., on the toxicity of technical DDT to wireworms, chiefly the Pacific Coast wireworm ( Llmonius canus Lec. ) . Preliminary tests in I9U4 indicated that DDT is not repellent to wire- worms and will not kill them within a few days of application but at dosages of l6 pounds or more per acre will cause appreciably mortalities at 5 weeks' exposure. In April I9UU a 10 percent DDT-pyrophyllite dust was thoroughly mixed in a concrete mixer with the top 9 inches of silt- loam soil. The soil was placed outdoors in cages 15 inches deep and U by 5 feet in size and constructed of metal walls and concrete bottoms. Cores of soil were taken from these cages 18 months later and placed in salve tint' in the laboratory. The mortalities of wireworms placed in these tins of soil are given in table 1* Table 1.— Mortality of wireworms placed in soil that had been exposed to weathering 18 months since treatment with DDT. DDT applied per acre t Mortality after indicated exposure 3 weeks : 5 weeks :13 weeks 320 U8 16 0 (check) Percent 66 32 Percent t Percent 1 100 S 26 12 100 100 88 60 32 After 18 months' exposure to weathering in soil the DDT was toxic to wireworms, but not so toxic as in similar tests made after 12 months. DDT was tested in 16 field plots 20 feet square and arranged in a latin square with /, replicates. The 10-percent DDT dust mixture was ap- plied during the early spring in dosages of 10, 20, and 40 pounds of DDT per acre. One-half of each dosage was applied to the soil surface, the soil was disked and plowed to a depth of 9 inches, then the remainder was applied and the soil was again disked in both directions and floated. -13- In each plot rows of the following crops were grown: lettuce, peas •car- rots, onions, cabbage, spinach, string beans, "beets, potatoes, and to- matoes, A row of lima "beans was grown later in the season. The DDT in the soil did not appear to injure any of the plants. As shown in table 2, the DDT reduced the number of old-brood wireworms in the soil, prevented the development of new-brood wireworms, and reduced the dam- age by wireworms to potato tubers. These results and those of laboratory tests demonstrate that DDT is much more toxic to the younger wireworms. Other preliminary tests indicate that DDT mixed with soil is toxic to wireworm adults that come in contact with the soil. The female beetles appear to he less susceptible than the males and are able to lay eggs before they die. Table 2. — Control of wireworms by applications of DDT to silt-loam soil to a depth of 9 inches. : Pounds of technical : Wireworms per square I Potato tubers at harvest: : DDT applied tfoot after 3p months : : Damage : \ Old brood \ {New "brood : Total :by wireworms s Uumber < > Number < Pounds » Percent : : to 0 « ; 0 J 157 . ; 1 \ : 20 i 0.5 0 ; 15U 1 X 10 • 0.5 : 0 i 110 10 t ? • 0 ; 2.2 j 0.7 s 129 s Suy^ar-heet wireworm. — At Ventura, Calif., a 25-pound-per-acre ap- plication of DDT to sandy-loam soil resulted in 7*HPercent control of the sugar-beet wireworm (Llmonius californicus Mann.) in 165 days and reduced the damage by wireworms to lima beans by 91 percent. A 10-percent DDT-pyrophyilite mixture was dusted onto the soil in two l/g-acre plots and disked and plowed into the soil to a depth of S inches. Dosages of 50 and 100 pounds of DDT per acre gave slightly better results than the 25-pound dosage. The lima beans were planted 2^ months after treatment. None of the treatments were detrimental to lima bean germination or to plant growth. Wireworms placed in samples taken from the top 6 inches of treated soil 5 months after treatment all died within a month* s tiraet whereas most of those placed in soil taken below the 6-inch depth lived. Tolerance of Plants to DDT Insecticides Applied to Foliage In tolerance tests at Madison, Wis., 28 crops were dusted with 10 percent of technical DDT in pyrophyllite. The temperatures were rela- tively low, accompanied sby frequent showers. Several applications were made beginning when the plants were 3 to 6 inches high. Only four crops -u- were injured - rhubarb (common variety), cucumber (variety A-500 and several others), pumpkin (Kentucky field), and squash (Table Queen, Warted Hubbard, Buttercup, and Early Straightneck Summer). The injury was to the foliage of the young plants and was so slight that the plants outgrew it and were not appreciably affected* In experiments in southern California a 10-percent DDT dust mixture applied in Banana squash for the control of the squash bug and to cucumber for the control of the striped cucumber beetle did not injure the plants* In experiments at Charleston, S. C, however, a 2-percent DDT dust applied at 10-day intervals to yellow and Co coze lie squash stunted the young plants, delayed the first harvesting about 2 weeks, and reduced the yield about 50 percent* In tolerance tests conducted at Beltsville, Md. , in cooperation with the Bureau of Plant Industry, Soils, and Agricultural Engineering, 10 truck crops were dusted with 20 to 30 pounds per acre-application of either 5 or 10 percent of DDT* Only one crop. Yellow Crookneck squash* was injured* The crops not injured included Bountiful beans, Early Wakefield cabbage. Bales Best cantaloups, Abbott and Cobb cucumbers, 2Tb sneezer onions, Alaska >eas, Irish Cobbler potatoes, Marglobe tomatoes, and Purple Top turnips* All except the cucurbits and tomatoes were early and were grown on rela- tively high land under irrigation* The cucurbits and tomatoes were grown in bottom land that was wet* There were frequent rains throughout the summer, and it is possible that these rains may have affected the results by washing the insecticide from the foliage or by keeping the plants in a succulent condition* The unusual weather conditions led to the presence of several diseases on cucumbers and cantaloups, and mads it difficult to determine whether DDT had caused injury* Two DDT dusts were used on the squash* Each contained 10 percent of technical DDT in pyrophyllite and one also contained 20 percent of sulfur* Each of these dust mixtures applied at about 20 pounds per acre-application injured the young squash plants so severely that the stand was almost de- stroyed* The plants that survived the early applications seemed to out- grow the injury, although they were stunted for some time* A total of nine applications were made, constituting about 18 pounds of DDT* A DDT spray suspension containing U pounds of a 10-percent DDT dust mixture in a tri- bes ic copper sulfate solution and applied at the rate of about 200 gallons per acre (about 7*2 pounds of DDT for the season) did not appear to injure the plants* The fungicide ferric dimethyl dithio carbamate was substituted at the rate of 2 pounds per 100 gallons of water for the trlbaslc copper sulfate in alternate applications of this spray* DDT suspensions have not been extensively tested on truck crops, but preliminary tests indicate that they are as safe on foliage as are the DDT dusts* -15- Solvents for DDT tend to injure plants; therefore solutions should not he applied to foliage of truck crops* Emulsions are safer than solutions, and those carefully formulated from methylated naphthalene, xylene, and toluene solutions of DDT appear to be fairly safe on pota- toes, beets, and onions, when not used in excess. Truck crops tolerant to DDT dust mixtures, — In tolerance experi- ments and in Insect control experiments conducted by the Division dur- ing the past 3 years with simple DDT dust mixtures, not containing oil or other conditioning agents, injury attributed to DDT applied to plant foliage has been reported on only four crops - squash, cucumbers, pump- kins, and rhubarb. Severe injury has been noted on squash only, but this crop sometimes escapes injury. Crops that have been dusted with unconditioned mixtures containing at least 5 percent of DDT without showing evidence of injury are given in the following list: Asparagus, unknown variety Madison, Wis* Beans Bush lima Black Valentine and Stringless Green Pod Bountiful Unspecified varieties Madison, Wis.; Borrolk, Va. Madison, Wis, Beltsville, Md. Ho r folk, Va.; Columbus, Ohio Beets Sugar Salt River Valley and Safford, Ariz.; Albu- querque, N. Mex.; Hemet, Calif.; Medford and Klamath Tails, Oreg.; Twin Sails, Idaho; Madison, Wis.; Heber Valley, Utah Table* Early Wonder Madison, Wis. Broccoli, Italian Green Sprouting Madison, Wis. Cabbage, many varieties* including two of red cabbage and one of Chinese cabbage Madison, Wis., Beltsville, Md.; Charleston, S. C; Baton Bouge, la.; Alhambra, Calif* Cantaloups, Hales Best Beltsville, Md. Carrots, Early Chant enay, unspecified varieties Madison, Wis. Phoenix, Ariz.; Charleston, S. C. -16- Cauliflower, Fortes ,Aberlee, and Green Hollander No. 8 Chard t Lucullufl Collards Dill, unknown variety Eggplant, Black Beauty Kohlrabi, Early White Vienna Lettuce, unspecified leaf variety Onion, several varieties Parsley, double curled Parsnips, Hollow Crown Peas, several varieties Peppers, California Wonder and Pimento Potatoes, several varieties Radish, Crimson Globe undesignated variety Sweet com, Biarcross and Golden Cross Bantam Sweetpotato Tobacco Madison, Wis* Madison, Wis, Charleston, S. C. Madison, Wis, Madison, Wis* Madison, Wis* Madison, Wis*; Phoenix, Ariz* Madison, Wis*; Phoenix, Ariz*; Belt8ville, Md.j Twin Palls, Idaho; Alhambra, Calif* Madison, Wis* Madison, Wis* Madison, Wis.; Beltsvllle, Md*j Phoenix, Ariz*; Moscow, Idaho Madison, Wis*; Alhambra, Calif* Houlton and Presque Isle, Maine; Yakima Talley, Wash*; Mad- ison, Wis*; Columbus, Ohio; Beltsvllle, Md. Madison, Wis* Phoenix, Ariz* Madison, Wis* Baton Houge, La. Florence, S.C.; Clarks- ville, Tenn. ; Oxford, N. C. -17- Tomato, jeveral Tarieties Turnip, Purple Topped Globe Madison, Vie.; Belts- ville, Md.j Alham- bra, Calif,; Colum- bus, Ohio; Logan, Utah Madison, Vis*; Belts- ville, M6U; Phoenix, Arii. UNIVERSITY OF FLORIDA^ U lls2 09238 7207 V