August 19^5 LIdKAKT STATE PLANT BOARD E-668 United. States Department of Agriculture Agricultural Research Administration Bureau of Entomology and Plant Quarantine RESULTS OF LABORATORY AND FIELD TESTS WITH DDT AGAINST THE PEA WEEVIL l/ By Tom A. Brindley and Ralph Schopp, Division of Truck Crop and Garden Insect Investigations As the result of preliminary tests with DDT against the pea weevil (Bruchue plsorum (L.)) during 19^ > it was reported 2j that this insec- ticide did not give promise of being a prospective remedy for the insect. The results obtained, however, in the more recent laboratory and field tests discussed in this circular indicate that DDT has considerable promise as a pea weevil insecticide. This brief discussion of the tests performed during the entire season at Moscow, Idaho, and vicinity has been prepared for early distribution to interested persons owing to the keen interest dis- played in DDT as a possible substitute for the rotenone -containing insecti- cides used ordinarily for the control of the pea weevil but which have been rendered scarce by wartime conditions. Laboratory Experiments Methods ; The technique in the laboratory tests was as follows: Five replicates were treated for each of the materials tested. Fifty weevils were dusted for each material in each replicate, making a total of 250 weevils treated with each material. The insects were treated under a dark- ened bell -Jar dusting chamber under five pounds air pressure and retained in the bell Jar five to seven minutes to allow the dust to settle. After ex- posure the treated weevils were transferred to clean cages to remove them from contact with the insecticide being tested. Immediately prior to and following the treatment, the insects were kept under constant temperature of 86° F. and 65 percent relative humidity, with one exception where a low- er temperature was used for comparison, which is described in the discussion of that experiment. The insects in each replicate were left under observa- tion for ten days. At the end of this period all weevils able to fly or walk were considered to be living and the others were counted as dead. l/ In cooperation with the Agricultural Experiment Stations of Idaho and Washington. 2/ Sohopp, Ralph, and Brindley, Tom A. Tests with DDT against the pea weevil. /Jour. Econ. Ent. 37: 150-151. 19^. 1- 13A5 - 2 - Results: Experiment I. In this test pea weevils were exposed to dosages of a 10 percent DDT mixture in pyrophyllite . The dosages were varied from 0.2 to 0.5 grams to determine if increased dosages would materially increase the kill obtained. A study of the data in table 1 shows that the kill obtained varied from 69-35 percent with a dosage of 0.2 grams to 97.16 percent with a dosage of 0.5 grams. A dosage of 0.5 grams in the laboratory dusting apparatus is equivalent to an application of about 35 pounds per acre on a field basis. % Experiment II. The influence of temperature on the toxicity of DDT to the pea weevil was tested in this experiment. Duplicate series of approximately 250 pea weevils collected from the same source * were exposed to a dosage of 0.2 gram each of the following insecti- cides for five minutes: (1) A dust mixture containing 5 percent of DDT, with pyrophyllite as the diluent. (2) A dust mixture containing a combination of 5 percent of DDT plus 0.25 percent of rotenone. (3) A dust mixture containing 0.25 percent of rotenone. After treat- ment, one series of weevils was held for 10 days exposed to each insecticide at a constant temperature of 86° F. and the other series at a temperature of 70° F. The humidity in all series was held con- stant at approximately 65 percent. The results of this test are summarized in table 2. These data indicate that the dust mixture containing 5 percent of DDT, whether used alone or in combination with rotenone, was more effective in killing the pea weevil at 70° F. than at 86° F. When used alone, the kill was approximately 60 percent greater at 70° F. than at 86° F. In combination with 0.25 percent of rotenone, the kill was approxi- mately 22 percent greater at 70 0 than at 86 0 F . The reason for the apparently increased toxicity of DDT to the pea weevil at the lower temperature has not been determined, and this point needs further investigation before definite conclusions can be drawn. This experiment also indicated that the kill obtained with a combination containing rotenone was increased slightly by the addi- tion of DDT at the lower temperature level but the reverse was true at the higher temperature tested. Experiment III. This series of tests was made to determine If the length of exposure to DDT would increase the kill obtained. The weevils were exposed to a uniform dosage of 0.3 grams of a 5 percent mixture of DDT and pyrophyllite in one series and to a 10 percent mixture of DDT in another series. After exposure the weevils were held in the chamber in the exposure cages from 5 to kO minutes before they were transferred to clean cages. The results of this experiment are given in table 3» A study of these data shows an increase in effectiveness with an increase in the length of exposure to the dust. In the case of the 5 percent mixture of DDT and pyrophyllite, the kill increased from 30.67 percent after 5 minutes exposure, to 63. 01 percent after kO minutes exposure.- With the 10 percent dilution, the kill increased - 3 - from ^9.60 to 78.OO percent. The difference in kill between con- secutive periods of exposure was not statistically significant, but between 5 and 20 minutes, 10 and kO minutes, and 5 and hO minutes, the differences were highly significant. Field Experiments The material used in the field plots was a 10 percent dust mix- ture of DDT in pyrophyllite supplied by the Bureau of Entomology and Plant Quarantine from material purchased in 19hh. No laboratory ex- periments have been made with this lot of material. The dust mixture used in these tests was prepared by diluting the 10 percent DDT dust mixture with pyrophyllite to contain 5 percent of DDT. The cube duet mixture used was a commercial brand labeled to contain 0.75 percent of rotenone. 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Check Living Dead Living Dead Living Dead Living Dead Living Dead Living Dead Living Dead Living Dead Living Dead 35 39 29 10 8 21 1*9 25 28 1 2k 18 3k 23 31 16 27 20 26 33 10 2k 17 39 31 3k 31 17 16 19 30 3 3 18 kQ kQ 11 23 Ik 38 26 36 23 7 12 27 k3 38 k& k9 k6 2 2k 3k 16 kO 10 32 19 3 kk 6 kk 13 37 1 50 9 kl k9 1 28 18 41 7 23 27 19 31 23 27 18 32 2 kS k k6 kO 10 165 73 183 60 lk3 109 91 155 125 123 67 183 51 198 55 195 232 19 238 2k3 252 2k6 2kQ 250 2^9 250 251 30.7 21*. 7 1*3.3 63.O 1*9.6 73.2 79.5 78.O 7.6 * Insects allowed to remain after dusting in the chamber for different periods of time before being transfearred to observation cages. Pyrophyllite was used as diluent. - 7 - Each of the plots used in this experiment were 1.10 of an acre in size. They were arranged singly along the edges of 6 pea fields in order to insure the presence of a high pea weevil population and to benefit from a minimum of interplot movement of the weevils. Each of the insecticides was applied with a power duster to 21 of these plots. The treatment order was according to a randomized block plan. Two 25 -sweep collections with an insect net were taken on each plot immediately before dusting and at 2l»-and k8 -hour intervals after dust- ing. The results of this experiment, as given in table U, indicate that on the basis of reduction of pea weevil populations there was very little difference in the degree of control obtained with a dust mixture containing 5 percent of HOT applied at a rate of 30 pounds per acre as compared with a duat mixture containing 0.75 percent of rotenone applied at a rate of 20 pounds per acre. At harvest, during the dry pea stage, an examination of samples of peas from 7 comparable plots where each of the insecticides had been applied indicated that, Judging from the samples examined, 1.8 percent of the peas were Infested by the pea weevil in the plots re- ceiving the DET insecticides while 2.7 percent were infested in the plots where the rotenone containing insecticides were used. Two samples of 1000 peas each were examined from each plot. Table 4. Results of dust applications on pea weevil populations in l/lO acre plots on edges of pea fields, June 18 to 29, 19kk , temperature 70°-8l°F. Insecticide Total weevils collection in 21 plots Reduction in population of weevil adults Number Percent Cube (0.75 percent rotenone) Before dusting After 2k hours After kQ hours 113^ 16 2k 98.8 98.2 Dnr (5 percent) Before dusting After 2k hours After kQ hours 1176 20 9 98.3 99.2 - 8 - Tolerance of Pea Plants of DDT Technique: Experiments have been made to determine whether pea plants might be affected by applications of DDT. Germination of seeds and growth of plants were recorded when the seeds were rolled in DDT prior to planting and when the DDT was placed in the seed furrow at planting time. Plants dusted with DDT were also tested as to growth and observations made for injury. Alaska peas were used in an experiment with six randomized blocks using plots consisting of 10 feet of row. The data on this experiment have been analyzed statistically and are summarized and presented in tables 5 to 8 inclusive. In these tables the treatments are recorded by symbols as indicated in the following list: Treatments : A. Seeds rolled in 10 percent DDT. B. DDT (10 percent) sprinkled in the furrow when planted at the rate of 1 ounce to 10 feet of row. C* DDT (5 percent) dusted on foliage at the rate of 80 pounds per acre. D. * DDT (5 percent) dusted on foliage at the rate of kO pounds per acre. E. Check, untreated. * Dusting was done under a canvas cover to prevent any drifting of the insecticide to other plots of the experiment. Table 5« Number of plants growing from 120 pea seeds in 19hk. Treatment Blocks Treatment totals 1 J § i 3 5 A 98 107 118 107 110 5^0 B 106 104 97 111 112 530 C 110 9^ 108 115 112 539 D 115 111 100 109 107 5^2 E 112 107 112 105 106 5^2 - 9 - The germination of the peas in the different treatments of the DDT experiment is recorded in table 5» The number of seeds planted was 120 in each plot. The treatment had no effect on the germinationl Table 6. Numbers of pea seed harvested from plots receiving different treatments with DDT in 19^. Treatments Blocks Treatment totals 1 1 2 1 3 | k 1 ^ 6 A 713 827 99h 951 1025 1036 55^6 B 990 607 883 865 1303 888 5536 C 753 972 12^0 723 1U92 1070 6250 D 1210 1207 1065 900 1030 9^5 6357 E 887 792 973 825 13^0 1087 5904 The number of pea seeds harvested from the treatment plots of the experiment with DDT, as shown in table 6, did not vary much with the treatments. The two treatments (A and B) where DDT was placed below the surface of the soil had slightly fewer peas, and the ones with DDT on the foliage (C and D) had slightly more than the untreat- ed check plots. The difference is small and can be attributed to chance. Table 7. Weight of pea seed harvested from plots redeiving different treatments with DDT in 19kk. Treatment Blocks Treatment Totals 1 l * I ? 1 k I * 1 ° Grams Grams Grams Grams Grams Grams A 169 118 210 202 216 215 1200 B 211 133 185 180 267 192 1168 C 169 218 26k 156 312 230 13^9 D 25^ 259 233 195 225 1360 S 195 168 215 175 285 22k 1262 \ UNIVERSITY OF FLORIDA 3 1262 09238 7421 The weights of pea seed harvested from the plots in the experi- ment on DDT are given in table f . The differences are small and may be chance variations. Those plots where the DDT was placed below the soil surface have a slightly lower yield than the checks, but the difference was not significant. Table 8. Weight of pea straw harvested from plots receiving different treatments with DDT in 19kk, Treatment Blocks Treatment Totals 1 2 .. 3 i 1 5 1 ^ Grams Grams Grams Grams Grams Grama Grams A 237 253 284 236 296 263 1570 B 263 229 225 253 315 255 l$kO C 281 26k 303 266 330 296 17^0 D 297 288 336 287 283 275 1766 E 236 265 306 261 313 256 1637 There is comparatively little difference between the yields of straw as recorded in table 8. Here, too, the yield was smaller on plots where DDT was placed below the soil surface and larger where it was dusted on the foliage. There was no difference in the pea plants that can be associated with the DDT treatments. The variation between blocks indicated differences of location which account for the differences between treatment yields. Conclusions Contrary to previously published information, these experiments indicate that DDT promises to be an effective insecticide for pea weevil control. Applied to the vines and in the soil in the seed furrow, it was not toxic to the pea plant. In laboratory tests, DDT showed a tendency to have greater toxicity to the pea weevil at 70° F. than at 86° F., a feature that will add to the effectiveness of this insecticide if the finding is substantiated in future field tests. Two important features remain to be determined concerning this insecticide. These are the duration of its effective period on the pea vines, and whether the material has any repellent effect on the pea weevil.