December 1948 E-765 QTatp ^ United States Department of Agriculture OlATE PLANT BOARD Agricultural Research Administration Bureau of Entomology and Plant Quarantine STUDIES WITH DDT AS A CONTROL / FOR WIREWORMS IN IRRIGATED LANDS— PROGRESS REPORT^ By M. C. Lane, M. W. Stone, H. P. Lanchester, E. w. Jones, and K. E. Gibson, Division of Truck Crop and Garden Insect Investigations During the course of investigations by the Walla Walla, Wash., and Ventura, Calif., laboratories on the control of the Pacific Coast wire- worm (Limonius canus Lee.) and the sugar-beet wireworm (L. californicus Mann.) in irrigated lands, DDT was found to be toxic to these wireworms and to offer a promising means of control to supplement soil fumigation and cropping practices. These two species have high moisture require- ments; therefore their reaction to DDT may be different from that of other species under other environmental conditions. The studies with DDT for wireworm control were begun in 19 A3 and are being continued. The purpose of this paper is to summarize the progress made during the last 5 years. For convenience the results obtained by the two laboratories are given in separate sections. Some of the results of these experiments have been summarized briefly by White (£). Experiments on the use of DDT for the control of wirerorms in the Eastern States have been reported by Greenwood (2) , Kulash (_2) , and Pepper et al. (^) . Wilson and Choudhri (6) have found that soil organisms and the accumulations of nitrate in the soil are not seriously modified by applications of 0.5 gram of DDT to 125 grams of soil containing 25 grams of moisture. Young and Gill (7j tested the tolerance of rye, tomato, strawberry, sweet pepper, collards, eggplant, and onion to DDT that had been applied to the top 3 inches of field plots in Tifton fine sandy loam soil. Rye, tomato, and strawberry were the most susceptible to the DDT, and were in- jured by 10, 25, and 100 pounds of DDT per acre, respectively. Fleming (1) reported that 25 pounds of DDT per acre worked into the top 3 or U inches of soil did not injure a large number of plant species, including rye. He also observed that the growth of a limited number of plants of strawberry, tomato, onion, soybean, lima bean, bush bean, and spinach was retarded by this dosage. The soil type in which each crop was grown was not specified. l/ The experiments by the Walla Walla laboratory are presented by M. C. Lane, H. P. Lanchester, E. W. Jones, and K. E. Gibson. Some of these ex- periments were conducted at Prosser, Wash., in cooperation with the Washing- ton Agricultural Experiment Station. The experiments at Ventura are pre- sented by Id. W. Stone. - 2 - Effect of DDT on Tireworms in the Eastern Washington Area In preliminary laboratory tests at Walla Walla late in 1943, rela- tively large dosages of technical DDT were mixed with soil. The action of the DDT on wireworms was slow but very effective. Apparently the DDT changed the moisture content of the wireworms, because those that vrere affected began to shrivel and then dried up. With smaller dosages of DDT it took longer to kill the wireworms, but these dosages vrere promising for practical control because of the long time a wireworm lives. Laboratory tests with dusts showed that DDT, either alone or when mixed with soil, was not sufficiently repellent to wireworms to keep them from crawling through the dusts. The DDT was cumulative in its action on wireworms and later work showed that intermittent exposures to DDT in the soil, 1 to 16 weeks apart, gave the same percentage of kill as an equal amount of continuous exposure. Early in the season of 1944 several outdoor soil cages (4 by 5 by 1^ feet with cement bottoms) were prepared with dosages of 16, 32, 48, and 320 pounds per acre of technical DDT mixed with Walla Walla silt loam soil in a cement mixer. The DDT was applied as a 10-percent DDT- pyrophyllite dust. Six inches of untreated soil was placed in the bottom of each cage and 9 inches of treated soil on top, Wireworms in untreated soil were caged in small screen containers, and the containers were then placed in the treated soil of the large cages. None of the wireworms were affected in any way, which shows that there was no fumigating effect from the DDT. At intervals composite samples of soil taken from these treated cages were put with wireworms in salve tins. Four years after treatment the 520-pound dosage of DDT killed all wireworms in 5 weeks, the 48-pound dosage killed 98 percent, the 32-pound dosage 61 percent, and the 16- pound dosage 37 percent. These results show that the outdoor weathering and the action of the soil for 4 years did not completely decompose the DDT. It should be emphasized, however, that some of these dosages vrere excessive, that the mortality tests were made at a constant temperature near the optimum for wireworm activity, and that the DDT was more thor- oughly mixed with the soil than would be possible under field conditions. Seeds of barley, wheat, beets, peas, beans, onions, carrots, spinach, cucumbers, and sweet corn, and potato seed-pieces and tomato plants were planted in the outdoor cages. Seedling counts in the soil treated with the 320-pound dosage showed a retardation of germination, and most of the crops were more or less affected by the DDT. Peas, beans, and barley were badly dwarfed. The growth of corn and wheat was definitely affected. Potatoes were the only crop apparently not damaged by this excessive dosage Germination and growth of all crops seemed to be normal in soil treated with the lower dosages. A year later, in 1945, barley, peas, corn and squash were planted in the cages containing soil treated at the 320-pound dosage, with about the same results. The peas and the squash were badly dwarfed and the barley and the corn were affected. Peas were planted in 1946 and 1947 and were again dwarfed by the 320-pound dosage, although less than in previous years. - 3 - In the spring of 1945 a series of field plots in Walla Walla silt loam soil were prepared with replioated dosages of 10, 20, and £.0 pounds per acre of technical DDT. The DDT was applied in the form of a 10-percent dust in pyrophyllite. One-half the dosage was broadcast on the surface of the soil, mixed by disking, and then plowed 9 inches deep. The other half was broadcast and disk-harrowed to a depth of 6 inches. The ten vegetable crops — lettuce, peas, beans, carrots, beets, spinach, tomatoes, cabbage, onions, and potatoes — were growi in each plot, the tomatoes, cabbage, and onions being grown from transplants. The growth and yield of these crops were not affected by the DDT. The reduction in wireworms at the end of the season was 80, 96, and 97 percent in the plots treated with 10, 20, and 40 pounds of DDT, respectively. The total potato damage at harvest was 35 percent in untreated plots as compared with 10, 4, and 3 percent in the plots treated with 10, 20, and 40 pounds of DDT. These figures are statistically significant and show a much re- duced population of wireworms and less damage to tubers 6 months after treatment. There was still further reduction in population and damage during 1946 in these same plots. Chemical tests to determine whether DDT was absorbed into the plant tissues gave negative results in all cases. In 1946 an experiment similar to the above and located adjacent to it was begun with dosages of 5, 10, and 20 pounds of DDT per acre. An identical experiment was begun in the same field in 1947. The 20-pound dosage was the only one that gave more than 50 percent reduction of wire- worms (exclusive of new brood) in 6 months. After 18 months, however, in the 1946 experiment, 42 wireworms were found in 40 quarter-square foot samples from the untreated plots as compared with 18, 16, and 0 in the plots treated with the 5-j 10^ and 20-pound dosages of DDT, respectively. The yields of potatoes in 1947, as given in table 1, show that although 20 pounds of DDT per acre was required to appreciably reduce wireworm damage (sufficient to throw tubers out of grade) the first season after treatment, the 10-pound dosage was effective for the second and third season after treatment. These results are in agreement with experiments in commercial fields and, as indicated by the toxicity data obtained in the laboratory, are due to the slow kill of the larger wireworms and the outstanding ef- fectiveness of the treatments in killing new brood. During 1945 considerable work was done with testing the effects of DDT on the adult and egg stages of wireworms. DDT was found to have no effect on the eggs and pupae in soil. The adult beetles reacted -rather slowly to DDT in the soil. DDT applied after formation of pupal cells by the full-grown largae did not significantly affect the production of adults. Tests with DDT dusts on the soil surface in the spring did not stop the female adults from laying eggs in the treated plots. Egg pro- duction might be somewhat reduced in adults that contact sufficient amounts of DDT on the surface, but it appears that rather large qiianti- ties would be required. It seemed more practical to mix the DDT with the soil and let it kill the much more sensitive newly hatched wireworms. Laboratory tests with DDT against the newly hatched wireworms showed that more than 80 percent were killed in a week's exposure to soil treated with 5 pounds of DDT per acre, and that 100 percent were killed at dosages of 10 pounds or more per acre. Soil samples taken in July from each of the field plots, and washed through screens to obtain a count of the newly - 4 - hatched wireworms of that season's brood, showed no survival of new-brood wireworms in any of the plots treated with 10 pounds of DDT or more per acre. During the season of 1946 more than 100 field tests were made with DDT dust formulations to find the most practical dosage to use against wireworms in the soil. In most cases a 50-percent DDT dust was considered the cheapest. The dust was applied to the surface by hand or with a ferti- lizer spreader and then plowed and harrowed to give as thorough a mix as possible within the plow depth of soil (6 to 9 inches), ViTireworm popula- tions were determined by baiting and by sifting samples of soil from treated and untreated portions of the same field. Reductions of 50 to 75 percent in the wireworm infestation were obtained with the 10-, 15-, and 20-pound dosages of DDT, with no significant differences between the dosages. The 5-pound dosage was inferior. Except for the 5-pound dosage, almost no new-brood wireworms survived. There were no appreciable differences either in yields or in the growth of the potato plants. The 10- to 20-pound dos- ages, however, reduced wireworm damage to the tubers 50 to 60 percent. During the 1947 season 58 field tests with DDT in sandy loam and silt loam soils gave results consistent with those obtained in 1946. New-brood mortality was 100 percent in the soils treated with 10 and 20 pounds of DDT per acre and 66 percent with the 5-pound treatment. Some differences were noted in the action of DDT in soils of different types. A slightly better and more consistent reduction in wireworms was noticed in the sandier types of soil than in loam types containing more clay. Laboratory tests on the relative susceptibility of sugar-beet and Pacific Coast wireworms to DDT failed to show differences. 2/ The Effect of DDT on lareworms in the Ventura, Calif., Area Tests to determine the effect of DDT on wireworms have been in pro- gress at the Ventura laboratory since 1944. In preliminary laboratory- tests wireworms were introduced into 2.7 cubic feet of Yolo fine sandy loam soil in 25-gallon containers. The soil had been treated with DDT at the rate of 15, 30, or 60 pounds per acre. Y/hen the scil was sifted 2 months later, 60, 93, and 98 percent of the wireworms found were dead, as compared with 4 percent in untreated soil. After 3 months very few wireworms remained alive in the treated soils. In field tests in Larch 1945 a 10-percent DDT dust in pyrophyllite was applied on the soil surface of l/8-acre plots in Yolo fine sandy loam at the rates of 25, 50, and 100 pounds of technical DDT per acre, in two 2/ The field tests reported were made possible through the kindness and cooperation of Messrs. Frank, Ralph, and Robert Borchard, who also threshed the beans from the individual plots in 1946 to 1947. H. Bruns, I.'. Vujovich, R. Bard, and 0. Kitchen assisted in the tests. G. Claypool, J. Pinkerton, S. Stuart, R. Hudson, and E. LEerritt were especially help- ful in obtaining data on wireworm populations in the field plots. - 5 - randomized blocks. The DDT was incorporated with the soil by disking and cross-disking to a depth of 8 inches and then dragging to pack the soil. The dragging reduced the loss of moisture from the soil and provided an optimum soil condition near the surface for the wire-worms to move around in. Lima beans were planted in these plots on lay 15, or 6 weeks after the treatments had been applied. The soil from each plot was sampled from time to time for wireworms by sifting 20 quarter-square-foot samples of soil to a depth of 16 inches. The plots treated with 25 pounds of DDT had, since 1940, been used for cover-crop experiments, and there had been an average of 3 wireworms per square foot during the spring of the 4 years. Prior to treatment in 1945 there were 2.3 -wireworms per square foot. Vfith- in 5-g- months the infestation dropped to 0.3 wireworm per square foot, and after 1 year no wireworms could be found. In untreated plots the infesta- tion from 1940 to 1945 had averaged 1.5 wireworms per square foot, the lower population being due to the lack of winter cover crops, and in the spring of 1946 there were 1.3 wireworms per square foot. The check plots had yielded more lima beans than the other plots, but in 1945 the yields of dry lima beans were 14.0 and 16,7 sacks per acre for the check and treated plots, respectively. The results with the 50- and 100-pound treat- ments were similar to those with the 25-pound treatment. In the laboratory wireworms were confined in samples of the upper 6 inches of soil from the 25-pound plots taken at random 5 months after treatment. All the "wireworms were dead in 1 month, wireworms placed in similar samples taken 1 year after the treatment died in 3 months. Nine- teen months after treatment a 5-month exposure caused 96 percent mortality. Mortalities in untreated soil in 3 months ranged from 6 to 14 percent. The pli of the soil averaged 7.9, In 1946 two experiments were conducted in one field of Yolo sandy to fine sandy loam. The dosages used were 5, 10, and 20 pounds of technical DDT per acre, the 5-pound dosage being omitted in the second experiment. The plots were 0.4 acre in size, and "were replicated four times in each experiment in randomized blocks. The pH of the soil averaged 7.8. The DDT was dusted on the soil surface and then incorporated vrith. the soil by disking and cross-disking to a depth of 8 inches. The soil surface -was then rolled -with a clod masher to reduce the loss of moisture in the soil. In one experiment DDT was applied 4 'weeks before irrigation and 8 -weeks before planting; in the other 2 weeks after irrigation and 2 weeks before planting. The wire-worm infestations and yields are given in table 2. The 5- pound dosage, although it did not give a high initial kill, greatly re- duced the wireworm population by the second year and continued to be effective into the third year. Both the 10- and 20-pound applications controlled the wireworms and gave outstanding increases in yield the first year after application. The wireworm infestation remained low the second year after treatment and the yields were good; but the infestation in the check plots also decreased the second year and these plots also produced good yields. -This decrease in infestation is not unusual, as studies of wireworm populations in experimental field plots over a period of years show that there is usually a decline in the number of wireworms the year following heavy infestations. The third year no wireworms were recovered - 6 in the soil samples taken, in plots treated with the 20-pound dosage. Only 9 wireworms were recovered in 64 4-inch by 4-inoh by 3-foot soil samples taken in the bean rows of the plots treated with the 10-pound dosage. There was no appreciable difference in the effectiveness of the 10- and 20-pound dosages of DDT or of the two times of application. There was a tendency, however, for the early 20-pound dosage to be the most effective. Samples of soil from the above experiments were taken from time to time and tested in the laboratory for toxicity to wireworms. These tests, which are summarized in table 3, show that the soil remained toxic to wireworms a year after treatment with DDT, but that it was not so toxic as it was after 6 months. In 1947 DDT was included in a randomized block experiment on Yolo very fine sandy loam. There were four replicate plots 0.5 acre in size for each treatment. The DDT was applied at the rate of 10 pounds per acre on Inarch 20 by dusting it onto a cover crop of sweetclover before it was plowed under preparatory to planting lima beans. The sweetclover was about 8 inches high. Lima beans were planted on Lay 10. The stand of lima bean plants in* the treated plots was about twice that in the untreated plots. The number of wireworms per foot of lima bean row was 0.3 in the treated plots as compared with 3.8 in the check plots, and the yield in the treated plots was 20.4 sacks per acre as compared with 11.6 sacks from the check plots. On Hay 20, 194~£ a tolerance experiment was initiated to determine whether DDT at 20 and 40 pounds per acre in Yolo fine sandy loam soil not infested with wireworms would affect the growth and yield of certain crops. The entire experimental area had been fumigated for wireworm and nematode control in February with 30 gallons per acre of a crude mixture of 1,2-dichloropropane and 1,3-dichloropropylene, and on I."ay 9 with 20 gallons per acre of 10-percent ethylene dibromide. The DDT was applied as a suspension spray to the soil surface and thoroughly worked into the soil to a depth of 6 to 8 inches on Lay 20. Each plot was 12-g- feet wide and 172 feet long and was planted with 10 crops between May 23 to 30. There were four replicate plots for each treatment. The yields of lima beans, potatoes, cabbage, and lettuce were significantly higher in the untreated plots than in the plots treated with 40 pounds per acre of DDT. Tomato growth appeared to be affected by this treatment, but the yields were within the limits of experimental error. A 20-pound dosage of DDT did not appear to affect the growth or yield of sweet potatoes, carrots, peppers, peas, and sugar beets, Winter cover crops of vetch, sweetclover, fenugreek, and peas planted in October 1947 were not injured by the DDT treatments. The results of this experiment illustrate the danger of using excessive dosages of DDT, and indicate the possibility of crop injury from accumulations of DDT in the soil when repeated applications of the insecti- cides are made to the soil and to crop foliage. Summary Laboratory tests on the toxicity of DDT to the Pacific Coast wire- worm (Limonius canus Lee.) and the sugar-beet wireworm (L, californicus Mann.) have been underway at Walla Walla, Wash., since 1943. Field - 7 - experiments on the control of these wireworms have been conducted by the Walla .Valla, Wash., and the Ventura, Calif., laboratories since 1944. L_j_ canus and L. californicus are the most common species of wireworms that attack crops' in western irrigated lands and have high moisture require- ments. The action of DDT on them is slow. After contact with the chemical the affected -wireworms live for weeks and appear to be desiccated slowly. Wireworms are not repelled by the chemical. Low concentrations of DDT in the soil are very toxic, and because of the apparent cumulative effect of the DDT, wireworms that repeatedly come in contact with the material in the soil die. Dosages of 10 to 20 pounds of DDT per acre remain toxic to wireworms the second season after application. DDT has no effect on eggs or pupae. Its action on adults is slow, as after exposure mating and egg laying take place before the adults die. The most sensitive stage of these insects is the first few weeks of their existence as young wireworms. Many field tests under a variety of soil conditions in the Columbia River area of eastern Washington and Oregon have shown that a dosage of 10 pounds of technical DDT per acre, either dusted or sprayed on the sur- face in the spring and thoroughly mixed with the soil to a depth of 6 to 9 inches, greatly reduces wireworm numbers. Even though this treatment has not protected crops planted immediately after treatment, it has re- duced the damage to late tuber and root crops for three seasons. No appreciable number of the new brood of wireworms survived for three sea- sons. Over a 4-year period the 10-pound dosage has not been detrimental to plants and soils when the DDT was thoroughly mixed with fine sandy loam, sandy loam, silt loam, or loam soils. In sandy loam to very fine sandy loam in Ventura County, Calif., the yield of dry lima beans was substantially increased when 10 to 20 pounds of DDT was applied per acre 3 to 6 weeks before planting. 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