January 1%6 E-740 United States Department of Agriculture Agricultural Research Administration Bureau of Entomology and Plant Quarantine WITH DDT FOR THE CONTROL OF THE PEA WEEVIL— PROGRESS REPORT By Ralph Schopp, Frank G. Hinman, and T. A. Brindley, Division of Truck Crop and Garden Insect Investigations^' Since 1937 rotenone-containing insecticides have been used and considered satisfactory for the control of the pea weevil (Bruohus pisorum L.). The wartime shortage of rotenone-containing materials made it necessary to search for a substitute for this material. Ac- cordingly an extensive program for testing insecticides was begun in 19U2 at Moscow, Idaho. Among various materials tested during 19U2 was a small sample of a dust mixture containing 2 percent of DDT. When tested in the laboratory, it appeared to have little or no affect on the weevils (l). In 19U3 a second sample of a dust mixture containing 10 percent of DDT was obtained and tested in the laboratory without further dilution. This material, applied at a heavier dosage, killed some weevils, although it did not compare favorably with rotenone. The lack of a promising insecticide other than rotenone for pea weevil control and favorable reports on the use of DDT on other insects led to its being included in field-plot tests* during 19M|. Favorable results in these preliminary field experiments led to more extensive trials in 19^-5 cuid 19^6 to compare this material with rotenone-dust ma- terials then in use for pea weevil control and to compare the effect of different concentrations of DDT and different rates of application. Comparisons were made in small-plot and in field experiments. Small -Plot Experiments Techni que— Throughout this series of small-plot experiments the randomized-block arrangement was used, except in a few instances as described. The plots were 1/10 acre, and were made 25 feet wide to permit them to be dusted with one swath of the power dusting machine. They were arranged along the edges of pea fields where" the weevils were usually concentrated. AH the plots of each block were grouped on the same field and treated consecutively. Since time is an important factor both in dusting and in population sampling, the blocks were treated as units. l/ In cooperation with the Agricultural Experiment Stations of Idaho and Washington. T* - 2 - The weevil population was sampled by use of a 15-inch insect- collecting net. Two 25-sweep collections were made on each plot. Each plot was swept before being dusted and on the first and second day- after being dusted when weather conditions permitted. When possible during 19^6 sweeping was continued over a longer period to gather data on the residual effects of DDT as compared with rotenone* A sample of 2000 dry peas was collected from each plot at harvest- time and examined to determine weevil infestation, 19hh Experiments — Five percent of DDT in pyrophyllite at 30 pounds per acre was compared in 21 replicates with cube dust containing 0,75 percent of rotenone in talc at 20 pounds per acre* Rotenone-dust mix- tures reduced weevil populations by 99 percent for the first 2 days after dusting as determined by sweeping. The DDT dust reduced the weevil populations by 98 percent 1 day after dusting and by 99 percent after 2 days. These results left little doubt as to the effectiveness of DDT, but the control with both insecticides was so high that the rela- tive value of the two could not be determined* 19^5 Experiments — Further experiments were made with dusts con- taining"!? ."5T3T""a n ~d~T0 percent of DDT in pyrophyllite, A dust containing 0*75 percent of rotenone in talc was included for comparison* All the materials were applied at the rate of 20 pounds per acre in each of 27 replicates* The weevil population was lowest the seoond day after application when the reduction was 99 percent for each of the DDT treatments and 98 percent for the rotenone treatment, the differenoe not being of mathematical significance. At harvesttime, however, the weevil infes- tations in the dry peas from the DDT treatments were lower than in those from the rotenone treatments* The percentages of peas infested were 9.2, 9.2, 10.1, and 13.5 for the 10-, 5-> and 2.5-percent DDT, and the 0«75-P©rcent rotenone treatments, respectively* Field tests were also made on large areas in 19U5* Four fields at Moscow, Idaho, were each divided into two parts* One part of each field was treated with the 5-percent DDT and the other with rotenone dust* In 20 locations on each treated area two sets of 25 sweeps were taken with a collecting net before dusting and 1 or 2 days after dusting to deter- mine the weevil populations. Samples of 2000 dry peas were collected at harvesttime from each of the areas previously swept* In these fields no weevils were found in any of the treated areas 2 days after application, but at harvesttime the weevil infestation in the dry peas was consistently lower in the DDT-treated areas. After the rotenone treatment 21 percent of the dry peas were infested, #iereas only 5»6 percent were infested after the DDT treatment, as shown in figure 1. - 3 - At Toppenish, Wash., airplene dusting on two fields was compared with ground -machine dusting on two other fields. The fields were divided into two parts, one part being treated with the 5~P e rcent DDT dust at about 50 pounds per acre and the other with rotenone dust at 25 to 35 pounds per acre. Two 25-sweep collections were made at each of 10 loca- tions in each division of each field before dusting and 1 day and 2 days after dusting. There were no appreciable differences in the effective- ness of the two kinds of treatments. In these collections a total of 689 weevils were netted before the application, 10 weevils 1 day after, and 1+ weevils 2 days after the application. 19i (6 Experiments — Experiments were planned to determine the minimum concentrations of DDT that would control the pest, the length of time DDT would remain effective under field conditions, and the most effective rate of application. Dusts containing 1 percent of rotenone were in- cluded for comparison with DDT. The first experiment was made on two fields early in the season while the weevils were still flying into the fields. It consisted of ill. replicates of h treatments— 5 percent of DDT in pyrophyllite at 10, 20, and Z4O pounds per acre, and 1 percent of rotenone in talc at 20 pounds per acre. Sweeping was continued to the sixth day after dusting to collect data on the residual effects of the treatment. The results, as shown in figure 2, were practically the same for the 20- and l+O-pound rates of application of DDT, Each of these treatments was superior to the 20 pounds of 1-percent rotenone dust for the first 6 days after application. The 10-pound application of DDT was also more effective than the rotenone after the first day. The light application of DDT was slower in action, but eventually was almost as effective as the heavier applications. Weevils had been*flying into the field throughout the 6-day period; therefore these results are a measure of the residual effect of the treat- ments. It is evident that under the conditions of this experiment, the residual effect of the DDT treatments exceeded that of rotenone, the effect of which had decreased considerably by the fourth day and still more by the sixth day. There were indications that the DDT treatments were also losing their effectiveness on the sixth day, but increases in weevil populations were still too small to be considered significant. In the second experiment five more replicates of the three dosages of 5-percent DDT and of rotenone dust were applied on a third field later in the season. Rains prevented sweeping on this field, except on the first day after dusting. The results obtained in the entire series of 19 replicates are presented in table 1. Tlie weevil infestation in the dry peas was much higher in field 3 than in the other two fields, and higher in the rotenone plots than in the DDT plots. The 10-pound application was almost as effective as the IiO-pound application, although it did not reduce the adult-weevil population as rapidly. - u- Table 1. Control of the pea weevil -with DDT applications at different rates as compared with control with rotenone, in 19^46 Insecticide DDT, 5 percent Rate per acre Pounds ko 20 10 _ . ■ . - , Weevily peas at harvest » Reduction of ♦ J r adult weevils' Fie ids 1 in 1 day ' ^ 2 Field 3 Percent 96 9U 89 Percent Percent 0.5 9 .7 -11 .5 13 Rotenone, 1 percent 20 90 1.3 17 Uniform rates of application of DDT dusts . — Dust mixtures con- taining 5» 2,5* and 1.25 percent of DDT in pyrophyiiite were tested, together with a dust containing 1 percent of rotenone in talc. These materials were applied on three fields in 22 randomized blocks at 20 pounds per acre. Rain fell immediately after one field of seven replicates had been dusted. That 16 to 28 percent of the weevils were surviving U days later shows that neither DDT nor rotenone can be relied upon to control the pea weevil if applied just before a rain. The f§sld was dusted with 1-percent rotenone k days after the experimental dusting, and weevily peas at harvest ranged from l+.l to 7*3 percent in the DDT plots and averaged 5*4 percent in the rotenone plots, the differences being within the limits of experimental error. The control obtained in this field is attributed to the redusting with rotenone. In the other two fields containing 15 replicates the weevil popu- lations were reduced by 93 and 97 percent in 1 day, and the weevily peas at harvesttime ranged from 12 to 16 percent, there being no appre- ciable differences in the effectiveness of the L\. treatments. Rains prevented sweeping on one of these fields after the first day, and it was possible to study the weevil population in only five replicates, all in one field, over an extended time. As shown in figure 2, throughout a period of 11 days there were fewer adult weevils in the plots dusted with 5 and 2.5 percent DDT than in those dusted with rotenone. There were as many weevils in the 1.25 percent DDT plots as in the rotenone plots on the first and third days after dusting but not during the remainder of the period. The 1.25 percent DDT dust was less effective than the higher strengths. - 5 - Field Experiments in 1%6 In the experiments on larger field areas DDT, 5 percent in talc, and rotenone, 1 percent in talc, were dusted on adjoining strips about 1,600 feet long and from 25 to UO feet wide on five fields. The rest of each field was dusted with either rotenone or DDT, At harvest 20 samples of 1000 peas each were examined from each strip. As in 19^+b* in each field the percentage of infested peas was lower after the DDT treatment than after the rotenone treatment (fig, 1), The average infestation was 1,5 and 2,5 percent, respectively, for the DDT and rotenone treatments, the difference being of mathematical significance* Cage Experiments in 19<^6 A part of a pea field was selected on which no weevils could be found by sweeping, A small part of this area was dusted with 1 percent of rotenone in talc and a similar adjacent area was dusted with 5 percent of DDT in pyrophyllite to compare residual effects of DDT and of rotenone. Both materials were applied with a 25-foot power duster at approximately 20 pounds per acre. One day later two screen cages, 3 feet high and covering 12 square feet, were set out on each area, and 25 weevils swept up from an undusted field were introduced into each cage. Four more cages were set out in the same manner on the second, third, fourth, and sixth days after dusting. No cages were set out on the fifth day because 0,16 inch of rain fell. The weevils were put in the cages as soon as possible after they were swept from the peas. The cages were set out in a predetermined randomized plan. Maximum temperatures while the cages were being set ranged between 68* and 80" F, Maximum temperatures for the following 3 days were between 83* and 86" F, At harvesttime all peas under each cage were collected separately and examined for weevil infestation. At the same time the peas were also harvested from the dusted area around them. The results of this experiment are presented in figure 3» From 3 to 15 times as many infested peas were found under the cages over the rotenone-dusted peas as were found under those dusted with DDT, The large increase in infested peas on the plots caged on the day after the rain may indicate that some of the dust had washed off. The data indicate that DDT may have retained more of its toxicity to the weevils than rotenone, even after 6 days and after a rainstorm* Survey of Fields Dusted Commercially in 19^(6 A survey was made of the Palouse area of Washington and Idaho in 19l»6 to determine the success of commercial dusting with DDT for weevil control as compared with rotenone. Information was obtained on 50 fields dusted with DDT, Each field was paired with the nearest field - 6 - dusted with rotenone on which data were available. In most of the fields in the DDT group a dust mixture containing 5 percent of DDT was applied* The fields of the rotenone group were about equally di- vided, a 0*75 percent dust being applied on one half and a 1*0 percent dust on the other. The rates of application ranged from approximately 7 to 20 pounds per acre, but usually from 10 to 15 pounds per acre* All insecticides containing DDT were applied at about the same rate per acre* Talc was the diluent for all commercial dusts used in the area* Infestation records of "the peas from the fields were obtained from the Federal pea-grading service of the Production and Marketing Administration. The DDT-treated fields had the smaller weevil infestations in kl of the 50 pairs. In the paired fields the 50 dusted with DDT had an average infestation of 2.3 percent while the 50 dusted with rotenone had an infestation of 5.5 percent. This difference in control was of high mathematical significance. Similar differences were obtained in 86 unpaired fields that were sampled. The data leave no doubt that the growers obtained better control with DDT than with rotenone dusts. Rec ommends t i ons On the basis of these experiments the following recommendations are made: The use of 15 to 20 pounds of dust containing 5 percent of DDT per acre applied in the same manner and under the same conditions as rotenone i6 recommended, except where pea ensilage or pea hay is to be fed to dairy cattle. In such cases rotenone-containing dusts should be apolifd as formerly recommended. Summary Results of field experiments from 19^ to 19^6 have shown that dusting with DDT is an effective control for the pea weevil (Bruchus pisorum L . )» Five-percent DDT dust was nearly as effective when applied at 10 pounds per acre as at 20 and J+0 pounds per acre but slightly slower acting. Applied at a uniform rate of 20 pounds per acre, a dust contain- ing 2.5 percent of DDT was superior to a dust containing 1.25 percent and as good as one containing 5 percent. Both DDT and rotenone dusts had lost much of their insecticidal effect after k days. Weevil populations were always lower on the second day after dust- ing with DDT than on the first day. - 7 - DDT, 5 percent, applied at the rate of 10 pounds per acre was more effective than rotenone, 1 percent, applied at 20 pounds per acre. DDT, 1.25 percent, applied at the rate of 20 pounds per acre, was more effective than rotenone, 1 percent, applied at 20 pounds per acre, except when applied just before a rain. Literature Cited (l) Schopp, Ralph, and Brindley, T. A. 194/.. Tests with DDT against the pea weevil. Jour. Econ. Ent. 37; 150-151. - 8 - 1945 ROTENONE 0,75% DOT 5% 1946 ROTENONE 1.0% DDT 5% B 25 S7T PERCENT INFESTED PEAS Figure 1. — Pea weevil infestations in dry peas from paired fields and areas dusted with DDT and with rotenone. , z , 75 - 70 ROTENONE 20 LBS/ACRE DOT 40 LBS/ACRE > DDT 20 LBS/ACRE DDT 10 LBS/ACRE 2 3 4 5 6 DAYS AFTER DUSTING 5 6 7 8 9 DAYS AFTER DUSTING Figure 2. — Reduction in pea weevil populations on l/lO-acre plots after ap- plications of DDT and rotenone dusts. - 9 - 1000 CO < UJ 900 CL •00 - ^ TOO » UJ UJ too 400 300 ■ 200 100 — .<. 12 3 4 5