August 1946 E-699 United States Department of Agriculture Agricultural Research Administration Bureau of Entomology and Plant Quarantine RESULTS OF FIELD EXPERIMENTS WITH DDT AGAINST INSECTS AFFECTING TOMATOES IN SOUTHERN CALIFORNIA By Joseph Wilcox and A. F. Howland, Division of Truck Crop and Garden Insect Investigations Tomato plants and fruits are attacked commonly in southern California by several species of insects and related pests. The more important are the tomato fruitworm (Heliothis armigera Hbn.), the tomato russet mite (Phyllocoptes destructor Keifer), the tomato hornworm (Protoparce quinquemaculata (Haw.)), the tobacco hornworm (P. sexta (Johan. )), the beet armyworm (Laphygma exigua (Hbn.)), and several species of cutworms which feed on the young plants and on the fruits during the fruiting stage, including the variegated cutworm (Peridroma margaritosa (Haw.)) and the granulate cutworm (Feltia subterranea (F.)). The tomato pinworm (Keiferia lycoper- sicella (Busck))> which was formerly a serious pest of tomatoes in southern California, was present in very small numbers in this locality during the 1944 and 1945 seasons. Tomato Fruitworm Tests Use of Hand Dusters. — Preliminary tests with dust mixtures con- taining DDT, calcium arsenate, or cryolite were made against the tomato fruitworm during 1944. The results of these tests were sum- marized by White l/. Additional tests were conducted during the 1945 season, and the results of the two seasons1 work are shown in table 1. The insecticides were applied with hand dusters to plots aver- aging about 0.035 acre in size (4 to 5 rows wide and 9 to 12 plants long). In 1944 the tests were made in triplicate in each of four fields, and in 1945 in quadruplicate in each of three fields. Three applications were made, the first when the fruit of the main crop was setting and the second and third applications at 2-week intervals thereafter. The dust was applied at the rate of 30 pounds per acre per application, making a total of 90 pounds per acre for the season. The data on fruit damage and control were obtained by picking and examining all tomatoes from 5 plants taken at random from the middle of each plot at the regular picking times throughout the season. 3/ White, W. H. A summary of the results of the work with DDT con- ducted by the Division of Truck Crop and Garden Insect Investigations during the season of 1944. U. S. 'Bur. Ent. and Plant Quar. B-642, 8 pp. 1945 [Jrocessed.j Table 1. — Results of insecticide applications with hand dusters for the control of the tomato fruitworm Insecticide 1944 1945 (Figures in percent) Damage Control Damage Control y Cryolite 70, sulfur 22 Percent 7.7 Percent 57 Percent 3.7 Percent 59 Calcium arsenate, undiluted 2/ DDT: 2.5 7.4 3.9 59 78 - 1.6 - 82 5 2.9 84 1.7 81 10 .9 95 .6 93 20 .3 97 DDT 4 fused with sulfur 85 1.1 88 DDT 10 fused with sulfur 85 .4 96 Check (no treatment) 17.9 9.1 1/ References to percentage of cryolite, unless stated otherwise, indicate the percentage of sodium fluo- aluminate. 2/ Mixed with pyrophyllite only in 1944; mixed with pyrophyllite and sulfur 25$ in 1945. -3- The results obtained in 1944 indicate that all DDT mixtures were superior to cryolite and calcium arsenate, and with DDT a pro- gressive increase in control was apparent as the strength was in- creased from 2.5 to 10 percent, with a decided improvement at the 10-percent strength. In 1945 the results were very similar, ex- cept that there was no difference between the 2.5- and 5-percent strengths of DDT. In 1945 tests were also conducted with a 20- percent DDT mixture, which gave only a slight (not significant) increase in control over a 10-percent mixture, and with two strengths of DDT fused with sulfur, which gave slightly but not significantly improved control over similar materials mixed with sulfur. In 1945 another series of experiments was conducted in three fields, each in quadruplicate. A mixture containing 10 percent of DDT, 25 percent of sulfur, and 65 percent of pyrophyllite was applied in each test at a total rate of 90 pounds per acre for the season, but at different intervals of application, i.e., 1, 2, and 3 weeks, and at two times of starting the dust applications , -when the fruit of the main crop was beginning to set, and 2 weeks later. Tomato fruit- worm infestation was low in all three fields; so the results cannot be considered as conclusive. The results indicate, however, that applications made at intervals of 1 and 2 weeks are superior to those made at 3-week intervals, and that applications started when the fruit of the main crop was setting are superior to those started 2 weeks later. In other words, even with a superior insecticide such as DDT, good results cannot be obtained unless the applications are started at the right time and repeated at regular intervals of not more than 2 weeks. Use of Power Dusters. — In other experiments the insecticides were applied with a 2-row power duster to plots in three fields. The plots were at least eight rows wide and varied in length, but averaged from 0.5 to 0.8 acre in size. The insecticides were rep- licated four to- eix times, and fruitworm damage and control were determined as in the hand-dusted plots. Three applications of insecticide were made at 2-week intervals, starting when the fruit of the main crop was beginning to set* In 1944 the experimental field contained jaix replicate plots * Calcium arsenate 70 percent plus sulfur 25 percent, applied at a total rate of 125 pounds per acre, gave 56 percent control; cryolite 70 percent plus sulfur 22 percent, applied at 123 pounds per acre, gave 70 percent control; and DDT 5 percent, diluted with pyrophyllite and applied at 131 pounds per acre, gave 92 percent control. The superior control given by the 5-percent DDT over that given by either of the other insecticides was highly significant. -4- In 1945 similar tests were conducted in two fields, each con- taining four replicate plots. The rates of application ranged from 101 to 125 pounds of the dust mixtures per acre for the three app- lications. The results are shown in table 2. A 4-percent DDT mix- ture was superior to the cryolite dust used, and 10-percent DDT was superior to the 4-percent mixture. DDT 4-percent fused with sulfur was used in one field, but the results are not shown in table 2. There was no significant difference between the two 4-percent DDT mixtures. Control of Cutworms, Hornworms, the Beet Armyworm, and the Tomato Russet Mite In experimental plots for the control of the tomato fruitworm, infestations of insects other than the fruitworm occurred, and records were kept of the fruit damage caused by these insects. As no separate experiments were conducted for the control of these insects, methods of checking results, rates of application, etc., were the same as previously described in the tomato fruitworm experiments • Cutworms . — These insects were important in one field during the 1944 season, and the results in triplicate plots are shown in table 3. DDT at 5 and 10 percent were both superior to DDT 2.5 percent, to cryolite 70 percent, and to undiluted calcium arsenate, the best control being obtained with the 10-percent DDT dust mixture. Hornworms . — Although hornworms are usually present in tomato fields in southern California, the percentage of fruit actually damaged by them is small, and the results given here are by no means conclusive, since they are based on experiments in fields -with low infestations. The results obtained in three fields, one in 1944 (triplicate, plots) and two in 1945 (8 replicated plots), are shown in table 3. With the materials used in both 1944 and 1945 the aver- age percentages of control were as follows: 70 percent cryolite, 48: 2.5 percent DDT, 66; 5 percent DDT, 72; and 10 percent DDT, 95. It therefore appears that, in order to obtain a satisfactory degree of control of hornworms with DDT, a 10-percent dust should be used. Beet Armyworm. — Thi 3 insect caused severe damage to the tomato fruits in several of the experimental fields during the 1945 season. The results obtained with power dusters are shown in table 2, and those with hand dusters in table 3. The best control was obtained with mixtures containing 10 percent of DDT and 25 percent or more of sulfur. It did not appear to make any appreciable difference whether the DDT was mixed with the sulfur or fused with it. -5- Table 2. — Results of insecticide applications with power dusters for the control of the tomato fruitworra and the beet armyworm. 1945 Insecticide Tomato fruitworm Beet armyworm (Figures in percent) Damage Control Damage Control Percent Percent Percent Percent Cryolite 67.5, sulfur 25 6.4 47 12.3 46 DDT 4, volcanic ash 46, sulfur 50 2.6 79 6.3 72 DDT 10, volcanic ash 40, sulfur 50 0.9 93 0.8 97 Check (no treatment in adjacent fields) 12.0 22.7 -6- cd vr\ H -4- o O rH -p c £3 o o r\ & CD -p g CO CD Q CD PCt rH O U +S B O o Os ~i CD CO O XJ u •H CD O P. •H -P O in 0) CO CO c © M •H &« -p c 0 CD CD O ^ CD (X, c CD O ^ CD (X, -P c CD c CD (X, -p c CD O Jh CD t? 0 O 0) cu -p c 0) o f-, CD •P •H rH O CD -P 0 CM 3 o rH sO vO {>- ON o to ON e o -3- to -4" to to CM vO o cm to to ON CD -P CO C CD CO u cd •riHl o cd o E-t CM UN 1 J 1 1 u-\ CM to ul u 01 rH 3 •> CO CD -P x: •H •p rH •H -o •E CD O CO «H P, -cf •% O E-< CM Q O. 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