May 1945 E " 658 United States Department of Arri culture Agricultural Research Administration Bureau of Entomology and Plant Quarantine EXPERIMENTS WITH DDT, SABADILLA, AND PYRE7HP.UM DUSTS FOR CONTROL OF LYGUS SPP. ON SEED ALFALFA By F. V. Lieberman Division of Cereal and Forage Insect Investigations In past years insecticide! control of Lygus spp. on alfalfa grown for seed has "been unsuccessful, largely "because immediate rein- festation resulting from continual hatching of nymphs and immigration of adults has required several applications of the contact poisons tested and has rendered effective control impractical and prohibitive in cost. In a field-plot test of three insecticidal dusts on seed alfalfa during the summer of 19^U at (randy, Millard County, Utah, DDT (l-trichloro-2,2-bis(p-chlorophenyl) ethane), one of the two new materials being given preliminary trial, was found to be highly toxic to Lygus spp., effective for weeks after application, and definitely promising as a satisfactory control for these bugs. Since cross-pollination of alfalfa flowers is now considered essential to profitable seed production, this experiment was made at an isolated ranch settlement where alfalfa-visiting wild bees, espe- cially Womia m^landeri Ckll., sometimes called the alkali bee, occur in numbers adequate for effective pollination of the crop* In this locality both Lygus hesperus Knight and Lygus ellsus Van D. occur, the latter heavily predominating. Twenty-five plots, 60 feet wide by lU%2 feet long, were laid out side by side in a single row without use of buffer strips. One plot in each consecutive 5 was selected at random for dusting with 10 percent of DDT in pyrophyllite, 10 percent of ground sabadilla seed in a 1:8 mixture of hydrated lime and Georgia talc, 20 percent of ground sabadilla seed in a 1:3 mixture of hydrated lime and Georgia talc, or 10 percent of pyrethrum extract in pyrophyllite (0.2 percent pyrethrins) ; the fifth plot was employed as a check. Applications were made with a duster mounted on the rear of a farm tractor and operated off its pulley. Its 22-foot boom was carried at plant-top level and trailed by a 15-foot canvas apron. The tractor was driven lengthwise of the plot 3 tir.es, each dusted strip being 1 armed somewhat to assure complete cover^gp. The speed of the tractor was approximately 3.7 miles per hour. Dusting and all subsequent operations in handling, harvesting, and threshing the crop closely approached actual farm practice. Population counts teken in the plots were based on 25 strokes of a 15-inch inoect net. ^ - 2 - To avoid repelling or killing alfalfa-visiting "bees, the insecticides were applied before many of the plants had produced flowers, or while they were essentially in the prebloom stage. Used at this time none of the insecticides showed repellent effects on the wild bees; they visited the plots in proportion to intensity of the bloom. No kill of wild bees is believed to have occurred, but mortality would be extremely difficult to observe, or determine in the field. No domestic honeybees are present in the Gandy area. Effectiveness of the different treatments in destroying Lygus is shown in table 1. Results of dusting with DDT are based on all five plots treated. Inadvertent irrigation prevented redusting of two plots treated, with each of the other insecticides used; data from these plots are therefore excluded in the presentation. Popula- tion data for three check plots are also omitted, because they were affected materially in varying degree by drift from DDT plots adjacent to their south borders. The best-yielding sabadilla and pyrethrum plots were those located next to plots dusted with DDT, which indi- cates that these plots also were affected by DDT drift. It was impos- sible, however, to distinguish the effect of the DDT from that of the sabadilla or pyrethrum. The data from these plots have therefore been included without correction for this effect. Of the two check plots for which data are given in table 1, the first is highly representative of the unaffected portions of all three check plots omitted. In check plot 2, which had a much lower initial Lygus population than any other plot, treated or untreated, the slower increase in population allowed bloom to occur and seed to set before infestation became severe. DDT was outstandingly successful in accomplishing Lygus control because of its ability to remain toxic throughout the period required for flowering and podding of the seed crop. One dusting of the alfalfa growth reduced and held the nymphs to negligible numbers. The sabadilla and pyrethrum dusts gave substantial population reductions, considering concentration and dosage of each, but with rapid hatching of eggs the population of nymphs was speedily rebuilt tc menacing strength, and economic destruction of reproductive part*-- of the plants was resumed. The fact that DDT was much more effective than the other insecticides in killing Lygus adults is difficult to appreciate from the data in table 1. Reinfestaticn by flight occurred daily in all plots, irrespective of treatment; it was greatest in the more succulent plots. Higher mortality of adults in the DDT nlots is indicated by the fact that early morning sweepings yielded fewer adults on these plots than on the others, whereas during the flight period l?ter in the day adult abundance on DDT-treated growth became -progressively closer to that on other treated alfalfa. Furthermore, since some adults swept from DDT plots were unable to coordinate leg movements, it was evident that mortality rather than emigration was responsible for their compara- tive scarcity in these plots in the morning. - 3- Table 1.— Bf feet Irenes s of DDT, sabadilla, and pyrethrum dusts In sup- pressing L ygus populations on alfalfa grown for seed, July-August 19HH. Treatments applied July 19 and 26 Dust Dosage ( pounds per acre) Jirst treat- ment Second treat- ment Stage Average number of Lygus bugs per net stroke Days after first dusting 3 7 9 22 22 Days after second dusting 2 15 26 DDT 10# 27.5 None Nymphs Adults 6.6 1.6 •2 .9 .6 0.1 1.0 0.1 .3 Sabadilla: 2C# 29 32.5 Nymphs Adults 5.1 2.1 1.2 .5 5.0 2.3 1.0 .1 li 5.1 .8 log 25 Pyrethrum 10^ lU 32.5 Nymphs Adults 6.1 2.1 1.5 .7 2.9 1.2 1.7 •2 5.8 1.7 3.9 l.l 16 Nymphs Adults 8.3 1.7 2.6 1.0 u.o .8 3.^ .7 8.2 2.0 3.6 .8 Checks (untreated)x No. 1 None None Nymphs Adults 6.1 1.3 9.3 1.7 9.7 1.0 - 6.6 .1 1.7 .0 No. 2 None None Nymphs Adults 2.7 2.U U.l 2.8 5.6 2.3 12.3 3.2 5.7 .5 — ' Duster failed to deliver this dust at desired dosage of 25 pounds per acre. - u- Weather may have Influenced the effectiveness of all dusts used. No precipitation occurred during the 59-day interval from dusting to cutting of the crop. However, the period was characterized "by exces- sive wind, frequently strong enough to "be suspected of removing insecticidal dust. All treatments were definitely "beneficial as measured "by the extent of flowering and setting of pods. After the pods had been formed in plots receiving two treatments of 20 percent sabadilla dust 7 days apart, the appearance of these plots indicated a prospective yield approximately equivalent to that of the plots dusted once with DDT. Lygus feeding on seed in immature pods and the sharply curtail- ing effect of an early severe frost substantially reduced the promising yields of these plots as well as those of most others. The entire plots were harvested for seed. Results of threshing, recleaning, and examina- tion of seeds from pod samples taken just "before frost are given in table 2. Yields from plot to plot under the same treatment varied widely. This variation is attributable mainly to a corresponding difference in plant succulency due to variations in soil moisture within the field and intensified "by precipitation deficiency. Late in August intensely drying winds reduced yield prospects in all plots, particularly in those already dried to excess. Variation in succulency and also in yield was greatest among the DDT plots. Tardy spot irrigation at vari- ous times during the flowering and podding period only added to the variation in plot condition without giving the "benefit that timely water- ing could have provided. Nevertheless, the "better yields under each treatment represent those that may "be expected under normal seasonal and field conditions. Without extensive tripping of flowers it would have "been impossible to obtain the very high yields realized from the two DDT plots that were only slightly affected by drought and frost. Observations showed that, in proportion to the bloom on the plants, pollinating bees were no more abundant in treated plots than they were on a large adjacent acreage of blooming alfalfa that was untreated* These high yields were produced under bee activity normal to the locality. They are not theoretical yields achieved by extreme concentration of pollinating insects onto small patches of blooming plants. - 5 - Table 2. — Yield and quality of seed from alfalfa field plots dusted with DDT, sabadilla, or pyrethrum, 19UU Treatment Yield . (pounds oer acre) Tn" hresher- run seed cleaned seed Percent shrink- age 1/ Number of seeds exam- ined Percent of seed in pod samples — Viable Normal color Off- color Hot viable Imma- ture Destroyed by— Lygus Chalcids DDT 10$ 3U5 650 725 285 135 Av. U28 Sabadilla: 20# S3 285 Av. 237 lOJ* 85 200 183 156 70 lhg 115 Av. Ill 30 260 Checks af- 188 fee ted by 310 DDT drift 175 Av. 22U Av. Pyrethrum Checks t No. 1 No. 2 322 593 629 26U 118 385 57 28J an 185 69 187 1U6 13U 50 128 89 89 23 179 169 285 1U7 200 6.6 8.82 / i3.3- ; 7.U 12.9^' 9.8 31.1^ 2U.5 19.U 6.U 20.? I5.H *.& 13.6 2/ 22.2^' 21.5 21.8 10.0 8.2 2/ 16. & 11.5 501 81 505 50U 79 79 502 58 505 86 77 U53 63 500 51 501 55 56 2H9 57 U99 39 500 55 50 U99 106 S 501 21 8 500 U36 513 500 31 28 65 69 U6 60 9 16 6 18 6 11 15 12 8 12 1U 16 9 13 18 12 5 12 25 13 15 12 12 13 7 3 11 Ik 1 7 10 22 21 18 12 2U 2U 20 31 22 UO 31 25 60 1U 13 18 15 3 2 3 9 6 5 12 13 15 13 16 18 11 15 17 26 26 3« 5 6 21 11 0.3 .6 .6 1.2 l.U 0.8 0.2 1.8 1.0 1.0 3.0 1.0 1.3 0.2 1.0 O.U O.U 0.5 3.U 1.3 1/ Determined from 100- to 300-gram portions of total plot yields. Yield of recleaned seed calculated by use of these percentages. 2/ Thresher operated with inadequate power; shrinkage was larger than it would have been otherwise. -6- The cleanliness of thresher-run seed due to the excellent protection of developing pods "by DDT is shown "by the lew shrinkage in recleaning and the small proportion of insect-damaged seed found in pod samples (table 2). Field observations indicated th-t the crops in all DDT-treated plots were maturing slightly faster than in the other plots. These observations were confirmed "by the smaller percentages of immature seeds found in pod samples as indicated in table 2. A slight tendency to dry out the alfalfa plants as they neared maturity appeared to he creditable to DDT, but further obser- vations under conditions less influenced by drought and drying winds are needed to decide this point. The remarkable -orotection given the check plots that received drift of DDT dust indicates that direct application of dosages of this insecticide lower than those used in this experiment will give adequate kill. The pods from DDT-treated plots contained an average of U.77 seeds per pod; pods from plots treated twice with 20 percent sabadilla, 10 percent sabadilla, or pyrethrum dust contained an average of 2.56, 2.92, and 2.91 seeds, respectively. The lower averages in the sabadilla and pyrethrum treatments were due to the more frequent occur- rence of pods with only 1 or 2 seeds. Samples from checK plots 1 and 2 averaged 1.33 a* 1 ^ 3*55 seeds per pod, respectively, the former being representative of the condition that would have prevailed in the treated plots if they had remained untreated. Many pods in these areas contained no seeds at all. In fact, the 8 seeds present in the sample from check plot 1 (table 2) represent the total production on 25 stems. Check plots affected by drift of DDT averaged 3.O6 seeds per pod. Seeds in pods from DDT plots were slightly smaller than those from the check or other treated plots, although no definite relationship to probable causal factors could be established, A composite sample of recleaned seed from the DDT plots had a commercial purity of 99*25 percent (premium grade); an official commer- cial germination test indicated viability to be within normal range for alfalfa seed of this area during the fall season. Quick-germinating seeds averaged U0.75 percent and hard seeds 53*75 percent to give a total estimated germination of 9^*50 percent. Samples of the alfalfa for analysis of DDT residues were taken from treated plots just before and after threshing, 95 to 96 days after the treatment was applied. No rain but much drying wind occurred dur- ing the 59""^ a y period between treatment and cutting. Three rains total- ing about 3/^- inch fell during the 36-day period between cutting and threshing. During this period the alfalfa was in cocks and was turned three times. The analyses indicated that the DDT residues were not over 17 parts per million (p. p.m.) in the alfalfa before threshing and less than 10 p. p.m. in the chaff after threshing. Although these resi- dues exceed the current tentative DDT tolerance cf 7 p. p.m., in viev: of the high degree of Lygus control secured in the lS^ik tests it seems likely that dosages can be reduced sufficiently to bring the DDT residues within the tolerance and still inaintain satisfactory Ly~us control ana economically profitable seed production. - 7 - In this experiment collection of actual data on reduction of insect population was restricted to Lygus . However, large aphid and thripa populations that existed in all plots at the time of dusting were practically eliminated in the DDT-trsated plots and slightly- reduced in the other treated plots. DDT was also observed to reduce populations (nymphs and adults) of Nabis ferus (L.) f ladybirds, end grasshoppers. Most of the specimens of these common alfelfa-field insects observed in the DDT plots after they were dusted probably were immigrants. Further information is needed on the best dosages and mixtures of DDT for use against Lygus spp. infesting alfalfa, on timing of applications to avoid the killing of honeybees and other alfalfa- pollinating bees, and on the livestock-poisoning hazard resulting from DDT residues on alfalfa, especially that which may be pastured or cut for hay instead of seed. UNIVERSITY OF FLORIDA 3 1262 09238 7033 )