STATE tL.AOii BOA PL) E-530 ^v ncnADTi/rMT // March 1941 CONTACT INSECTICIDES FOR CHINCH BUG CONTROL ON CORN By E. V. Walter and Curtis Benton, Division of Cereal and Forage Insect Investigations Introduction The protection of growing corn from the ravages of chinch bugs by the use of insecticides has generally been considered impractical because of the high cost of the treatment as compared with the relatively low per- acre value of the crop. When a portion of a field becomes seriously in- fested by chinch bugs migrating into it before a barrier has been con-' structed, the usual custom has been to locate the barrier ahead of the worst of the invasion and to abandon the more heavily infested portion of the crop. A well-constructed barrier built in advance of the arrival of the bugs is still the cheapest and most effective means of protection. Occasionally, however, a crop of more than usual value, such as inbred or hybrid seed corn or market sweet corn, becomes infested and an attempt to save as m-uch of it as possible becomes desirable. Nicotine, soap, or kerosene emulsion sprays, and nicotine or calcium cyanide dusts have been recommended but have not been found entirely satis- factory on account of their high cost, low efficiency, or caustic effect on the plants. An effort has therefore been made during the period of 1938- 1940, inclusive, to find a safe, effective, and readily prepared insecticide that would be cheap enough to use at least on particularly valuable corn. To date an emulsion of a highly refined v/hite mineral oil to which has been added either nicotine sulfate or derris extract has given the best results. This spray is a modification of the lubricating oil emulsion and nicotine sulfate spray recommended by Horton and Satterthwait in U. S. Department of Agriculture Farmers' Bulletin 1223. The investigations are not complete, but the results thus far indicate that so much protection can be obtained at a moderate cost that immediate publication of the information seems desirable. Habits of the Chinch Bugs Chinch bug infestations usually originate from eggs laid in the spring or early in the summer by the overwintered adults that have migrated to the fields of srcall grain. As these grains mature or are preiraturely killed, the partly grown nymphs leave them in search of food and so invade nearby fields of young corn. Thus the bugs are usually first observed late in June and early in July on a few rows of corn nearest the grain fields. At this time the corn usually is from li to 3 feet in height. The bugs normally remain to feed on these first rows until they have killed the plants, or are ready to molt. Infested corn should be treated as soon as possible after the bugs are found. However, if the bugs are still migrating, it is advisable that a barrier be constructed first to prevent more bugs from coming into the field. The sprays mentioned have some repellent action, consequently, unless a barrier is constructed, many of the bugs coming in after spraying is done will go farther into the field to the unsprayed plants, thus spreading the infestation. In some of the tests, plants unprotected by a barrier were sprayed. In spite of the fact that they afterward became densely covered with bugs, many lived and produced ears, thus indicating that the bugs did not feed readily on the sprayed plants. When ready to molt, especially in very hot weather, the bugs often crawl under clods or trash, where it is cooler and more humid than on the plants, and remain there for several hours. Also, many hide in such places during very windy weather. Thus itany bugs may be missed by insecticidal treatment during such times, making a second application necessary in extreme cases. Type of Oil The oils thus far tested as insecticides for the control of chinch bugs on corn were white mineral oils having an unsulfonatable residue of at least 96 percent. There did not seem to be any significant difference in the effectiveness of oils of the different viscosities used, which were within the range from 85 to 210 seconds Saybolt at 100° F. Since the lighter oils are usually cheaper than the heavier oils, they are recommended for the purpose . Commercial miscible and summer spray o ils are effectiv e but should be used with caution^ as cases of severe injury f rom their appl icati on to corn h ave been ob served . Home-made Emulsion Mineral oils of the type mentioned are easily emulsified according to the following formula: 1 pound potash (yellow) laundry soap ■^ gallon hot soft water 1 gallon oil 3 - Dissolve the soap in the water and, while still hot, add the oil slowly and beat until emulsification is complete. If a spray pump of the bucket or wheelbarrow or power type is available, the mixture can be easily emulsified by pumping it through the nozzle and back into the container. Small quan- tities may be readily emulsified by use of a rotary egg beater. Well-prepared emulsions made by this formula contain approximately 62 percent of oil. They are quite stable and have shown no breakdown or separation when allowed to stand for a year or more. Commercial Emulsions Several brands of ready-prepared emulsions of mineral oils of the types used in these experiments are on the market. They may be as effective as the home-made emulsion but are usually much higher in price. One such brand containing 83 percent of oil and intended for greenhouse and garden spraying has been used with good results. Materials Used Many different dilutions of the oil emulsions, and combinations of them with varying amounts of nicotine sulfate, rotenone, pyrethrum, and other insecticides, were tried experimentally. The cheapest mixture that gave satisfactory results was one in v/hich 1 part by volume of the home- made oil emulsion containing 62 percent of oil was mixed with 30 parts of water and fortified by the addition of one-eighth ounce, or approximately 1 teaspoonful, of nicotine sulfate (40 percent nicotine) per gallon. A number of tests indicated that a similar quantity of a derris extract containing 5 percent of rotenone can be substituted for the nicotine sulfate with equally good results. Oil emulsion to which different amounts of pyrethrum extract were added gave but little better results than the oil emulsion alone. The strength of the spray material mentioned above is the minimum that gave satisfactory results when used against migrating chinch bugs when many of them were in the fourth and fifth instars or adult stage. Slightly better kills were secured by increasing either the oil or nicotine content of the spray, but the results did not appear to be enough better to com- pensate for the increased cost. Water dilutions of the oil emulsion alone and also of the nicotine sulfate, rotenone, pyrethrum, and many commercial insecticides have been tried, but none has given as good results as the combination of oil emulsion with either nicotine or rotenone. When very hard water is used for spraying, enough of a good water softener should be used to break it before the emulsion is added. This prevents separation of the oil and the formation of a scum of insoluble soaps that might interfere with the spraying. The kind and amount of the water softener used will depend on the degree of hardness of the water and - 4 the minerals involved. Do not use too much, as an excess of some types may injure the plants. Cost of Treatment Several of the mixtures which showed the best results in the experi- mental tests were mixed and applied under field conditions to determine the cost of materials and the time required to treat an acre of corn. It was thus found that approximately 70 to 75 gallons of spray mixture would be required to treat 1 acre of corn from 2 to 4 feet in height. It was also found that a man using a knapsack type of compressed-air sprayer could treat approximately 3,200 hills, or slightly less than 1 acre, per 8-hour day. (There are 3,556 hills per acre when planted in hills 3|- feet each way. ) The cost of materials for mixing 70 gallons of spray, when purchased at common retail prices, may be distributed approximately as follows: Oil, 1.4 gal. at $0.50 $0.70 Soap, 1.4 lb. (about 2 bars) 08 Nicotine sulfate, 12 oz. at $2.25 per 1-lb. bottle _1^69 $2.47 The comparative cost and insecticidal value of a number of the sprays used in these tests are shown in table 1. The cost is based on retail purchase of the quantity of each article necessary for 70 gallons of spray. It is almost impossible to arrive at a definite percentage of the chinch bugs killed, so ratings based on observations in from 6 to 16 rep- lications of each combination and strength were used. These are as follows: P. poor, under 30 percent dead; F. fair, 30 to 60 percent dead; G, good, 60 to 90 percent dead; and VG, very good, 90 to 100 percent dead. - 5 Table 1. — Comparative cost and insecticical value of insecticides tested for the control of chinch bugs on corn, based on the retail price of enough of each material to mix 70 gallons of spray. ture Home-made oil emulsion 1-30 2.0 Do. 1-16 3.6 Do. 1-40 1.5- Do. 1-30 2.0 Do. 1-16 3.6 Do. 1-30 2.0 Nicotine sulfate (40% nicotine) 1-200 . - - --■■ Liquid soap (40% dry soap) 1-51.2 Do. . 1-25.6 — Do. 1-51.2 __ Dilution Approx. Nicotine in water percent sulfate of oil (40% nicotine) in mix- oj:. per gal . 3/16 Esti- Cost mated per kill acre (70 gal.) P $0.78 G 1.40 G 2.27 G-VG 2.47 VG 3.09 VG 3.31 G . 4.66. F 3.28 i G 10.14 1 VG 8.40 Injury to Plants No burning or other injury was observed when emulsions of the highly refined white mineral oils described above were applied, in the dilutions recommended, to the leaves or lower parts of the stalks of corn that was 12 inches and over in height. Some occasional injury was observed on very small corn, especially when the spray material was allowed to collect in the top of the plant. Occasional slight injury was also observed on heavily sprayed suckers of 12 inches or less in height. Plants 3 feet or more in height were apparently uninjured v;hen experimentally sprayed to haJ^ their height with the pure oil, l.^^ Summary STATE* PU^T boabJ^ An emulsion of a highly refined mineral oil combined with either nicotine sulfate or rotenone was found effective for killing chinch bugs on growing corn. Such a spray is rather expensive for use on corn grown for feed but is cheap enough for use on crops of inbred or hybrid seed or market sweet corn, it is not recommended as a substitute for the creosote barrier UNIVERSITY OF FLORIHA but only to kill the bugs that reach the corn in spite of the barrier or before it is constructed. Emulsions of all the highly refined mineral oils so far used have been found safe for application on corn 12 inches or more in height. Com- mercial emulsions of similar oils gave good results but the cost was greater. Commercial miscible or summer spray oils should be used with caution. Properly prepared oil emulsions are stable and will keep for a year or more without separation. An easily follov/ed formula is given. A combination of the oil emulsion with either nicotine sulfate or rotenone gave much better results than either alone, even at very much greater strengths. The addition of pyrethrum extract did not materially increase the effectiveness of the oil emulsion. The minimum strength of spray that gave satisfactory results con- tained 2 percent of oil and had one-eighth ounce of nicotine sulfate (40 percent nicotine) or of derris extract (5 percent rotenone) per gallon. Approximately 70 gallons of such spray, costing $2.47 when materials are purchased in small lots at retail prices, were required to spray 1 acre of corn that was from 2 to 4 feet in height. A man using a knapsack-type sprayer can spray slightly less than an acre in an 8-hour day.