Insecticides for Cotton by R

Insecticides for Cotton by R. W. HARNED c./OT.TON, an important crop since before the dawn of history and one that has been grown in North America since the earliest colonial days, has always had serious insect enemies. Against them, most of the materials known to control insects attacking plants in the United States have been tried. The boll weevil, bollworm, cotton leafw^orm, cotton aphid, cotton flea hopper, and other pests continue, however, to reduce yields and destroy cotton crops. The first potent weapon was paris green, the use of which was recommended in 1872 by Prof. C. V. Riley, then the State Entomologist of Missouri, to control the cotton leafworm. Its use spread rapidly, especially in the Gulf Coast States, and during the next 40 years thousands of tons were used against the cotton leafworm and the bollworm. Next came the discovery in 1916 by workers in the Department of Agriculture that calcium arsenate dust of certain specifications was efïective and practical for the control of the boll weevil. Later investigations showed it to be useful against the bollworm, and in mixtures with sulfur to be of great practical value in the control of the cotton flea hopper, tarnished plant bug, and other plant bugs and stinkbugs that attack cotton. Because of its wide use for boll weevil control and its lower cost, it largely replaced paris green for the control of the cotton leafworm and bollworm. For the control of cotton insects about 50 million pounds of this arsenical insecticide are now used each year. The need for applying calcium arsenate to large acreages of cotton stimulated improvement of dusting machines, but no ground machines could be satisfactory for use when the soils were saturated with water. In 1922 airplanes were first used for the application of calcium arsenate to cotton. This method was so satisfactory 655 656 YEARBOOK OF AGRICULTURE' that within a few years there were several dozen companies operating hundreds of airplanes used largely for dusting cotton. During the decade 1920-30 two other materials, sulfur dust and nicotine, were first used extensively for the control of cotton insects. Sulfur had long been known as an insecticide, but was not used on cotton until it was found to be helpful in reducing losses caused by the red spider and the cotton flea hopper. Sulfur later was found to be useful in reducing infestations of the tarnished plant bug and rapid plant bug and its use helped to check the reduction in quality of cotton in fields where injurious stinkbugs were abundant. Mixtures of sulfur and arsenicals, such as calcium arsenate and paris green, were found to be more effective against the cotton flea hopper and the other plant bugs and stinkbugs that attack cotton, and in recent years millions of pounds of these mixtures have been dusted on cotton in Texas, Arizona, New Mexico, and California. The cotton aphid had always been a minor pest of cotton until the extensive use of calcium arsenate for boll weevil control, about 1920, caused it to become a major problem. The heavy aphid infestations develop because calcium arsenate decreases the acidity of the plant juices and there is some evi- dence that this favors the rapid development and reproduction of the aphids. The arsenical dust also kills the beneficial parasitic and predaceous insects that normally destroy the aphids and prevent serious infestations. Nicotine was found to be fairly satisfactory for the control of the cotton aphid and its use for this purpose has steadily increased during the past 25 years. The method of application generally used is to mix nicotine sulfate with calcium arsenate or with lime for dusting cotton. During recent years the supply of nicotine available for use on cotton has not been sufficient to meet the demand. During the 1930's the insecticides recommended and widely used for the control of cotton insects were calcium arsenate, sulfur, lead arsenate, paris green, nicotine sulfate, and mixtures of them. Recent research has developed several materials that may be as effective as the insecticides developed in the years since paris green was first used in the cotton fields, or even more effective. Experiments have shown that mixtures of basic copper arsenate and sulfur have good dusting qualities and when applied at the rate of 12 to 15 pounds an acre are more effective than calcium arsenate for the control of the boUworm. They are more effective than sulfur or mixtures of sulfur and calcium arsenate for the control of the cotton flea hopper, nearly equal to calcium arsenate against the boll weevil and less likely to cause injurious aphid infestations, and better than calcium arsenate and equal to lead arsenate against the cotton leaf worm because they adhere to the foliage and are effective for weeks after application. Their chief disadvantage is that they cost more than calcium arsenate or mixtures of calcium arsenate and sulfur. INSECTICIDES FOR COTTON 657 Cryolite applied at the rate of 8 to 10 pounds an acre is more effective against the bollworm than calcium arsenate, but is much less effective against the boll weevil and cotton Icafworm. Research had shown that rotenone has possibilities against the cotton aphid, but the w ar interfered with investigations with this material and it was not available for use on cotton. Additional research may disclose that it may be useful against aphids and possibly other insects on cotton. Sabadilla is another material that shows promise against the stinkbugs and plant bugs. Another period in the development of insecticides for the control of cotton insects occurred during and immediately following the war, when many synthetic organic chemical compounds were tested to determine their toxicity to insects. In 1943, preliminary tests indicated that DDT was effective against such cotton pests as the bollworm, the tarnished plant bug, and other plant bugs, the Say stinkbug, the brown stinkbug, the conchuela, the onion thrips, and the tobacco thrips, but was comparatively ineffective against the boll weevil, the cotton leafworm, and the cotton aphid. During 1944 extensive tests conducted with DDT against cotton insects confirmed the results obtained in 1943. In addition, it was found to be eíTective against six other cotton pests, the pink bollworm, the cotton flea hopper, the red-shouldered plant bug or stinkbug, the superb plant bug, the small darkling beetle, and the beet armyworm, and not effective against the red spider mite. In fact, the use of DDT dust on cotton sometimes causes an increase in the red spider and cotton aphid populations, probably because it destroys their natural enemies. Many experiments in 1945 indicated that in DDT an insecticide had been discovered that is of practical value for the control of the pink bollworm ; that is more effective for use against the bollworm than cryolite, basic copper arsenate, lead arsenate, and calcium arsenate, the materials that previously had given the best results against this insect; and that in Arizona gave notable increases in yields of cotton when used for the control of sucking bugs. As a result, during 1946 many thousands of acres of cotton, especially in Texas and Arizona, were dusted by farmers with mixtures containing DDT for the control of the cotton flea hopper, bollworm, and other insects, and the Governments of Mexico and the United States cooperated in applying thousands of pounds of 10-percent DDT dust by airplane for the control of the pink boUworm to hundreds of acres of cotton on dozens of farms on both sides of the river in the lower Rio Grande Valley. Another organic chemical, benzene hexachloride, which was first made by Michael Faraday in 1825, w^as shown to be a promising insecticide in England in 1942. Preliminary tests with this material against cotton insects in 1945 indicated that it might be more efïective against the 7038;;U)°—47 43 658 YEARBOOK OF AGRICULTURE boll weevil, cotton leafworm, cotton aphid, and certain stinkbugs than any insecticide that had previously been used. Experiments with benzene hexachloride in 1946 indicate that it may be the most potent insecticide thus far discovered for use against the boll weevil, cotton aphid, and cotton leafworm. It may be equal to DDT in efiFectiveness against thrips, the cotton flea hopper, tarnished plant bug, and some of the other cotton insects, but is less effective against the boUworm, pink boUworm, and beet armyworm. Experiments conducted by the Department in cooperation with the State agricultural experiment stations in South Carolina, Mississippi, Louisiana, Texas, and Arizona in 1946 indicate that the proper use of DDT or benzene hexachloride or mixtures of these materials may greatly increase the yields and improve the quality of cotton in all areas where insects are seriously injurious to this crop. At Waco, Tex., a mixture of DDT and benzene hexachloride gave remarkable results in increasing the yields of cotton by controlling heavy combined infestations of the boll weevil, cotton aphid, boUworm, and cotton leafworm. Another new organic compound tested at Waco in small plots and large-scale field experiments showed very promising results against heavy infestations of boll weevils, cotton aphids, bollworms, and leafworms. Laboratory and cage tests also indicated it would control the cotton flea hopper, stinkbugs, loopers, and garden webworms. This material is a chlorinated camphene known as Hercules 3956. It does not have some of the objectionable features connected with benzene hexachloride. Other less promising organic compounds are hexaethyl tetraphosphate and another chlorinated hydrocarbon known as Velsicol 1068. THE AUTHOR R. W. Harned, entomologist in charge of the Division of Cotton Insects of the Bureau of Entomology and Plant Quarantine, has been associated with cotton insect investigations since 1907, when he joined the staff of the Mississippi Agricultural Experiment Station.

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