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 effective 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 bollworm 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, was 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 effective against the 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 effectiveness against thrips, the cotton flea hopper, tarnished plant bug, and some of the other cotton insects, but is less effective against the bollworm, pink bollworm, 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, bollworm, 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. From 1908 until 1931 he was in charge of entomological activities in Mississippi. He organized the State Plant Board of Mississippi in 1918 and was in charge of the regulatory work until he assumed his present position in 1931.