Some Results of Recent Work on the Newer Insecticides 1

Abstract

Summary Several kinds of tests have been employed in the selection of compounds effective as clothing treatments for protection of individuals against infestations of fleas and ticks. A number of the chemicals showed a high degree of protective value against both of the test species, and others were effective in repelling one of the two. The evaluation of these materials is being continued to determine the compounds most suitable for practical use. In investigations of clothing treatments for protection against chigger mites, a number of effective materials were found to be more durable than benzyl benzoate, the present standard. Eighteen of these compounds were selected for impregnation into uniforms for rigid field testing in chigger-infested areas. Five that remained effective through 7 to 10 launderings were benzil, p-cresyl benzoate, 2-thenyl benzoate, diphenyl carbonate, and 2-thenyl salicylate. It is thought that all of them would be of value for large-scale practical use. The names of several compounds are given, which as clothing treatments were effective and long-lasting repellents to salt-marsh mosquitoes. As repellents applied to the skin, dimethyl phthalate and N,N-ethyl succinamate appeared to be the best of a series of compounds tested against subarctic species of Aedes and Simulium. One of the new compounds, parathion (O,O-diethyl O-p-nitrophenyl thiophosphate), has shown a high degree of toxicity to several species of insects. In laboratory tests it was indicated to be several times as toxic as chlordane to cockroaches, and about 10 times as toxic as DDT to anopheline larvae. Field results against anophelines, however, were not so striking. In a residue type of application against house flies and mosquitoes it compared very favorably with DDT. It also appeared to be a promising insecticide for the area control of chigger mites. Since this compound is quite toxic to warm-blooded animals, special attention must be given to the maximum concentration that can be safely used under different conditions. Methoxychlor (methoxy analog of DDT) and two “DDT byproduct oils” appeared, from laboratory tests, to be of possible usefulness as residue treatments. Chlordane and benzene hexachloride were effective for this purpose for a short time after application, but did not show the long-lasting qualities of DDT. A strain of DDT-resistant flies developed in the laboratory proved to be resistant also to other insecticidal sprays. With chlordane, pyrethrins, or chlorinated camphene, as well as DDT, about twice as much material was required to cause mortality equal to that obtained with flies from a normal colony. A subculture from the special strain lost most of its resistance when reared for 12 to 15 generations without further exposure to DDT. In tests of cockroach dusts, several of the newer compounds were equal to Lethane 384, and better than benzene hexachloride, in speed of knock-down. DDT, chlordane, and chlorinated camphene were about equally effective in the area control of the lone star tick, when applied as sprays at rates of 1 and 2 pounds of the toxicant per acre. Thorough treatment of the infested ground litter was necessary to produce good results. Heavier dosages of the materials in dust form were required to insure effective control. In one large-scale dusting test an application of DDT at about 4 pounds per acre gave a high degree of control over an extended period of time. Against adults of the American dog tick treatments with the same three materials, as dusts, at rates of 2 and 3 pounds per acre gave better than 90 per cent control. Chlordane and chlorinated camphene as emulsions gave good results in the control of chigger mites at dosages of 2 pounds per acre. Benzene hexachloride had about equal immediate, but less residual, effectiveness. Parathion showed distinct promise for use against these mites. The prehatching, or preflooding, treatment of mosquito breeding places was indicated to be an important method of control of salt-marsh and other floodwater species of Aedes. DDT gave the best results of any of the materials tested. TDE was highly effective as a black fly larvicide when applied to infested streams at a dosage of 0.2 p.p.m. over a period of 10 minutes. This dosage is believed to be well below the tolerance limit for fish.