DDT-Resistant House Flies and Mosquitoes12

Abstract

Tests were conducted in dairy barns in two localities in Florida against Musca domestica. Chlordane emulsion at dosages of 100 and 200 mg. of toxicant/sq. ft. was the most consistently effective material, followed in order by dieldrin emulsion at 50, 25, and 10 mg.; lindane wettable-powder spray at 50 mg.; toxaphene emulsion at 200 mg.; and a DDT-methoxychlor wettable-powder spray at 200 mg. (100 mg. of each material). Most of the tests with the last 2 materials alone were failures, as were also all of 4 tests with pyrethrum extract. A few preliminary tests with space sprays applied at frequent intervals appeared of promise. In tests with 21 lots of flies from different barns, exposed to DDT-treated plywood panels, the exposure time required to produce 70% mortality of females ranged from about 4 to 17 times (avg. of 11 times) that of a normal laboratory colony. For 9 lots tested on methoxychlor, the LT_70 was slightly over twice as long as that for the laboratory colony, whereas for lindane, toxaphene, and dieldrin, in smaller numbers of tests, the LT_70,s were less than twice as long. Of 14 lots of flies from natural sources which were reared in the laboratory, all showed a great deal of variation in susceptibility to DDT in consecutive generations, but most of them gave evidence of a definite loss of resistance after a few generations. A highly resistant laboratory colony was much less resistant to several other insecticides. This colony showed a partial loss of resistance to DDT after 9 generations without further exposure to the compound. Evidence was obtained that increased resistance to DDT was developing in salt-marsh mosquitoes, Aedes taeniorhynchus and A. sollicitans, in areas that have been subjected to intensive treatment with this material for several years. In laboratory tests both larvae and adults from such areas were much more difficult to kill with DDT than were similar stages from untreated areas. This was also true for TDE but not for lindane, chlordane, parathion, or dieldrin. In large-scale field tests, DDT was considerably less effective against adults than it had been in the same locality in previous years. With other materials the best results were obtained with lindane applied at rates of 0.1 and 0.05 lb./acre. Parathion at these dosages and technical benzene hexachloride (12% gamma isomer) at dosages of 0.4 and 0.2 lb. gave results comparable with those obtained with lindane in open marshes, but were somewhat less effective in the woods. Chlordane at 0.2 and 0.3 lb. and toxaphene at 0.2 lb./acre were not highly effective. In laboratory tests with adults of Anopheles pseudopunctipennis from a Mexican village where all premises had been treated with DDT residual sprays once each year during the previous 3 years, there was no indication of increased resistance to this material. Human body lice, Pediculus humanus corporis, have not as yet shown increased resistance to DDT after exposure to sublethal dosages.