Failure of DDT to Control House Flies1

Abstract

In cases of failure of DDT residues to control Musca domestica under conditions in which the treatments had formerly been highly successful, the chief factor involved appears to be an increased resistance of the flies, probably a result of exposure in treated buildings over a period of 1-4 yrs. Samples of flies obtained from 7 localities in 5 states all showed a greater resistance to DDT residues than that of 2 laboratory colonies that had never been exposed to this compound. All samples, however, were much more susceptible than a special resistant colony that has been developed experimentally at Orlando during the past 32 mos. No evidence has been obtained that inferior quality of DDT used this yr. was involved, or that freshly applied residues on the walls of buildings were less toxic than formerly. Heavy deposits of DDT (0.8 to 4 g. per sq. ft.) showed some repellency to flies, but the more usual dosage of 0.2 g. was practically nonrepellent. Heavy deposits of wettable chlordane and methoxy-chlor were nonrepellent or even slightly attractive. Technical benzene hexachloride was somewhat more repellent and a partially refined product somewhat less repellent than DDT. Preliminary tests indicated that the increased resistance to DDT was much greater than it was to methoxychlor, chlordane, and benzene hexachloride. Respraying of 2 barns with chlordane emulsions, and 2 with suspensions of methoxychlor provided a high degree of control for several weeks.