Specific DDT-Resistance in Houseflies

Abstract

Two strains of houseflies (Musca domestica L.) have been derived by selection for a number of generations from a DDT-susceptible colony which originated from wild flies collected in 1939. The colony had been mass-reared in the laboratory for fourteen years without exposure to insecticides. Selection was by two different methods. One strain (D) was derived by incorporating DDT wettable powder in the larval medium at 80 p.p.m. DDT for the first three generations, 160 p.p.m. for generations 4–9, and 320 p.p.m. for all subsequent generations. The other strain (L) was started ten generations later than D; it was selected on the basis of slow preadult development, that is, by breeding only from flies emerging from the cultures towards the end of the emergence period, and its breeding stock was not exposed to insecticides. Pimentel et al.¹ had found that late-emerging flies were more tolerant of DDT than early emerging ones, and that, in four DDT-resistant strains, resistance was associated with prolonged larval period.