FACTORS AFFECTING RESISTANCE TO INSECTICIDES IN HOUSE-FLIES, MUSCA-DOMESTICA L (DIPTERA, MUSCIDAE) .2. CLOSE LINKAGE ON AUTOSOME-2 BETWEEN AN ESTERASE AND RESISTANCE TO TRICHLORPHON AND PYRETHROIDS

Widespread slight pyrethroid-resistance in M. domestica L. on animal farms in southern England [UK] was correlated with strong resistance to trichlorphon and to malathion, and with the presence of an esterase, E0.39, detected by electrophoresis. In the laboratory, the frequency of E0.39 increased in response to selection with either pyrethroids or trichlorphon. Genetic analysis confirmed that this esterase, controlled by a gene on autosome 2, was closely linked with moderate resistance to trichlorphon and malathion and weak resistance to pyrethroids. When autosome 2 with the gene for E0.39 was introduced into a strain homozygous for the resistance mechanism super-kdr, resistance to pyrethroids increased by a factor equivalent to the weak resistance conferred by autosome 2 with E0.39 alone. Homozygosity for both mechanisms of resistance, and E0.39, was obtained by selecting the progeny of this cross with permethrin alone, permethrin and trichlorphon, or DDT and trichlorphon, demonstrating that very strong pyrethroid resistance can be achieved through the use of non-pyrethroid insecticides. E0.39 was absent from insecticide-resistant strains of M. domestica from Denmark but was present in several multi-resistant strains from other European countries.