The factors of resistance to many insecticides were located and isolated in a homozygous condition from the diazinon-selected SKA strain of house-flies (Musca domestica L.) by inbreeding each of the five autosomes of the SKA strain into a susceptible multi-marker strain ac;ar;bwb;ocra SRS. The insecticides were: DDT, methoxychlor, tri-butyl tin, the methoxy and ethoxy analogues of parathion, chlorthion and malathion and their corresponding phosphates, diazinon and diazoxon.
The SKA flies' autosomes had the following factors of resistance:—autosome I—very slight resistance to ethyl-chlorthion; autosome II—gene a for low ali-esterase activity which conferred weak to moderate resistance against all organophosphates tested, and Deh (DDT-dehydrochlorinase) which gave great resistance to DDT, but none against methoxychlor; autosome III—Pen, which delayed knock-down by slowing down the entry of insecticides into the flies and had a negligible effect at death except against tri-butyl tin and methoxychlor, and a factor of resistance to Zectran unaffected by sesamex; autosome IV—Dld, the major factor of resistance to dieldrin; and autosome V—Ses, a sesamex-suppressed factor, which gave weak to moderate resistance against ethyl-malaoxon, diazoxon, diazinon, DDT and methoxychlor, but was ineffective against the other organophosphates tested.
TBTP (S,S,S tri-butyl phosphorotrithioate), an ali-esterase inhibitor, greatly synergised organophosphates only against insects with gene a. Pretreatment with sesamex, an inhibitor of microsomal activity, elicited two types of response: antagonism with most of the thioates, most pronounced in flies with gene a, and synergism with the phosphates, especially evident with flies with factor Ses. The possible reasons for differences in the response to organophosphates after pretreatment with TBTP or sesamex, and the nature of the resistance factors are discussed. High resistance in SKA flies against organophosphates arises through the interaction of resistance factors which singly give only weak to moderate resistance.