Interaction between the factor delaying penetration of insecticides and the desethylation mechanism of resistance in organophosphorus-resistant houseflies

The desethylation (gene a) and penetration delaying (pen) factors of resistance to organophosphorus insecticides isolated by genetical methods from the diazinon-selected SKA strain of houseflies (Musca domestica L.) were inbred into a strain whose resistance was compared with that of flies with single factors of resistance and the SKA strain. Pen alone decreases kill marginally, and gene a alone raises LD₅₀ to 1·4–48 times that of susceptible flies, depending on the insecticide. The two factors together increase resistance greatly to many organophosphorus insecticides (up to 5–10 times or more to diazinon, malathion-ethyl and chlorthion-ethyl than of flies with gene a only) indicating that the two factors interact. Interaction is greater against the thionates than the corresponding phosphates, probably because pen delays entry of thionates more than of the corresponding phosphates. The role in resistance of each factor when hetero-and homo-zygous, and the reasons for interaction, are discussed.