Characteristics of Methyl Parathion Resistance in House Fly Larvae

Abstract

A comparative study of two laboratory strains of the house fly, Musca domestica L., one that is generally susceptible (S) to all insecticides and the other selected for high levels of resistance to methyl parathion (Rmp), indicated that resistance to this compound in third instars of the Rmp strain was attributed to enhanced metabolic detoxification and decreased target site sensitivity. Susceptibility to topical applications of methyl parathion was 44.1-fold greater in S strain larvae (LC50 = 0.23 µg per insect) than in those of the Rmp strain (LC50 = 10.14 µg per insect). In vitro studies of the interaction of methyl paraoxon (the toxic principle of methyl parathion poisoning) with solubilized preparations of membrane-bound acetylcholinesterase (AChE) from whole body homogenates of larvae indicated the AChE of S larvae was about three times more sensitive than that of the Rmp larvae. This difference in sensitivity resulted from a higher affinity of S AChE for methyl paraoxon and a slower rate of phosphorylation of the AChE of the Rmp larvae. In vivo studies of the fate of topically applied [14C] methyl parathion in third instars demonstrated that the rate of cuticular penetration was rapid and was about the same for both strains. Concentrations of methyl paraoxon were found to be about two times higher at all sample times in S strain larvae after treatment.