In Vitro Metabolism of Parathion in Susceptible and Parathion-Resistant Strains of the Greenbug, Schizaphis graminum (Rondani) (Homoptera: Aphididae)

Results of in vitro metabolism studies in a susceptible and two parathion-resistant greenbug strains indicate that hydrolytic enzymes are active toward paraoxon, but not parathion, and are partially responsible for resistance. The resistance does not involve reduced levels of oxidative metabolism and activation of parathion since NADPH-dependent microsomal formation of paraoxon in resistant and susceptible strains was similar. Paraoxon is a potent inhibitor of greenbug esterases, and the resistance mechanism apparently involves rapid and tight binding of enzyme and insecticide that prevents the insecticide from reaching its target site. Native polyacrylamide gel electrophoresis also supported this view since staining of enhanced bands associated with resistance was inhibited by incubation with paraoxon. The resistant strains apparently produce more of the isozymes that are inhibited by paraoxon and are able to sequester a greater number of paraoxon molecules than susceptible greenbugs.