Investigations into the biochemical basis of azinphosmethyl resistance in the light brown apple moth, Epiphyas postvittana (Lepidoptera: Tortricidae)

Target site and metabolic enzyme studies were carried out on azinphosmethyl susceptible and resistant strains of light brown apple moth, Epiphyas postvittana (Walker). Acetylcholinesterase from the two strains was inhibited in vitro to the same degree by azinphosmethyl oxon or by the carbamate eserine. There was also no difference between strains in the kinetics ( K m and V max ) of the reaction with acetylthiocholine iodide. The penetration and in vivo metabolism of topically applied [ 14 C]azinphosmethyl was greater by resistant moths after a 6-hr incubation. In vitro , metabolism of [ 14 C]azinphosmethyl was greater with homogenates of resistant moths and, for both strains, was influenced by the presence of glutathione but not NADPH. Assay for total nonspecific esterase activity showed significantly higher levels in resistant compared to susceptible strains for both field-collected and laboratory-reared samples. Electrophoretic analysis of esterases indicated that increased activity of many isoenzymes was involved. Assay for glutathione S -transferase activity, using 2,4-dichloronitrobenzene as substrate, also showed significantly higher levels in both field-collected and laboratory-reared resistant moths compared to the corresponding susceptibles. A kinetic study on laboratory-reared strains revealed that in the presence of independently varying concentrations of 2,4-dichloronitrobenzene and glutathione, the V max for both substrates was two fold higher in the resistant strain while the K m 's were similar to those of susceptible moths. High glutathione S -transferase activity in the resistant strain was, therefore, probably due to presence of a greater amount of the enzyme rather than due to a different form. In conclusion, modification of the target site was not a factor in resistance, but increased metabolism was involved.