Selection with azinphosmethyl influences glutathione S-transferase activity in the light brown apple moth, Epiphyas postvittana (Lepidoptera: Tortricidae)

Abstract

Light brown apple moth genotypes were sampled from the field by allowing wild males to cross with tethered females and by collecting mated wild females. The progeny were used to estimate the phenotypic frequency of azinphosmethyl resistance from the two sources: susceptible (laboratory) female × unknown genotype (wild) male and unknown genotype (wild) female × unknown genotype (wild) male. The crosses occurred within a known zone of azinphosmethyl resistance and the offspring were screened with diagnostic insecticide concentrations to determine the frequency of susceptible × susceptible crosses in the population. The frequency of resistant male parents within the population was estimated as 83% ( n = 18) from the first type of cross, while the frequency of resistant male and/or female parents was estimated as 100% ( n = 17) from the second type of cross. The offspring were also selected with azinphosmethyl at the LC 99 of susceptibles, and the survivors were compared with their unselected siblings for glutathione S-transferase activity. Adults, selected as larvae, had significantly higher glutathione S-transferase levels than their unselected siblings ( P < 0.05) and this resulted from a positive shift in the normal distribution of log glutathione S-transferase activity. This could not be explained by the removal of susceptible individuals with the insecticide or by induction of the enzyme by insecticide treatment. Insertion of the resistance into a susceptible genome through five generations of backcross and selection with azinphosmethyl resulted in significantly higher glutathione S-transferase activity in larvae of the resistant strain compared to the susceptible strain which shared a ca. 97% common genome.