EFFECTS OF HOST PLANTS ON THE TOXICITY OF AZINPHOSMETHYL TO SUSCEPTIBLE AND RESISTANT LIGHT BROWN APPLE MOTH (LEPIDOPTERA, TORTRICIDAE)

Responses of resistant and susceptible strains of light brown apple moth, Epiphyas postvittana (Walker), to azinphosmethyl varied significantly depending upon the diet that larvae consumed. Responses of resistant larvae reared on blackberry and susceptible larvae reared on artificial (general purpose) diet were similar. Susceptible larvae on artificial diet were significantly more tolerant compared with susceptible larvae fed any of the natural host plants, whereas resistant larvae reared on blackberry were significantly less resistant than resistant larvae reared on any other diet. Activity of nonspecific esterases of resistant larvae fed blackberry was significantly lower than activities in resistant larvae fed general purpose diet, gorse, apple, or broom, and not significantly different from nonspecific esterase activities of susceptible larvae reared on general purpose diet, gorse, apple, blackberry, or broom. In the presence of high glutathione S-transferase activity that characterized all resistant larvae regardless of diet, depressed nonspecific esterase activity may be responsible for increased toxicity of azinphosmethyl to resistant larvae on blackberry. In resistant strains of other polyphagous species, responses of individuals tested with pesticide may reflect metabolic effects related to the host plant rather than to the magnitude of genetically based resistance to the pesticide.