Monooxygenase, Esterase, and Glutathione Transferase Activity Associated with Azinphosmethyl Resistance in the Tufted Apple Bud Moth, Platynota idaeusalis

Azinphosmethyl-selected tufted apple bud moths were compared to susceptible and reverted strains with respect to possible metabolic mechanisms of resistance within the third instar, fifth instar, and adults. Based upon bioassays conducted by topical application with azinphosmethyl, LD50s were as high as 867-fold in the selected strain as compared to that seen in susceptible bud moths. The LD50 of the reverted strain was intermediate to that of the susceptible and selected insects at all stages studied. Glutathione transferase activity measured with 1-chloro-2,4-dinitrobenzene was elevated in the selected strain 1.6- and 2.2-fold as compared to third and fifth stadium susceptible bud moths, respectively. No consistent strain differences were noted for 1,2-dichloro-4-nitrobenzene. Cytochrome P450 content and P450 mRNA was not significantly different in fifth instars of the susceptible and selected strain. However, there was a 2.7- and 1.9-fold increase in benzphetamine and p-nitroanisole metabolism, respectively, in the guts of azinphosmethyl-selected fifth instars. Benzo[a]pyrene metabolism was elevated 2.4-fold in the carcass of selected bud moths and no differences were noted for methoxyresorufrin in either gut or carcass. Susceptible fifth instars demonstrated a reduced rate of metabolism of azinphosmethyl to the oxon and methyl benzazamide. Piperonyl butoxide failed to synergize azinphosmethyl toxicity. Esterase activity measured with 1-naphthyl acetate and p-nitrophenyl acetate was elevated in selected larvae compared to that seen in susceptible tufted apple bud moths in both larvae and adults. This increased esterase activity was attributed to several isoforms as resolved by analytical isoelectric focusing. One of these forms was consistently overexpressed in all of the life stages examined. Pretreatment of selected fifth instars with S,S,S-tributylphosphorotrithioate increased the toxicity of azinphosmethyl 400-fold and had minimal effect on toxicity in susceptible insects. It appears that multiple hydrolases are the primary metabolic factor in azinphosmethyl resistance in the tufted apple bud moth.