Alterations in esterases are associated with malathion resistance in Habrobracon hebetor (Hymenoptera : Braconidae)

Biochemical mechanisms of malathion resistance were investigated in a malat hion-resistant strain of the parasitoid Habrobracon hebetor Say collected f rom a farm storage in Kansas. General esterase activities were significantl y lower in the resistant strain compared with those in a susceptible strain . However, no significant differences were found in activities of malathion specific carboxylesterase (MCE),glutathione S-transferase and cytochrome P 450 dependent O-demethylase activities, cytochrome P450 contents, and sensi tivity of acetylcholinesterase to inhibition by malaoxon between the 2 stra ins. Because MCE was not elevated in the resistant strain, the weak malathi on resistance in H, hebetor may result from a different mechanism compared with that hypothesized for some insect species in which reduced general est erase activity is accompanied by an elevated MCE. Decreased. esterase activ ity in the resistant strain suggested that null alleles of some esterases w ere associated, with the resistance. Indeed, E1 and E2, major esterases in the susceptible strain, were not present in the resistant strain on polyacr ylamide gels that were stained for esterase activity using the model substr ate 1-naphthyl acetate. In contrast, the activity of esterase E3 on the gel s was much higher in the resistant strain as compared with that of the susc eptible strain. These findings indicate that malathion resistance in H, heb etor is associated with both an increased activity of the esterase E3 and n ull alleles of the esterases E1 and E2.