Characterization of esterases in malathion-resistant and susceptible strains of the pteromalid parasitoid Anisopteromalus calandrae

General esterase, malathion-specific carboxylesterase, phosphotriesterase, glutathione S-transferase, cytochrome P-450-dependent monooxygenase activit y, and target site sensitivity were compared in malathion-resistant (R) and malathion-susceptible (S) strains of the parasitoid Anisopteromalus caland rae (Howard) (Hymenoptera: Pteromalidae). Activity against alpha-naphthyl a cetate was not significantly different in male and female wasps for either strain. General esterase activity ranged from 1.2-fold to 2.5-fold higher i n the R strain compared with the S strain, but these differences between st rains were not consistent. Based on V-max/K-m ratios estimated for a number of analogs of four substrates (alpha-naphthyl acetate, beta-naphthyl aceta te, 4-methylumbelliferyl acetate, and p-nitrophenyl acetate) there was no e vidence that general esterase activity was elevated or reduced in the R str ain. Malathion-specific carboxylesterase (MCE) activity, determined by usin g 2,3-C-14-malathion as substrate, was 10- to 30-fold higher in the R strai n compared with that in the S strain. The MCE. has a pH optima at about pH 7, is cytosolic, and is labile upon storage at - 80 degrees C. MCE activity could be recovered from native 10% PAGE gels and IEF-PAGE gels (pI = 5.2), but the peak of MCE activity also contained the major peak of activity aga inst cy-naphthyl acetate. There was no evidence for major involvement of ph osphotriesterase, glutathione S-transferase, monooxygenase, or altered acet ylcholinesterase in the resistance. These data suggest that an increased ac tivity of a MCE in the R strain is the probable major mechanism conferring resistance to malathion in A. calandrae. This study provides the first char acterization of a biochemical resistance mechanism in a parasitoid with a h igh level of resistance to an organophosphate insecticide. (C) 1998 Elsevie r Science Ltd. All rights reserved.