ACTION OF ESTERASE-B1 ON CHLORPYRIFOS IN ORGANOPHOSPHATE-RESISTANT CULEX MOSQUITOS

Esterase B l activity in Culex pipiens mosquitoes was strongly inhibited by oxidized organophosphates (OP), but not by nonoxidized forms or by carbamates. Inhibition by chlorpyrifos oxon and paraoxon remained total during the 2 hr following the removal of free insecticide molecules, indicating that hydrolysis by esterase B l is either very slow or absent. This hypothesis was confirmed by comparing the fate of [14C]chlorpyrifos in larvae of strains TEM-R (with the over-produced esterase B1) and MSE (lacking an overproduced esterase). As expected, large quantities of chlorpyrifos oxon were observed in the two strains, but no other metabolite was found in TEM-R. It is concluded that esterase B1 confers resistance at least to diethyl OPs through sequestering rather than metabolism, as is also the case with the overproduced esterase E4 of Myzus persicae. © 1993 Academic Press.