Acetylcholinesterase activity in organophosphorus and carbamate resistant and susceptible strains of the Culex pipiens complex

Abstract

In vitro studies of acetylcholinesterase (AChE) activity and inhibition by insecticides have been carried out on larvae of four strains of mosquitoes of the Culex pipiens complex, two of which (MSE, DAR D ) were resistant to both carbamates and organophosphorus (OP) compounds, one (RANGOON) resistant only to OPs and the other (CfCA) susceptible to both. Activity of AChE in MSE was substantially lower than in CfCA and DAR >D while in RANGOON it was intermediate. The AChE of MSE was much less sensitive than other strains to inhibition by the carbamates propoxur and carbaryl and the OPs paraoxon and fenthion ( I 50 ratios with CfCa: >1250, >500, >10000, >11.1). Resistance to propoxur in MSE and DAR D was high (LC 50 ratios with CfCa: MSE >667; DAR D = 2133 ) but resistance in both strains was unstable and declined when laboratory selection was relaxed. It is concluded (a) that resistance to carbamates and OPs in larvae of the MSE strain is associated with a mutant form of AChE of broad insensitivity, (b) that resistance to OPs in RANGOON is possibly associated with a change in AChE giving specific insensitivity to fenthion, and (c) that resistance to carbamates and OPs in DAR D is not explained by a change in AChE. The basis of resistance in these strains is discussed in relation to metabolic studies and the pattern of insecticide use in the field.