The efficacy of acetylcholinesterase in organophosphorus and carbamate resistance in Culex pipiens L. from Italy

In response to intensive use of organophosphorus insecticides against mosquitoes, resistance in Culex pipiens from Lucca, Italy, has increased, largely thwarting control efforts. In 1985, the field population was segregating for both altered acetylcholinesterase and elevated nonspecific esterase-based resistance mechanisms. Strains colonized in the laboratory at this time were selected for resistance or susceptibility to organophosphorus and carbamate insecticides. Microtiter plate assay analysis of genotypes was used to identify when strains had reached homozygosity for the altered acetylcholinesterase (AChE) mechanism of insecticide resistance. In Italian C. pipiens, the alteration in the enzyme, AChE, associated with resistance had no significant effect on either the Km or Vmax values with the substrate acetylthiocholine iodide. In vitro measurement of bimolecular rate constants showed that the altered form of AChE was more insensitive to inhibition by certain insecticides, than the AChE variant in the line selected for susceptibility. The cross-resistance conferred by this mechanism to a number of organophosphorus and carbamate insecticides was determined by bioassay, the contribution of nonspecific esterase activity to insecticide resistance being quantified by use of a nonspecific esterase inhibitor. Altered AChE conferred greater resistance to carbamate than to organophosphorus insecticides, the contribution of nonspecific esterase activity being greater in resistance to the latter. Data indicated that a third resistance mechanism may be present in this population conferring moderate levels of resistance to pirimiphos methyl and fenitrothion.