Different insecticides select multiple carboxylesterase isoenzymes and different resistance levels from a single population of Culex quinquefasciatus

Amplification of two carboxylesterases, Estα21 and Estβ21, is the major insecticide resistance mechanism in the field-collected Pel strain of the mosquito Culex quinquefasciatus. Three colonies were selected from the Pel strain with three different insecticides. Both esterases Estα21 and Estβ21 were partially purified from each colony and their biomolecular rate constants (k(a)'s) for the interaction with insecticides were measured. Significant intercolony differences were observed for the k(a) values suggesting that different esterase isoenzymes or mixtures of isoenzymes occur within each colony. This may be due to different proportions of the nonamplified carboxylesterases Estα3 and Estβ13 being present in the predominantly Estα21 and Estβ21 preparations. An altered acetylcholinesterase mechanism, which was present at a very low initial frequency in the parental colony, was also differentially selected and contributed to the differences in resistance in the colonies. Neither glutathione S-transferases nor monooxygenases appear to be involved in this resistance.