Mechanisms of resistance to fenthion in Culex pipiens fatigans Wied.

One of the insecticides of choice to control the mosquito Culex pipiens fatigans, a vector of Bancroftian filariasis, is fenthion. The mechanism of resistance to this insecticide which could develop in C. p. fatigans was investigated in a strain from Rangoon, Burma, made 8 times more fenthion-resistant than normal by laboratory selection, by exposing the larvae to (32)P-fenthion and examining the metabolites.Larvae of the resistant strain were found to absorb about half as much fenthion as those of the corresponding normal strain, and they degraded proportionately about twice the amount absorbed to water-soluble metabolites. Thus, the larval content of chloroform-soluble toxicants remaining was only one-third to one-seventh as much in the resistant as in the normal strain. The production of the non-toxic hydrolytic (water-soluble) metabolites was 70% greater in the resistant strain in absolute terms, despite the lower amount absorbed into the larvae. The greatest increase (4-fold) was in the oxonase activity, although the thionase activity was more important in both strains.Esterase zymograms from agar-gel electrophoresis revealed a principal band which was much more intense in the resistant than in the susceptible strain, as judged by its hydrolysis of alpha- and beta-naphthyl acetates and phosphates. This band was comparatively insensitive to the toxicant fenoxon, and could hydrolyse it.