The Mechanism of Negative Correlation between DDT and Pyrethroid Resistance Found in a Housefly Strain

Abstract

A housefly strain (PR) collected from Pakistan was resistant to p, p'-DDT but supersusceptible to pyrethroids, when compared with a standard susceptible strain (CSMA), demonstrating a negative correlation between DDT and pyrethroid resistance. No significant difference was found between the strains in the rate of cuticular penetration of 14C-DDT, the microsomal degradation of 14C-DDT and the electrophysiological response of the nervous system to DDT. In contrast, the soluble fraction of the PK strain fortified with GSH produced many fold more DDE than that of the CSMA strain, suggesting an increased degradation of DDT by DDT-dehydrochlorinase being responsible for the DDT resistance in the PR strain. With regard to the mechanism of the supersusceptibility of the PK strain to pyrethroids, a comparison of in vivo kinetics of 14C-fenvalerate applied topically implicated a combination of faster penetration and less metabolism of the compound in the PK strain than in the CSMA strain. Based upon the in vitro degradation study, the difference in metabolism was assumed to be due to the cytochrome P450-dependent monooxygenase system in the PK strain, which is less effective in degrading the pyrethroid than in the CSMA strain. The assumption was substantiated by a lower degree of synergism exhibited with piperonyl butoxide in the PK strain as compared to the CSMA strain. Since the negative correlation between DDT and pyrethroid resistance in the PR strain is conferred by independent mechanisms, it is concluded to be of apparent, not intrinsic, nature.