In vitro Studies on the Mechanism of Pyrethroid Resistance in the German Cockroach

Abstract

A strain (R) of German cockroach originally collected in Ginza, Tokyo, and selected with permethrin in our laboratory showed cross resistance to various pyrethroids and DDT, and moderate resistance to organophosphate and carbamate insecticides as compared to a susceptible (S) strain. In vitro studies on the degradation of ¹⁴C-fenvalerate by subcellular fractions suggested that the major activity was in the soluble fraction and the microsomes, but no significant difference was found in either of these fractions between the S and R strains. Little, if any, difference in the synergistic effect of metabolic inhibitors, DEF and PBO, on the fenvalerate toxicity to the R and S strains supported the conclusion that metabolism is not an important factor for the pyrethroid resistance in this strain. Although in the R strain the content of cytochrome P450 in the microsomes was 2.6-fold larger than in the S strain, it may be due to cytochrome P450 species which are not involved in fenvalerate degradation. Electrophysiological studies on the exposed abdominal nerve cord clearly demonstrated that the nerve system of the R strain was much less sensitive than that of the S strain to all the pyrethroids tested and DDT, indicating that the so-called KDR was a predominant factor for the pyrethroid resistance in this strain.