Effects of Cypermethrin Selection on Expression of Insecticide Resistance Mechanisms in the German Cockroach (Blattaria: Blattellidae)

A cross-resistant strain of German cockroach, Blattella germanica (L.), (Marietta) expressing multiple resistance mechanisms was subjected to selection pressure with cypermethrin. Resistance to cypermethrin increased incrementally from 3.6-fold in the parental strain to 35-fold after 4 rounds of selection. No significant changes were observed in cytochrome P450 content, aldrin epoxidase activity, α-naphthyl acetate hydrolysis (cytosolic fraction), or glutathione S-transferase (CDNB conjugation) activities. Although no significant differences were observed in cypermethrin metabolism, a trend toward greater detoxification among each successive generation was observed. Furthermore, methoxyresorufin O-demethylase activity (often associated with pyrethroid resistance) increased incrementally with each successive selection iteration. The average knockdown time increased incrementally from 37 min (parental strain) to 177 min, and there was a corresponding increase in the kdr allele frequency from 19% to 99% after 4 rounds of selection. The data indicated that kdr-type nerve insensitivity and enhanced metabolism by cytochromes P450 and hydrolases were the principle mechanisms of resistance after selection with cypermethrin.