Mechanisms responsible for propoxur resistance in the german cockroach, Blattella germanica (L.)

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Abstract
The mechanisms of propoxur resistance were studied in a strain (Baygon‐R) of the German cockroach (Blattella germanica (L.)) displaying high levels of resistance by topical application compared to a susceptible laboratory strain (CSMA). The penetration of [14C] propoxur was significantly slower in the resistant strain, and resistance was reduced 3.4‐fold when the insecticide was injected compared to topically applied, implicating decreased cuticular penetration as a resistance mechanism. In‐vivo metabolism studies employing [14C]propoxur indicated that propoxur was metabolized at a faster rate in the resistant strain. Resistance to propoxur was partially suppressed by both piperonyl butoxide and S,S,S‐tributyl phosphorotrithioate suggesting the involvement of both cytochrome P450‐dependent mono‐oxygenases and hydrolytic enzymes in the resistance mechanism. In vitro, the Baygon‐R strain displayed higher rates of NADPH‐dependent microsomal metabolism of [14C]propoxur associated with the formation of o‐hydroxy propoxur and N‐hydroxymethyl propoxur, further implicating the monooxygenase system in the resistance. In‐vitro metabolism of [14C]propoxur with cytosolic fractions indicates that the substrate is lost from the incubation mixture at a higher rate in the Baygon‐R strain, possibly due to hydrolytic enzymes that result in production of expired [14C]carbon dioxide. The bimolecular inhibition constants for propoxur against acetyl‐cholinesterase from resistant and susceptible strains were not different. These results indicate that increased levels of metabolism together with decreased rates of penetration are mainly responsible for resistance to propoxur in this strain.