Scharf_1997_Pestic.Biochem.Physiol_59_67

Insecticide resistance in the German cockroach can be mediated by a number of mechanisms, the most common being enhanced enzymatic metabolism. Changes in metabolic enzymes following insecticide selection of a hybrid German cockroach strain (JM) were investigated. The JM strain originated from matings of the Munsyana (multiple resistance) strain and Johnson Wax (susceptible) strain. Two subpopulations of the JM strain were selected over three generations: one with a pyrethroid insecticide (cypermethrin) and one with an organophosphate insecticide (chlorpyrifos). Significant increases in resistance levels were observed in the first generation following selection. These trends continued for three generations and resistance levels to each insecticide approached those of the original Munsyana strain by the third generation of selection. Cypermethrin selected for cross-resistance to chlorpyrifos; however, chlorpyrifos selected for negative cross-resistance to cypermethrin. Increased body weight was weakly but positively correlated with increases in insecticide resistance to each insecticide. Examinations of metabolic enzymes indicated no change in total esterase (1-NA, 2-NA, and PNPA hydrolysis) or glutathione S-transferase (DCNB and CDNB conjugation) activity. Native PAGE of esterases identified one electromorph (E2) which stained more intensely following chlorpyrifos selection and which was inhibited by both chlorpyrifos and chlorpyrifosoxon. Examination of cytochromes P450 by carbon monoxide difference spectra indicated that each insecticide selected for increases in total cytochrome P450 content. However,in vitrodemethylation and Western analysis results suggest that different cytochrome P450 isoforms were selected by each insecticide.