Valles_1996_Pestic.Biochem.Physiol_54_172

Final instar nymphs and adult males of the Village Green strain of German cockroach were investigated to determine the biochemical mechanisms responsible for conferring stage-dependent propoxur tolerance. Propoxur susceptibility was dependent upon age within the final instar. Final instar nymphs (1 day old) were as susceptible to topically applied propoxur as adult males (7 to 14 days old) while 7- and 12-day-old nymphs were up to 16-fold more tolerant. Piperonyl butoxide, a cytochrome P450 monooxygenase inhibitor, almost completely eliminated the tolerance of propoxur in male and female nymphs compared with adult males. The rate ofin vitromicrosomal oxidation of propoxur was three times higher in male and female final instars than in adult males. Nymphs had significantly higher cytochrome P450 content and methoxyresorufinO-demethylase and ethoxyresorufinO-deethylase activities than adult males. Similarki(bimolecular rate constant) values for propoxur in nymphs and adult males indicated that acetylcholinesterase insensitivity did not contribute to the enhanced nymphal tolerance. Cuticular penetration studies using [14C]propoxur revealed that there was no marked difference in the rate of penetration of propoxur in nymphs and adults. However, compared with adult males, less radioactivity was observed in nymph internal extracts and more was observed in nymphal excreta. The results indicated that nymphal tolerance toward propoxur was largely due to enhanced microsomal oxidation.