Effects of plant flavonoids and other allelochemicals on insect cytochrome P-450 dependent steroid hydroxylase activity.

The plant flavonoids flavone, chrysin, apigenin, kaempferol, morin, quercetin, myricetin and phloretin were found to inhibit in a dose-dependent manner the cytochrome P-450 dependent ecdysone 20-monooxygenase activity associated with adult female Aedes aegypti, wandering stage larvae of Drosophila melanogaster, and fat body and midgut from prewandering and wandering stage last instar larvae of Manduca sexta. The concentrations of these flavonoids required to elicit a 50% inhibition of the steroid hydroxylase activity in all the insects ranged from ca 1 x 10(-5) to 1 x 10(-3) M. In addition, lower concentrations (1 x 10(-6) to 1 x 10(-5) M) of the flavonols kaempferol, morin, quercetin and myricetin significantly stimulated (50-100% above control) M. sexta fat body ecdysone 20-monooxygenase activity. Other plant allelochemicals examined and found to significantly inhibit insect ecdysone 20-monooxygenase activity include corynanthine, quinidine, and quinine; whereas, indican and mimosine were found to significantly stimulate M. sexta fat body steroid hydroxylase activity. Several allelochemicals were without effect at all concentrations tested. Although none of the compounds tested in this study elicited effects at very low concentrations (1 x 10(-9) to 1 x 10(-8) M), the in vitro monooxygenase radioassay does hold considerable promise as a screening tool for the detection and identification of plant allelochemicals which may function as biopesticides affecting insect ecdysteroidogenesis.