Selective induction of abamectin metabolism by dexamethasone, 3-methylcholanthrene, and phenobarbital in Colorado potato beetle, Leptinotarsa decemlineata (Say)

Abstract

Oxidative metabolism of abamectin as a principal resistance mechanism in the Colorado potato beetle, Leptinotarsa decemlineata (Say), was validated using dexamethasone (DEX)-, 3-methylcholanthrene (3-MC)-, and phenobarbital (PB)-induced beetles of susceptible (SS) and abamectin-resistant (AB-F) strains compared to non-induced strains, respectively. Significantly decreased mortality to abamectin in DEX- and 3-MC-induced versus non-induced SS beetles indicated that induction by DEX and 3-MC resulted in increased abamectin tolerance. The toxicity of abamectin in PB-induced SS and AB-F beetles compared to non-induced SS and AB-F beetles, respectively, was not significantly different, indicating induction by PB in either strain did not affect abamectin tolerance. Significant synergism by piperonyl butoxide (PBO) in DEX- and 3-MC-induced SS beetles was suggestive of increased oxidative metabolism as a mechanism responsible for the increased tolerance to abamectin. Mortality to abamectin was not significantly synergized by PBO in PB-induced SS beetles, indicating that oxidative metabolism of abamectin is not likely increased by PB induction. Three major metabolites of [ 3H]avermectin B 1a (24-OH, 26-OH (tentative identification), and 3″DM) were produced under both in vivo and in vitro conditions. DEX-induced SS beetles and microsomes prepared from DEX-induced SS beetles produced significantly more 3″DM versus non-induced beetles and microsomes, respectively, indicating that DEX was involved in a specific induction pathway but less involved in production of the two hydroxylated metabolites (24-OH and 26-OH). 3-MC-induced SS beetles produced significantly greater amounts of all three metabolites than non-induced beetles, which suggests that 3-MC was involved in multiple induction pathways. The enhanced production of 3″DM by DEX-induced microsomes was significantly inhibited by PBO and troleandomycin (TAO) in vitro. Because DEX is a specific inducer and TAO is a specific inhibitor of CYP3A family of cytochrome P450 monooxygenases, it appears that this family or a related family of P450s is involved in oxidative metabolism of abamectin to 3″DM and may be responsible for this aspect of resistance. The specific and significant increase in the production of 3″DM by DEX induction and its selective inhibition by TAO may provide an efficient means to obtain sequence of this monooxygenase by differential display or isotope-coded affinity tagging/mass spectrometry techniques.