Age-specific expression of a P450 monooxygenase (CYP6CM1) correlates with neonicotinoid resistance in Bemisia tabaci

The whitefly, Bemisia tabaci (Hemiptera: Aleyrodidae), is an invasive insect pest of agriculture and horticulture worldwide. The neonicotinoid insecticides, which target nicotinic acetylcholine receptors in post-synaptic nerve membranes, play a major role in controlling B. tabaci although the increasing incidence of resistance to these compounds is now undermining their effectiveness. Neonicotinoid resistance has been shown to be caused by up-regulation of a cytochrome P450-dependent monooxygenase gene, CYP6CM1, involved in insecticide detoxification. The main aim of this study was to further investigate the finding that expression of resistance to the neonicotinoid imidacloprid is largely restricted to adults of B. tabaci, whereas immature stages retain susceptibility. Imidacloprid resistance was first characterised in four strains of B. tabaci (B- and Q-biotypes) using bioassays, biochemical and molecular-based techniques. A relative gene expression assay was then developed to measure mRNA levels of CYP6CM1 in whitefly adults, pupae and nymphs. We show that CYP6CM1 expression correlated with age-specific imidacloprid resistance in adults and nymphs from each strain. Furthermore, in adult whiteflies, the up-regulation of this gene was correlated with imidacloprid metabolism into a hydroxy metabolite as determined by liquid chromatography-mass spectrometry (LC-MS). These findings strengthen the involvement of CYP6CM1 imidacloprid detoxification and suggest that transcriptional changes in CYP6CM1, possibly constrained by the role of P450 enzymes in insect development, are the cause of an age-specific resistance phenotype in B. tabaci.