A genomic approach to investigating avermectin resistance in the diamondback moth, Plutella xylostella L. (Lep: Plutellidae)

BACKGROUND: Plutella xylostella (L.) causes a severe damage to crop production worldwide. This study aimed to elucidate the mode of inheritance, cross-resistance, mechanisms and synergism of emamectin benzoate resistance in a field-derived multi- resistant Japanese population of P. xylostella. Furthermore, the evolution of avermectin resistance was modelled under insecticide rotation and mixture scenarios to examine which of these two strategies are more effective in slowing resistance development and inform future insecticide resistance management strategies. RESULTS: A field collected resistant strain of P. xylostella from Japan was established in the laboratory and repeatedly exposed to emamectin benzoate - however, selection did not considerably affect the resistance levels, with the level of emamectin resistance in the Aichi-R non selected (Aichi-R-ns) and selected populations both >230-fold compared with the susceptible reference strain. Genetic analysis showed that resistance in this strain is inherited as an autosomal, incompletely recessive trait, and is conferred by a single or a few closely linked loci. Aichi-R-ns was found to be resistant to emamectin benzoate, abamectin, lepimectin, chlorantraniliprole, lufenuron, spinetoram, indoxacarb, fipronil, dieldrin, endosulfan and lambda-cyhalothrin, demonstrating the strain’s remarkable multi-/cross-resistance profile. Finally, piperonyl butoxide (PBO), a P450 inhibitor, increased the toxicity of emamectin benzoate in the resistant strain by 10-fold, suggesting the involvement of P450s in resistance. Cloning and sequencing of the GluCl channel revealed the absence of target-site mutations and NGS analysis revealed upregulation of P450 genes that may contribute to metabolic resistance. CONCLUSIONS: No significant effect on resistant levels due to selection in the laboratory may be an indication of a high initial resistance allele frequency. A clear cross-resistant phenotype to abamectin and lepimectin was confirmed in our emamectin benzoate resistant strain. No target-site mutations were detected in the GluCl channel, the primary site of action of avermectins. Synergism studies, NGS analysis and qPCR validation in the near-isogenic resistant strain indicate the potential involvement of cytochrome P450 monooxygenases in avermectin resistance. Keywords: Plutella xylostella; avermectins; resistance; inheritance; GluCl; cross- resistance; synergis