A novel potential nAChR variation and the upregulation of CYP304A1 contribute to resistance against lambda-cyhalothrin and spinosad in Bactrocera dorsalis

Oriental fruit fly, Bactrocera dorsalis (Hendel), is a vital quarantine pest worldwide. Currently, the application of chemical insecticides is still the primary measure to prevent and control B. dorsalis. However, the frequent and extensive use of insecticides contributes to the evolution of insecticide resistance. To investigate the mechanism of insecticide resistance of B. dorsalis, we initially detected the susceptibility of 19 typical insecticides using the residue film bioassay. The results indicated that spinosad and lambda-cyhalothrin (λ-cyhalothrin) exhibited high toxicity. Transcriptome analysis showed that 6 genes related to detoxification metabolism were differentially expressed between laboratory and λ-cyhalothrin resistant population. RNA interference (RNAi) was performed to verify the function of one differentially expressed gene, CYP304A1, and its association with metabolic resistance of B. dorsalis to spinosad and λ-cyhalothrin was confirmed. Moreover, we discovered a novel nonsynonymous mutation, I345V, in the exon of the nicotine acetylcholine receptor (nAChR) subunit alpha 9 gene, which might be the potential target of spinosad and λ-cyhalothrin. There was a strong correlation between the allele frequency of the I345V mutation and the resistance level of spinosad (r2 = 0.912) or λ-cyhalothrin (r2 = 0.889). These findings indicated that up-regulated CYP304A1 and I345V mutation of nAChR were associated with spinosad and λ-cyhalothrin resistance of B. dorsalis. The results provide a new perspective for in-depth research on the resistance mechanism of B. dorsalis to spinosad and λ-cyhalothrin. This study also provides insights for developing a new molecular detection method for monitoring resistance to these two insecticides.