Multiple down-regulated cytochrome P450 monooxygenase genes contributed to synergistic interaction between chlorpyrifos and imidacloprid against Nilaparvata lugens

Insecticide mixtures are an effective strategy in pest resistance management. The synergistic chlorpyrifos and imidacloprid mixture could significantly increase toxicity against rice pest, Nilaparvata lugens, despite their high levels of resistance. However, synergism mechanisms to explain this phenomenon remain unknown. Chlorpyrifos and imidacloprid at a 1:0.5 ratio showed significant synergism on N. lugens with a combination index value of 0.18 after topical exposure. We constructed a genetic database of the genes expressed in individual and synergistic chlorpyrifos and imidacloprid treatments of N. lugens using Illumina Hiseq (TM) X Ten, and 17 co-downregulated genes putatively involved in synergism were detected by comparative transcriptome analyses. Expression patterns of the 17 candidate synergistic genes matched with transcriptome sequencing data by quantitative real-time PCR analyses. Feeding of dsRNAs further reduced the expression levels of 10 of these candidate synergistic genes (from 1.68 to 4.13-fold). Nymphs fed with only dsRNAs of CYP4DE1, CYP6AY1v2, CYP353D1, and CYP439A1 experienced more high mortality rates (81.45-90.34%) to improve synergism between chlorpyrifos and imidacloprid. Multiple reductive expressed P450 genes were potentially associated with synergism of a mixture of chlorpyrifos and imidacloprid, as confirmed by comparative transcriptome analyses and RNAi assays. Our findings suggested that synergistic interactions between chlorpyrifos and imidacloprid might be controlled by P450s.