Odorant Binding Protein 39 Contributes to the Resistance of Bradysia odoriphaga to Phoxim

Bradysia odoriphaga, a major soil-dwelling pest, has developed substantial resistance to numerous pesticides, severely limiting the efficiency of chemical control. Emerging evidence suggests that insect odorant binding proteins (OBPs) may contribute to insecticide resistance, yet their functional roles remain unclear. Here, we identified BodoOBP39 as a larval-specific and cuticle-enriched OBP in B. odoriphaga, which was significantly upregulated in the phoxim-resistant strain. Ligand-binding assays revealed that recombinant BodoOBP39 exhibited the highest binding affinity for phoxim. Three-dimensional modeling and site-directed mutagenesis demonstrated that single mutations (T59A, I62A, and S120A) and the double mutation T59A/S120A all substantially diminished phoxim-binding affinity, validating their critical roles. BodoOBP39 expression was further induced by sublethal phoxim exposure, and RNAi-mediated knockdown increased larval susceptibility to phoxim, suggesting its functional involvement in resistance. These findings advance our understanding of resistance evolution and propose BodoOBP39 as a promising molecular target for developing novel control strategies to manage B. odoriphaga.