Synergistic insecticidal mechanism of 2-undecanone and decanal against Tribolium castaneum larvae via dual suppression of GST and FDa pathways

Tribolium castaneum (Herbst) is a major stored-product pest, necessitating safer alternatives targeting its destructive larval stage. This study investigated the synergistic insecticidal effects of 2-undecanone and decanal, two active constituents from Houttuynia cordata essential oil, against T. castaneum larvae and elucidated the underlying mechanisms via quantitative real-time PCR (qRT-PCR), RNA interference (RNAi), histopathology, molecular docking, ELISA, and enzyme activity assays. Contact toxicity assays identified the 5:5 mixture as optimally effective, exhibiting significant synergy (R ≥ 1.5) and the lowest LD 50 (14.8 μg/larva). Whereas 2-undecanone alone upregulated farnesol dehydrogenase ( FDa ), glutathione S-transferase ( GST ), and cytochrome P450 ( CYP6BQ7 ), the 5:5 mixture significantly suppressed the expression of FDa and GST (>60 %), accompanied by decreased GST enzyme activity and lower juvenile hormone (JH) III levels. RNAi silencing of FDa and GST confirmed their important roles in larval susceptibility. Molecular docking predicted interactions of both compounds with FDa and GST, implying potential cooperative inhibition. Histopathology revealed more severe midgut damage under the mixture than under either compound alone. In conclusion, the synergy arises from coordinated suppression of the GST-mediated detoxification and FDa-linked JH biosynthesis, offering a promising strategy for developing potent, eco-friendly botanical formulations for stored-product pest management.