Plant-Derived Isoferulic Acid and RNAi Synergize to Overcome Detoxification Defenses in the Bird Cherry–Oat Aphid (Rhopalosiphum padi)

Sustainable management of insect pests is threatened by insecticide resistance, driving demand for effective, environmentally safe tools. The bird cherry–oat aphid (Rhopalosiphum padi), a major cereal pest, shows resistance to pyrethroids, neonicotinoids, and other insecticide classes. We identified isoferulic acid, a phenolic compound from Prunus padus, as a potent aphicide with lethal and sublethal impacts. Bioassays showed dose-dependent lethality (LC50 = 1.74 mg/L at 48 h), while sublethal exposure shortened adult longevity (17.7 to 10.2 d) and reduced fecundity (81 to 53 nymphs/female). Transcriptomic and enzymatic analyses revealed broad induction of detoxification pathways, with RpLipase consistently upregulated. RNAi-mediated knockdown of RpLipase enhanced susceptibility, lowering the LC50 to 0.85 mg/L and confirming its role in tolerance. Nontarget assays with the predator Harmonia axyridis revealed no acute toxicity. Collectively, isoferulic acid is a promising plant-derived insecticide, and silencing induced detoxification genes enhances its potency, providing a foundation for developing novel resistance-breaking strategies.