Enhanced fumigant toxicity of methyl benzoate and phosphine combination against phosphine-resistant Rhyzopertha dominica (Fabricius) (Coleoptera: Bostrichidae)

The rapid evolution of phosphine resistance in stored-product beetles necessitates novel fumigants. We assessed the fumigant activity of methyl benzoate (MBe, a plant-derived ester) against phosphine-susceptible (PS) and phosphine-resistant (PR) Rhyzopertha dominica adults. Adults from both strains were exposed to varying concentrations of phosphine (0–20 mg/L) and MBe (0–30 mg/L) for 24 h in 10 L glass desiccators. The LC₅₀ values for phosphine were 0.0106 mg/L (PS) and 1.58 mg/L (PR), demonstrating a 149-fold increase in the PR strain's resistance. In contrast, the LC₅₀ values for MBe were 10.12 mg/L (PS) and 12.04 mg/L (PR), suggesting no cross-resistance. Near complete Mortality (>99 %) at 18 mg/L MBe was observed in both strains. Notably, co-fumigation treatments revealed a strong synergistic effect. A sublethal dose of MBe, 6 mg/L, alone caused 0 % mortality in PR adults, but when combined with phosphine at 1.4 mg/L (phosphine's LC₃₀), mortality increased to 96.0 %. Likewise, 0.50 mg/L of phosphine alone caused 0 % mortality, but when combined with MBe at 10.8 mg/L (MBe's LC₃₀), this concentration induced 100 % mortality. MBe is an effective resistance-breaking fumigant and a compatible synergist for phosphine, offering a practical option for resistance management in stored-grain IPM. Moreover, its synergistic interaction with phosphine suggests that MBe can be effectively integrated into resistance management strategies, enhancing the control of resistant pest populations in stored grain environments.