Synergistic Insecticidal Efficacy of the Natural Fumigant Methyl Benzoate Against Phosphine-Resistant Sitophilus oryzae

The widespread emergence of phosphine-resistant populations of stored-product pests, such as the rice weevil (Sitophilus oryzae), poses a significant challenge to effective grain protection. This study investigated the fumigation toxicity of methyl benzoate (MBe), an eco-friendly aromatic ester, both alone and in combination with phosphine against phosphine-susceptible (So-PS) and phosphine-resistant (So-PR) strains of S. oryzae. Molecular analysis identified an asparagine-to-threonine substitution in the dihydrolipoamide dehydrogenase (dld) gene in the So-PR strain, which exhibited a 141-fold higher resistance to phosphine than the So-PS strain of LC50 = 1.13 and 0.008 mg/L, respectively. In contrast, MBe exhibited negative cross-resistance, with greater toxicity toward the So-PR strain (LC50 = 6.10 mg/L) than toward the So-PS strain (LC50 = 10.08 mg/L). Co-fumigation assays based on a Bliss independence model revealed strong synergistic fumigation toxicity across the tested concentration matrix. Combining MBe at the strain-specific LC30 with reduced phosphine concentrations resulted in complete mortality of So-PR adults under the tested conditions. The observed synergy is consistent with the involvement of distinct toxicological targets of the two fumigants, although this study did not directly examine the underlying mechanism. These findings indicate that MBe has potential as a co-fumigant for managing phosphine-resistant S. oryzae and warrants further evaluation under grain-bulk and storage conditions.