Contact Toxicity, Grain-Protective Efficacy, and Behavioral Effects of Methyl Benzoate and Acetophenone Against Four Major Stored-Product Insect Pests

Although the fumigant activity of the naturally occurring compounds methyl benzoate and its analogues has been extensively evaluated against stored-product insect pests, their contact toxicity when applied as residual surface sprays and grain protectants, as well as their repellent effects, remains poorly studied. The current study evaluated the contact toxicity of methyl benzoate (MB) and a related natural product, acetophenone (AP), on glass surfaces and on wheat grains, along with their repellent activity, against adults of the lesser grain borer, Rhyzopertha dominica, the red flour beetle, Tribolium castaneum, the rice weevil, Sitophilus oryzae, and the khapra beetle, Trogoderma granarium. Both compounds were significantly toxic as residual films on glass surfaces, with S. oryzae and T. granarium being more susceptible to both compounds than R. dominica and T. castaneum. Dose-mortality studies found that the LC50 values for MB were 0.265, 0.233, 0.257, and 0.149 mg/cm2, while those for AP were 0.328, 0.156, 0.233, and 0.126 mg/cm2 for R. dominica, S. oryzae, T. castaneum, and T. granarium adults, respectively. Furthermore, both compounds were highly toxic to all insect species on wheat grain, achieving complete adult mortality at 2.0 g/kg after 3 days of exposure, with no adults emerging (0.0 adults) after 65 days of treatment and with no reduction in wheat grain weight (0.0% loss). However, MB and AP exhibited negligible to weak repellent or attractive effects toward all tested insect species, even at the highest concentration tested (50 µg/cm2). Overall, these findings suggest that MB and AP could be viable candidates for stored-product protection as residual applications and may serve as alternatives to, or reduce reliance on, synthetic insecticides.