Toxicity Assessment of Four Commonly Used Insecticides to the Corn-Infesting Rice Weevil (Coleoptera: Curculionidae)

Rice weevil, Sitophilus oryzae (L.) (Coleoptera: Curculionidae), is one of the most significant pests of corn (Zea mays L.) and other stored grains. Historically, synthetic insecticides have been widely used to control pest populations due to their effectiveness, convenience of storage and application, and persistent activity. This study aimed to assess the toxicity profiles of four commonly used insecticides, including pirimiphos-methyl, deltamethrin, deltamethrin plus (S)-methoprene, and malathion, to rice weevil adults. The weevils were exposed to a range of concentrations of each insecticide to generate concentration–mortality response curves and median lethal concentrations (LC50) of each insecticide. Pirimiphos-methyl exhibited the highest toxicity to the adult weevils, with an LC50 of 0.74 ppm (95% confidence limit [CL], 0.57–0.92). Based on nonoverlapping 95% CLs, we determined that the LC50 of pirimiphos-methyl was significantly lower than the LC50 of malathion (7.08 ppm [95% CL, 5.44–9.01]), deltamethrin (9.11 ppm [95% CL, 6.89–12.6]), and deltamethrin plus (S)-methoprene (13.94 ppm [95% CL, 9.71–18.60]). Thus, the relative toxicity of these four insecticides to adult rice weevils was pirimiphos-methyl > malathion > deltamethrin > deltamethrin plus (S)-methoprene. We subsequently compared the LC50 value of each insecticide with that of the label-recommended application rate and found that only malathion aligned with the recommended label rate. These findings could prove useful in refining application rates for the effective control of rice weevils infesting stored grains.