Toxicity of Phytochemical Monoterpenes against the Rice Weevil (Coleoptera: Curculionidae)¹

Postharvest insect pests pose a significant threat during product storage, diminishing both the quantity and quality of stored goods. Recently, plant-based pesticides have emerged as a promising alternative for managing storage pests. Plant essential oils are rich in biologically active monoterpenes, which have shown considerable potential as pest control agents due to their toxicity to insects. The purpose of the study was to assess the toxicity of six monoterpene compounds (citral, β-citronellol, eucalyptol, geraniol, β-myrcene, and thymol) against rice weevil, Sitophilus oryzae L. (Coleoptera: Curculionidae). Among the tested monoterpenes, thymol and geraniol exhibited the highest contact toxicity, with 50% lethal concentration (LC₅₀) values of 63.16 and 70.77 μl/cm² against S. oryzae, respectively, at 72 h after treatment, followed by β-citronellol and citral. The fumigant activity of the compounds tested at concentrations ranging between 20.0 and 100.0 μl/ml of air was highest with β-citronellol (LC₅₀ of 22.12 μl/ml of air), followed by citral (LC₅₀ of 32.23 μl/ml of air) at 72 h after exposure. β-Myrcene and geraniol also exhibited fumigant toxicity but at levels lower than β-citronellol or citral. Among the six monoterpenes, geraniol showed highest repellent activity of 90.00% ±1.44%, followed by β-citronellol (75.00% ± 2.88%) and citral (71.75% ± 1.25%) at 4 h of exposure at 7.5 μl/cm². In sum, the six tested monoterpenes exhibited moderate to high toxicity against S. oryzae, suggesting that they have the potential to be developed and used as effective alternatives for managing S. oryzae in stored products.