Stored-grain insects cause substantial quantitative and qualitative losses during storage, and the rice weevil, Sitophilus oryzae L., remains one of the most damaging pests of cereal commodities worldwide. Because reliance on synthetic grain protectants raises concerns about resistance, residue persistence, and environmental safety, plant-derived essential oils are increasingly being examined as low-risk alternatives. This study evaluated the contact toxicity and repellency of lemongrass essential oil against adult S. oryzae under laboratory conditions. Contact toxicity was assessed at 23.58, 31.44, 39.30, 47.16, and 55.02 µg/cm², and mortality was recorded after 24, 48, and 72 h. Repellency was assessed at 9.82, 19.65, 39.30, 78.60, and 157.95 µg/cm² over 5 h. Toxicity data were analyzed using a binomial generalized linear mixed model (GLMM) and time-specific probit models, whereas repellency data were analyzed using a binomial GLMM. Mortality increased with both dose and exposure time, rising from 0.00% at the lowest dose after 24 h to 76.67% at the highest dose after 48 and 72 h. The GLMM showed a strong positive dose effect on mortality, and the dose-response slope was significantly steeper at 48 and 72 h than at 24 h. Probit analysis showed that LD50 values declined from 49.86 µg/cm² at 24 h to 45.60 µg/cm² at 48 h and 45.27 µg/cm² at 72 h. Repellency was also dose-dependent, with the highest dose producing 60.00%-80.00% repellency over the 5 h observation period. These findings indicate that lemongrass essential oil possesses biologically relevant toxic and repellent activity against S. oryzae and may serve as a promising botanical component for stored-grain pest management, although formulation and storage-scale validation are still required.
