Fumigant toxicity and enzymatic effects of citronella oil and citronellal on Sitophilus oryzae

Sitophilus oryzae , a major stored grain pest, causes significant quantitative and qualitative losses worldwide. Growing concerns about insecticide resistance and the environmental risks have increased interest in plant-derived biofumigants. This study investigated the toxicity and underlying biochemical and molecular mechanisms of citronella ( Cymbopogon winterianus ) essential oil (CTO) and its major component, citronellal (CTP), against S. oryzae . Gas chromatography–mass spectrometry analysis identified citronellal as the primary compound in CTO. Fumigation bioassays were conducted using CTO or CTP at concentrations of 100-500 µL/L air, with exposure periods ranging from 24 to 120 h. Both CTO and CTP caused mortality in a concentration– and time–dependent manner, with CTP showing significantly higher potency. The LC 50 of CTP was 180.49 µL/L air (95% CI: 151.01–207.56), which was lower than that of CTO at 262.17 µL/L air (95% CI: 231.25–296.55). CTP also produced shorter lethal times across all concentrations. Biochemical assays revealed substantial inhibition of acetylcholinesterase (AChE) and glutathione S-transferase (GST) activity in fumigated insects, with higher inhibition by CTP (AChE: 42.69 ± 0.42%, GST: 40.54 ± 0.13%) than by CTO (AChE: 19.98 ± 0.40%, GST: 15.37 ± 0.20%). Molecular docking and enzymatic assays confirmed a dual-target mechanism for CTP: primary neurotoxic disruption of AChE (-6.0 kcal/mol) coupled with a secondary metabolic impact on GST (-3.9 kcal/mol). Overall, even at lower concentrations, CTP demonstrates strong potential as a biofumigant and an eco-friendly alternative to synthetic insecticides for managing S. oryzae during grain storage.