Volatile Compounds Emitted by Ocimum basilicum Repel Grapholita molesta , Reducing the Egg-Laying Activity and Apple Fruit Infestation Rate

Grapholita molesta, which causes fruit infestation damage, produces overlapping generations, making chemical control ineffective. Excessive pesticide use causes the “3R” problem, necessitating eco-friendly alternatives. Although Ocimum basilicum repels many pests, its effects on G. molesta are unclear. Olfactory, fruit infestation, and oviposition tests showed that O. basilicum repelled G. molesta and significantly reduced fruit infestation and egg laying. To identify the repellent compounds in O. basilicum, volatile compounds were extracted using headspace adsorption. Gas chromatography–mass spectrometry and gas chromatography–electrophysiological detection identified four key components. Electroantennogram and behavioral assays showed that 6-methylhept-5-en-2-one (10 μL/mL), 2-ethyltoluene (0.1 μL/mL), styrene (0.1 μL/mL), and ethyl carbonate (1 μL/mL) mediated the repellent effect, significantly reducing G. molesta fruit infestation and egg production. 6-Methylhept-5-en-2-one was most effective, decreasing fruit infestation in the field and lab and total egg production. This study reveals the repellent mechanism of O. basilicum volatiles, supporting plant-based repellent development for sustainable pest control.