| Summary: | Herbivore-induced plant volatiles (HIPVs), chemicals produced by plants infested by herbivorous insects, can act as kairomones that recruit natural enemies of the pest herbivore. <i>Agrotis segetum</i> (Denis and Schiffermüller) is a common, important pest of seedling cotton in Xinjiang Province, China, and the braconid <i>Microplitis mediator</i> (Haliday) is an important mortality factor of this pest’s larvae. In olfactometer tests, which included healthy foliage, infested foliage, or infested roots, <i>M. mediator</i> preferred <i>A. segetum</i>-infested cotton plants to healthy cotton plants. In GC-MS analyses of plant-emitted volatiles, we found that compounds emitted increased 14.9- and 13.3- fold after leaf infestation and root infestation, respectively, compared to healthy control plants. The volatiles were mainly p-xylene, nonanal, tetradecane, decanal, benzaldehyde, β-caryophyllene, and humulene, while linalool was only present in the leaf-infestation treatment. In addition, principal component analysis indicated that all 18 compounds were associated with the infested plants, especially β-caryophyllene, p-xylene, and decanal. Based on the above studies and previous functional evaluations of the volatile compounds, it can be demonstrated that these compounds play a crucial role in modulating the interactions between <i>A. segetum</i> and <i>M. mediator</i> and regulating parasitoid behavior. It may be possible to enhance the biological control of <i>A. segetum</i> by <i>M. mediator</i> through the application of HIPVs.
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