Clove and Lemongrass Essential Oils as Repellents Against Halyomorpha halys: From Behavioural Responses to Molecular Interactions and Controlled Release

Halyomorpha halys is a highly polyphagous invasive pest whose management still relies heavily on broad-spectrum insecticides, creating a need for more sustainable behaviour-based alternatives. This study evaluated clove leaf and lemongrass essential oils (EOs) as potential repellents against adult H. halys by integrating behavioural bioassays, controlled-release formulations, molecular modelling and in silico ecotoxicological assessment. Male and female adults were tested in a two-choice Y-tube olfactometer; the individual EOs were encapsulated in calcium-alginate microspheres, and selected constituents were assessed using molecular docking and computational ecotoxicity models. Clove EO, lemongrass EO and their binary combination induced strong avoidance responses in both sexes, with pooled avoidance ranging from 90.0% to 96.7% and repellency indices from 0.80 to 0.93. No significant differences were detected among the three EO treatments when responses were pooled across sexes (χ2 = 2.14, p = 0.343). Both EOs were successfully incorporated into calcium-alginate microspheres, with encapsulation efficiencies exceeding 90% (91.57 ± 0.67% for clove EO and 96.42 ± 0.29% for lemongrass EO). Lemongrass EO showed higher encapsulation efficiency and loading capacity and reached near-complete cumulative release, whereas clove EO release plateaued at approximately 70%. Phylogenetic, structural and physicochemical analyses supported the selection of H. halys HhOBP21 for comparative molecular modelling with the structurally characterised Drosophila melanogaster DmOBP28a. Molecular docking indicated that selected EO constituents could be accommodated within the binding cavities of these OBPs, with β-caryophyllene, α-humulene and limonene among the most favourably scored ligands, suggesting potential interactions with these proteins. Predicted acute toxicity values for the standard non-target test organism Tetrahymena pyriformis, Daphnia magna and Pimephales promelas remained below the applied thresholds for all evaluated compounds, although several constituents showed elevated predicted bioconcentration potential and limited biodegradability. Overall, the results demonstrate strong avoidance responses to the tested EOs under laboratory Y-tube conditions and support their further evaluation in semi-field and field studies to determine whether these responses translate into spatial repellency under practical conditions. Experimental validation of predicted molecular interactions and environmental fate is also needed.