Epicuticular Wax Composition and Wax Crystal Microstructure Shape Oviposition Preference in the Diamondback Moth, Plutella xylostella, on Cabbage

The diamondback moth Plutella xylostella is a major pest of cabbage and exhibits distinct host preferences among cabbage cultivars. Glossy cabbage cultivars, characterized by reduced epicuticular wax crystal coverage on the leaf surface, are generally more susceptible to oviposition by females than waxy cultivars with normal wax bloom. However, the roles of wax components in shaping oviposition behavior remain unclear. We examined this issue using the waxy cultivar 'Kinkei 201 (Kinkei)' and the glossy cultivar 'YR Yuryoku (YR)'. In two-choice tests, females significantly preferred YR over Kinkei for oviposition. Kinkei leaves were densely covered with well-developed epicuticular wax crystals dominated by n-nonacosane (nC29), whereas YR leaves had sparse, granule-like wax crystals dominated by triacontanal (C30al). Mechanical removal of wax crystals significantly increased oviposition on Kinkei leaves but not on YR leaves. Application of 0.1-10 mg nC29 to wax-removed Kinkei leaves significantly suppressed oviposition and, at higher doses, induced dense wax structures across the leaf surface. Similarly, application of 1 mg nC29 to intact YR leaf surfaces increased nC29 coverage and significantly reduced oviposition. In contrast, application of C30al to wax-removed YR leaf surfaces produced sparse wax structures and did not affect oviposition. These results suggest that oviposition deterrence is associated with dense nC29-associated wax structures rather than simply with the presence of epicuticular wax itself. Differences in oviposition preference between the two cabbage cultivars may reflect variation in both epicuticular wax composition and wax crystal microstructure, although their relative contributions remain unclear.