Reduced body mass and divergent developmental responses under larval competition between acorn‐sharing moths and weevils

Abstract Novel interactions on highly localized resources can strongly reshape developmental trajectories and competitive balances among co‐occurring herbivores. Acorns were found in which oviposition channels drilled by Curculio glandium females contained eggs of Cydia amplana . This unusual phenomenon may lead to direct competition between the two species. Therefore, the outcomes of direct larval competition between acorn weevils ( C. glandium ) and rusty oak moths ( C. amplana ), naturally co‐occurring on Quercus robur , were examined. The identity of C. amplana from C. glandium egg galleries was confirmed using the COI marker. In a competition test, larvae of moths and weevils were kept in single‐species and mixed‐species acorns, and development was assessed. The results showed a significantly reduced body mass in both species from mixed groups compared to single species. Under competition, C. amplana exhibited shorter development (13.2 vs. 16 days), whereas C. glandium developed longer compared to rearing in isolation (15.5 vs. 11.6). The results are discussed in the context of high abundance of C. amplana in a habitat previously unoccupied by this moth. Together, these findings illustrate how new competitive interactions on restricted resources can generate strong developmental shifts with potential consequences for local herbivore assemblages.