The Lesser of Two Weevils: Differential Susceptibility of Chinese–American Chestnut Hybrids to Curculio sayi

The American chestnut (Castanea dentata (Marsh.) Borkh.) was a foundation species whose loss reshaped eastern North American forests. Ongoing breeding with blight-resistant Chinese chestnut (Castanea mollissima Blume) represents the leading strategy for its restoration. However, breeding programs have focused almost exclusively on pathogen resistance, leaving susceptibility to native seed predators unexamined—a critical gap, because nut production underpins the ecological function that restoration seeks to recover. Here, we investigate how hybridization level affects infestation by the lesser chestnut weevil (Curculio sayi (Gyllenhal, 1836)), monitoring 25 genetic lines across four genetic categories (Chinese, F1, backcross, and American) for larval emergence, weight loss, damage, and emergence timing over eight weeks. Hybridization dramatically altered susceptibility: F1 hybrids were the most susceptible category. No larval emergence was observed from American chestnuts, and backcross hybrids remained substantially susceptible despite three–four generations of backcrossing. These results expose a fundamental trade-off for restoration: blight resistance derives from the Chinese genome, whereas weevil resistance appears to be associated with the American genome. Backcross lines bred for blight resistance retain weevil susceptibility, and F1 hybrids risk functioning as pest sinks that amplify local weevil populations. Incorporating weevil resistance screening into breeding programs could help us to ensure that restored chestnuts can fulfill their historical role as mast-producing foundation trees.