American chestnut (Castanea dentata[Marsh.]Borkh.) was once the dominant hardwood species in Eastern North America before an exotic fungal pathogen,Cryphonectria parasitica (Murrill) Barr,functionally eliminated it across its range.One promising approach toward restoring American chestnut to natural forests is development of blight-tolerant trees using genetic transformation.However,transformation and related processes can result in unexpected and unintended phenotypic changes,potentially altering ecological interactions.To assess unintended tritrophic impacts of transgenic American chestnut on plant-herbivore interactions,gypsy moth (Lymantria dispar L.) caterpillars were fed leaf disks excised from two transgenic events,Darling 54 and Darling 58,and four control American chestnut lines.Leaf disks were previously treated with an LD50 dose of either the species-specific Lymantria dispar multiple nucleopolyhedrovirus (LdMNPV) or the generalist pathogen Bacillus thuringiensis subsp.kurstaki (Btk).Mortality was quantified and compared to water blank controls.Tree genotype had a strong effect on the efficacies of both pathogens.Larval mortality from Btk-treated foliage from only one transgenic event,Darling 54,differed from its isogenic progenitor,Ellis 1,but was similar to an unrelated wild-type American chestnut control.LdMNPV efficacy was unaffected by genetic transformation.Results suggest that although genetic modification of trees may affect interactions with other nontarget organisms,this may be due to insertion effects,and variation among different genotypes (whether transgenic or wild-type) imparts a greater change in response than transgene presence.