Interactions between an insect and a fungus pest: how American chestnut is impacted by two exotics

American chestnut, Castanea dentata, was a dominant component of eastern North American forests, until the exotic chestnut blight fungus, Cryphonectria parasitica, functionally eliminated it. Today chestnut is experiencing a resurgence, thanks to American chestnut restoration efforts, and the use of chestnut in mine reclamation and nut production. However, this resurgence is being threatened by a second exotic invader, the Asian chestnut gall wasp, Dryocosmus kuriphilus (Hymenoptera: Cynipidae). The gall wasp causes abnormal shoot and twig growth, prevents normal flowering and nut production, and can kill trees. I evaluated the interactions between the gall wasp and a stem cankering fungus on gall wasp fitness, fungal lesion growth, and host plant productivity in a greenhouse experiment. Potted seedlings were used to evaluate 1) infestation by the gall wasp (N=5), 2) infection by a natural occurring, lesion-forming stem fungus (N=10), 3) gall wasp infestation + fungal infection (N=15), and 4) untreated control (N=8). Seedling height and diameter growth and number of leaves, canker expansion and gall wasp survival were evaluated. Chestnut seedlings with both the fungus and the gall wasp had significantly lower (P=0.03) relative height growth, and the expansion of healing callus on cankers was slower (P=0.004) on galled seedlings. My results suggest that infestation by the Asian chestnut gall wasp compromises the hosts’ ability to defend itself against fungal-induced stem cankers, exacerbating the impact of cankers on seedling fitness. This work contributes to our understanding of evolving interactions of two exotic invaders on a forest tree.