Christopher J. Greyson-Gaito, Sarah J. Dolson, Glen Forbes, Rosanna Lamb, Wayne E. MacKinnon, Kevin S. McCann, M. Alex Smith, Eldon S. Eveleigh

A major pest of eastern North American forests is spruce budworm, Choristoneura fumiferana Clemens (Lepidoptera: Tortricidae), which outbreaks every 30–40 years and causes large scale tree mortality. Researchers have established that hardwood content reduces the defoliation and mortality of balsam fir and spruces during spruce budworm outbreaks. One mechanism posited to explain these patterns is that hardwood content positively impacts the parasitoids of spruce budworm. Researchers have found that parasitism of spruce budworm by individual parasitoids is impacted by hardwood content. Yet, more research is needed to understand how hardwood content impacts the parasitoid community as a whole. In this study, we used DNA barcoding and stable isotope analysis of Malaise trap sampled parasitoids to examine how hardwood content influenced parasitoid community composition, structure, and trophic interactions. We found that although composition did not significantly differ along a hardwood content gradient, phylogenetic community structure did differ. Furthermore, the trophic relationships between several parasitoids and caterpillars on balsam fir or hardwood trees changed over time. Our study highlights the importance of hardwood trees for spruce budworm dynamics through influencing the parasitoid community.