White-tailed deer, Odocoileus virginianus, have induced direct and indirect changes in hardwood forest ecosystems by over-browsing. As a result of browsing, direct influences include significant changes in the composition of the understory seedling and sapling composition, abundance and diversity. Although direct effects of white-tailed deer browsing are well studied indirect effects of herbivory are not well understood. From 1979-1990, deer density was manipulated in regions of Allegheny hardwood forests in NW Pennsylvania and ultimately resulted in significant changes in understory composition. Roughly thirty years after the initial research indirect effects of deer browsing are being studied, in particular insect composition, abundance and diversity. We hypothesized that tree species most impacted by deer would have the greatest insect density because these species are likely to be poorly defended by plant chemicals. In 2009, we sampled insects from 261 branch tips of the most common tree species using a pole-mounted tree pruner and collection bag. Bags were sealed and brought to the lab, where all arthropods were collected, counted, identified, photographed using a calibrated digital camera and preserved for identification. Leaves from each branch were removed, dried and weighed and caterpillar abundance was standardized to number per kg leaf mass. Differences in caterpillar density among tree species were analyzed with general linear mixed models in SAS 9.1.
Results/Conclusions
Dominant tree species were red maple (Acer rubrum), sugar maple (Acer saccharum), striped maple (Acer pensylvanicum), sweet birch (Betula lenta), yellow birch (Betula alleghaniensis), American beech (Fagus grandifolia), pin cherry (Prunus pensylvanica), and black cherry (Prunus serotina). Tree species significantly (P = 0.0056) differed in mean caterpillar density per kg foliage. Pin cherry supported over five times the number of caterpillars as black cherry, the species with the next-highest caterpillar density. Caterpillar density on black cherry did not significantly differ from density on other tree species. These results are consistent with the hypothesis that plants most impacted by deer would have the highest caterpillar density. Data collected in summer 2010 will determine if insect communities that occur on different tree species differ in composition and if the relative proportions of host generalists and specialists differ on each tree species. Results of this study will enable a better understanding of the indirect effects deer have on insect populations in forest ecosystems that were previously exposed to white-tailed deer over-browsing.