Climate change drivers such as drought are predicted to have important implications for forest ecosystems, including the interactions between insect herbivore pests and their host trees. These relationships are further complicated for outbreak defoliators because the consequences of drought may vary with respect to herbivore density. To explore these relationships, we developed the Defoliator Response Integrated Temperate Rainfall Exclusion Experiment (DRI-TREE), a manipulative forest drought experiment established in Atlantic Canada to study changes in feeding by the spruce budworm (Choristoneura fumiferana Clemens) on its primary host, balsam fir (Abies balsameaL.). A series of rainout tarps were installed 2 meters from the base of each tree to divert throughfall rain water and induce tree drought. In the canopy of each tree we enclosed spruce budworm larvae at densities of 0, 25, 50, and 100 individuals in sleeve cages on balsam fir shoots. Sleeve cages were processed at the end of the field season to quantify defoliation and insect fitness responses.
Results/Conclusions
Rain exclusion significantly reduced soil moisture in the drought plots, as well as moisture potential in the canopy foliage. Although spruce budworm caused significant defoliation to balsam fir shoots, there were no differences among the drought or budworm density treatments. Consequences for insect fitness included a reduction in adult female dry weight, but only in the drought treatment. This interaction likely reflects an increase in budworm survival in response to increasing density that was also only observed in the drought plots. Greater survival would have resulted in increased resource competition, and thus reduced average individual weight. Overall, our results suggest that both drought and insect density can have important implications for tree-defoliator relationships that are mediated predominantly through feedbacks on insect performance, although short term consequences for tree defoliation appear to be minimal.