Eruptive insect pests have significant impacts on the structure and function of forest ecosystems. In addition to the density-dependent relationships that drive insect outbreaks, rising populations often coincide with drought events that are expected to increase in both frequency and intensity in response to climate change. However, as population approach the eruptive phase, consequences of intraspecific competition may outweigh the benefit of host water stress. The objective of our study was to quantify defoliation and insect performance responses to the interactive effects of drought and density of spruce budworm (Choristeneura fumiferana), a significant pest of the spruce-fir forests of eastern North America. To test for these interactions, we established a manipulative field experiment in a mature, balsam-fir dominated forest stand using a combination of single-tree rainout shelters and sleeve caged insect larvae at four different densities; 0, 25, 50 or 100 individuals.
Results/Conclusions
Defoliation of one-year old shoots, but not current-year shoots, significantly increased in response to higher insect densities. Density also had a significant, negative effect on budworm %survival, although the total number of recovered adults remained highest in the high density treatment. Adult female body mass was significantly reduced in response to increased density, with effects particularly pronounced under the drought treatment. Lastly, male wing length was significantly decreased in response to increased density. Overall, our results demonstrate that across a broad range of outbreak densities, rain exclusion had a very minor impact. Accordingly, we anticipate that as insect pest populations approach epidemic levels, the influence of density on insect feeding and fitness will far outweigh the impact of drought stress.