Insects play several important roles in forest ecosystems including serving as natural enemies, pollinators, and nutrient cyclers. However, outbreaking species can cause damage beyond the limits of forest tolerance. Forest insect outbreaks can be extremely catastrophic events that lead to large scale tree mortality over a relatively short period of time, causing cascading negative effects including reductions in wildlife habitat and increased carbon emission. Insects are also strongly affected by climate and changes in climate because many of their physiological processes are regulated by temperature. Consequently, warming temperatures have critical ramifications to insect populations, including more frequent and/or more intense outbreaks. We used historical aerial detection survey data (1960-2018) from the coniferous forests of Oregon, Washington, and British Columbia to quantify the spatial dynamics of bark beetles, folivores, and their interactions across local and regional scales, and to measure how these dynamics have changed through time. To complement these analyses, we established field plots across the Washington Cascade Range to measure how natural enemy communities influence the dynamics of two ecologically-important native forest insect species on Douglas-fir trees, Douglas-fir beetle (DFB) and western spruce budworm (WSB), both of which outbreak in eastern Washington but not in western Washington.
Results/Conclusions
Among bark beetles, the degree of spatial autocorrelation has increased over the past two decades, indicating that their collective outbreaks have become larger in spatial extent, more clustered, and thus more contiguous across the landscape. In contrast, the degree of spatial autocorrelation among folivores has remained fairly constant through time. At local spatial scales, bark beetles and folivores were temporally autocorrelated for several years after the onset of the outbreak, but were not cross-correlated, suggesting that bark beetles and folivores persist within areas for several years, but that these areas do not overlap in space or time. In field plots from the Washington Cascade Range, we observed different relationships with natural enemies in DFB and WSB even though their sampled densities were similar across all plots. Clerid beetles and snakeflies, important predators of DFB, were found in high densities in eastern Washington but rarely observed in western Washington. In contrast, tachinid and wasp parasitoids, natural enemies of WSB, were found in high densities in western Washington but less so in eastern Washington. This research highlights the importance of identifying ecologically important interactions and understanding the processes that drive these patterns to better mitigate the consequences of forest insect outbreaks.