The tale of two regions: Understanding the factors affecting Dendroctonus pseudotsugae outbreak dynamics in the western and eastern Washington Cascade Mountain Range

Coniferous forests of western North America are hosts to numerous biotic disturbance agents, some of which cause widespread tree damage and mortality with important ecological and economic consequences. One species of particular interest in the Pacific Northwest is Dendroctonus pseudotsugae Hopkins (Coleoptera: Curculionidae), which is a native subcortical bark beetle species that attacks Pseudotsuga menziesii. In Washington, D. pseudotsugae occurs east and west of the Cascade Mountain Range; however, outbreaks typically occur more often in the east. We established field sites across the Washington Cascades over four years to measure the influence of natural enemies, regional climatic trends, and forest stand characteristics on the population dynamics of this ecologically important species. We observed that the natural enemy communities (e.g., Braconidae, Ichneumonidae, Cleridae, and Dolichopodidae species) were similar across all sites in eastern and western Washington; however, higher numbers of all species were generally observed from eastern sites. Moreover, wood borers (e.g., Cerambycidae and Buprestridae species) appear to only be competing with D. pseudotsugae in eastern sites. While competitors may not pose a constraint to D. pseudotsugae populations in the west, the community of natural enemies may play a role in limiting D. pseudotsugae outbreak development. In the east, where outbreaks are more common and where natural enemies and competitors tend to be more abundant, other factors such as climate and stand characteristics may play a larger role in facilitating outbreak development. This research highlights the importance of identifying ecological interactions and their role in affecting forest insect outbreaks.