Bark beetle and defoliator interactions across Pacific Northwestern forests

Abstract

The dynamics of irruptive forest insects are changing rapidly in response to climate warming,altering many important ecosystem services and often overwhelming budgetary and logistical resources allocated to mitigate the ramifications of outbreaks. Changes in outbreak dynamics have been reported for many insects across the world; however, the patterns are not consistent among or within taxa. Increases in outbreak size, severity, and duration have been largely attributed to warming temperatures, increases in drought, and changes in forest structure and composition for bark beetles and defoliators. In my dissertation, I used historical aerial survey data (1960-2019) from the coniferous forests of Oregon, Washington, and British Columbia to quantify the spatial dynamics of bark beetles, defoliators, and their interactions across local and regional scales, and to measure how these dynamics have changed through time. I was able to identify common patterns of increasing spatial autocorrelation through time for insect species that inhabit higher latitudes and elevations, and identify that there is greater spatial overlap of bark beetle and defoliator outbreaks in more recent decades. I also investigated drivers of population dynamics in Douglas-fir beetle, Dendroctonus pseudotsugae Hopkins (Coleoptera: Curculionidae: Scolytinae), which shows a greater propensity for outbreaks in the interior Douglas-fir Pseudotsuga menziesii (Mirbel) regions east Cascade Range than in the coastal Douglas-fir regions west of the Cascades. I evaluated how Douglas-fir beetle abundance in eastern and western Washington was influenced by a suite of weather, natural enemy, and forest metrics, finding that climate and Douglas-fir beetle outbreak proximity and size appear to best explain the differences in the abundance of Douglas-fir beetle populations in eastern and western Washington. Lastly, I investigated the role of invertebrate natural enemies and competitors in providing top-down pressure on low density Douglas-fir beetle populations in eastern and western Washington by mapping and quantifying interactions beneath the bark using ImageJ software, and identifying all emerged species. Findings suggest that the robust interspecific competitor community in the interior Douglas-fir forests did not appear to play a major role in limiting Douglas-fir beetle population densities and suggests competition does not likely play a role in western Washington. Medetera aldrichii Wheeler (Diptera: Dolichopodidae) may play a key role in limiting endemic Douglas-fir beetle populations in coastal Douglas-fir forests as it prefers moister habitats, while predaceous Coleoptera may perform better in the drier interior forests, but be limited in their top-down pressure on Douglas-fir beetle populations as many are generalist predators. Understanding insect outbreak dynamics and their interactions over broad spatial and temporal scales, identifying the top-down and bottom-up factors influencing these changes, and quantifying how these patterns may change under a warming climate is vital to developing effective management plans for forest ecosystems.