Theses and Dissertations Collection

Western spruce budworm is a common defoliating insect that caused extensive damage to a number of tree species across the western United States. Outbreaks of WSBW have been linked to drought stress of host trees in some studies, yet other studies have yet to use aerial detection survey and growth rate of an insect population to study the effect of drought in the Western US. This study compared seasonal drought stress metrics (Palmer Drought Severity Index (PDSI) and climatic water deficit (CWD)) in multiple locations of WSBW outbreaks in the western US during 1997-2015 identified from aerial detection surveys. Superposed epoch analysis and insect population growth rates as represented by tree defoliation area change were assessed to quantify the drought condition associated with initiation and continuation of outbreaks. We found that initiation of outbreaks was associated with a short period of drought before initiation in the northwestern US. This study also indicated that the budworm outbreaks of southwestern US are less associated with drought. For the continuation stage, this study indicated that other non-climate factors might be influenced by non-climate factors. Improved understanding of the role of drought given future climate change will lead to improved predictions and management of future outbreaks of WSBW.