Drought and Moisture Availability and Recent Western Spruce Budworm Outbreaks in the Western United States

Western spruce budworm (WSBW) is a common defoliating insect that has caused extensive damage and mortality to a number of tree species across the western United States (US). Past studies have linked outbreaks of WSBW to increased moisture stress of host trees in the Northwest and decreased moisture stress in the Southwest. Our study analyzed seasonal drought stress metrics with WSBW outbreaks within Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests in the western US during 1997–2015. Superposed epoch analysis and defoliation area growth rates (representing insect population growth rates) were assessed to quantify the drought conditions associated with the initiation and continuation of outbreaks, respectively. We found that multiple years of drought occurred prior to and during outbreak initiation in the Northwest, and that outbreak initiation in the Southwest was associated with only weak drought or neutral conditions. During the outbreak continuation stage, there was a weak positive correlation between May moisture availability and defoliation area growth rates in the Southwest (R2 = 0.12), but no clear relationship was identified in the Northwest. Increased frequency of summer droughts such as these expected from climate change may increase WSBW outbreaks and promote tree dieoff. Improved understanding of the role of different influences of drought and moisture availability across landscapes will lead to improved predictions and management of future outbreaks of WSBW.