Comparing Outbreak Regimes of Western Spruce Budworm at Low- and High-Elevation Sites in Idaho using Dendrochronology

The western spruce budworm (Choristoneura freemani; WSB) is the most damaging defoliating insect in the Pacific Northwest. Despite general knowledge about climate influences on WSB, few studies have evaluated how outbreak dynamics are affected by local variability in climate. We used dendrochronological techniques to reconstruct WSB activity in Douglas-firs (Pseudotsuga menziesii) at three low- and three high-elevation sites (representing climate variability) in Idaho, USA. We first tested different thresholds used in an established algorithm for determining WSB activity and found substantial variability in outbreak metrics. We then compared the timing of our reconstructed outbreaks with activity reported by aerial surveys, historical reports, and other reconstructions. Some agreement occurred in non-outbreak periods but significant disagreement existed in the timing of outbreaks. Our assessment of topo-climatic influences on reconstructed WSB activity revealed that defoliation frequency was lower at low-elevation sites, with some inconsistencies depending on metric and threshold choice. Finally, we examined the influence of interannual variability of drought on outbreak initiation, finding no consistent effects. Our results reveal sensitivity to the choice of threshold of the reconstruction algorithm and suggest more investigation is needed to better understand the role of climate given future conditions that will likely be warmer and drier.