Temperature fluctuations influence predictions of landscape-scale patterns of spruce budworm defoliation

The geographic range of insects is heavily influenced by their tolerance for stressful abiotic conditions, including temperature. While many studies on insect thermal tolerance consider temperature exposure, the frequency of temperature exposures is emerging as an important and generally overlooked driver of insect fitness. The (eastern) spruce budworm (Choristoneura fumiferana) is a lepidopteran defoliating pest of coniferous forests across Canada whose outbreaks lead to large-scale tree mortality. Studies have shown the frequency of temperature fluctuations affects spruce budworm overwintering survival rates in the laboratory; however, the influence of temperature fluctuations on spruce budworm defoliation at the landscape level has not been investigated. We used a species distribution model approach to evaluate the influence of temperature fluctuations on the distribution and severity of spruce budworm defoliation. We combined publicly available maps of spruce budworm outbreaks between 2006-2016 in Quebec with climate, temperature fluctuation, and forest composition predictors to train a species distribution model. Our model evaluated how predictors influence spruce budworm defoliation, and compared these results to a model trained without temperature fluctuations. Additionally, we predicted future spruce budworm defoliation under 2041-2070 climate change conditions using the models trained with and without temperature fluctuation predictors and compared the results to determine the effect of temperature fluctuations on future defoliation predictions. We found that model performance improved with the inclusion of temperature fluctuation predictors, and these predictors ranked highly, relative to predictors in other categories. The model trained with temperature fluctuation predictors also predicted vastly different defoliation distribution and severity across Quebec and Ontario than the model trained without them under climate change conditions. These results reveal the previously overlooked importance of temperature fluctuations on landscape-scale spruce budworm defoliation and support their inclusion in insect species distribution models.