Abstract
Aim The biogeography and distribution of irruptive forest insect herbivores are driven by population level responses to host availability, temperature, and precipitation. Because the irruptive dynamics of tree‐killing insects such as spruce beetle are regulated by climate, it is imperative to quantify biogeographic drivers and limiters to develop informed predictions and forecasts of future outbreaks. Here, we: (1) identify subcontinental drivers of spruce beetle outbreak distributions, (2) compare drivers and limiters of outbreaks across regions, and (3) develop geospatial hindcasts and forecasts to evaluate the potential for shifts in outbreak activity through time. Location Western North American spruce forests spanning 33 to 68 degrees latitude. Taxon Spruce beetle ( Dendroctonus rufipennis ) and spruce trees ( Picea spp.) Methods We compiled aerial surveys of spruce beetle‐caused tree mortality from 1962 to 2023, forest inventory, and downscaled climate data to develop ensemble machine learning models to investigate patterns of recorded outbreaks and predict the historical and projected distributions of spruce beetle outbreaks. Results The distribution of spruce beetle outbreaks are best explained by minimum winter temperature, precipitation as snow, and mean growing season temperatures across western North American. Limiters of outbreaks varied regionally. Host occupancy limited the southern distribution, abundant moisture and diffuse host availability in mixed species forests constrained the western coastal distribution, and minimum winter temperature constrained the northern distribution. Hindcasts and forecasts indicate that the outbreak distribution has expanded in all regions since 1900 and will continue to expand poleward due to projected warming across the boreal. Conclusions Understanding how the distributions of forest insect outbreaks respond to fluctuations in climate will dictate the location and severity of outbreaks over the next century. Large‐scale irruptions of spruce beetle in forests with historically limited activity will lead to substantial impacts on spruce‐dominated landscapes and unknown ramifications for forest processes or long‐term disturbance regimes.