Influence of temperature on historic and future population fitness of the western spruce budworm, Choristoneura occidentalis

Climate change affects the geographic range and outbreak behavior of forest insects. Both the range and dynamics of insect populations are linked to physiological responses to abiotic conditions and trophic interactions via their effects on individual fitness. We develop a process-based simulation model of population fitness for the western spruce budworm, Choristoneura occidentalis (Freeman) to examine the effects of temperature on historical range and observed outbreak behavior in the complex topography western North America. Model predictions are consistent with observed changes in the distribution and frequency of this insect's outbreaks over the past 100 years. These changing patterns are the result of several direct and indirect responses of the insect to temperature. Overwintering survival is negatively affected by warming and determines its southern and lower-elevation limit. Ability to complete its life cycle before killing frosts limits its range to the north and at higher elevations. Interactions affecting synchrony between the insect's feeding life stages and host foliage development also determines fitness and will increase the area favoring fitness of western spruce budworm in the future, especially in western Canada where host trees extend much farther north than the insect currently does.