Insect outbreaks are significant disturbances in forests of the western United States, with infestation comparable in area to fire. Outbreaks of mountain pine beetle (Dendroctonus ponderosae Hopkins) require life cycles of one year with synchronous emergence of adults from host trees at an appropriate time of year (termed "adaptive seasonality") to overwhelm tree defenses. The annual course of temperature plays a major role in governing life stage development and imposing synchrony on mountain pine beetle populations. Here we apply a process-based model of adaptive seasonality across the western United States using gridded daily temperatures from the Vegetation/Ecosystem Modeling and Analysis Project (VEMAP) over the period 1895--2100. Historical locations of modeled adaptive seasonality overlay much of the distribution of lodgepole pine (Pinus contorta Douglas), a favored host, indicating that suitable temperatures for outbreak occurred in areas of host availability. A range of suitable temperatures, both in the mean and over an annual cycle, resulted in adaptive seasonality. Adaptive seasonality typically occurred when mean annual temperatures were 3¿--6¿C, but also included locations where mean temperatures were as low as 1¿C or as high as 11¿C, primarily as a result of variability in winter temperatures. For most locations, years of adaptive seasonality were uncommon during 1895--1993. We analyzed historical temperatures and adaptive seasonality in more detail in three northern forest ecoprovinces. In the Northern and Middle Rockies, areas of adaptive seasonality decreased at lower elevations and increased at higher elevations during warmer periods, resulting in a movement upward in elevation of adaptive seasonality. In contrast, the Cascade Mountains exhibited overall declines in adaptive seasonality with higher temperatures regardless of elevation. Projections of future warming (5¿C in the western United States) resulted in substantial reductions in the overall area of adaptive seasonality. At the highest elevations, predicted warmer conditions will result in increases in the area of adaptive seasonality. Our findings suggest that future climate change may alter forest ecosystems indirectly through alteration of these important disturbances.