Warming climate is helping bark beetles spread in the West
Like most insects, bark beetles cannot metabolically regulate their own body temperatures. When temperatures drop too low, they die from cold; when temperatures get warmer, they grow more quickly. According to the Canadian Forest Service (CFS), for example, winter low temperature at or below minus 40°F will reduce their spread (the CFS has a comprehensive publication on the biology of the mountain pine beetle in particular).
Overall, the climate in the western US has been getting warmer. This helps bark beetles spread in several ways.[[Raffa, K.F., B.H. Aukema, B,J.Bentz, A.L. Carroll, J.A. Hicke, M.G. Turner, and W.H. Romme, W.H. “Cross-scale drivers of natural disturbances prone to anthropogenic amplification: dynamics of biome-wide bark beetle eruptions.” (PDF) BioScience 58, no. 6 (2008): 501 - 517.]] First, warm temperatures can increase population size: fewer beetles die in the winter, and beetles mature and reproduce faster in summer. Second, beetles fly farther during warm weather, so they can invade new territory more easily. Third, warm temperatures can increase summer drought stress, and trees that are stressed are more prone to infestation than healthy trees. Recent research in Washington State elaborates this third point.
For more information see this beetle information page by the US Forest Service.