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The mountain pine beetle is a native insect of western U.S. pine forests. It survives by killing infested trees, usually individually, but occasionally in epidemics. Mountain pine beetle epidemics are particularly associated with lodgepole pine, a common western tree that typically grows in dense, even-aged stands. The beetle is a seasonally adapted species that thrives in areas where it can complete its life cycle in one year. The beetle has evolved a mass-attack approach to overwhelm tree defenses through large numbers, and adults congregate on large trees under stress. Widespread stress (e.g., a regional drought) sets the stage for an epidemic. Mountain pine beetle epidemics are recurrent events in western forests. The current epidemic can be separated into three distinct events: the central U.S. Rocky Mountains, interior British Columbia (Canada), and high-elevation pines. Two aspects of the current epidemic are widely believed to have been exacerbated by climate change: (1) increased temperatures farther north and at higher elevations (allowing complete life cycles in areas previously not susceptible to the beetle) and (2) possibly regional drought (making trees more susceptible to beetle attacks). Controlling a mountain pine beetle epidemic can be problematic. Individual trees can be protected by insecticide sprays, but ...