Mountain pine beetle develops an unprecedented summer generation in response to climate warming.

The mountain pine beetle (MPB; Dendroctonus ponderosae) is native to western North America, attacks most trees of the genus Pinus, and periodically erupts in epidemics. The current epidemic of the MPB is an order of magnitude larger than any previously recorded, reaching trees at higher elevation and latitude than ever before. Here we show that after 2 decades of air-temperature increases in the Colorado Front Range, the MPB flight season begins more than 1 month earlier than and is approximately twice as long as the historically reported season. We also report, for the first time, that the life cycle in some broods has increased from one to two generations per year. Because MPBs do not diapause and their development is controlled by temperature, they are responding to climate change through faster development. The expansion of the MPB into previously inhospitable environments, combined with the measured ability to increase reproductive output in such locations, indicates that the MPB is tracking climate change, exacerbating the current epidemic.