The mountain pine beetle epidemic that has spread through the Rocky Mountains for the last two decades originated from not one but many points of origin, according to new research from the University of Colorado, Boulder. Smaller outbreaks blossomed independently, coalesced and continued their spread through lodgepole and ponderosa pine, leaving vast swaths of red-tinged forest behind them.

That pattern of simultaneous eruption from many regional epicenters "really highlights the importance of the broad-scale drivers of the pine beetle epidemic, like climate and drought," said lead author and CU Boulder doctoral student Teresa Chapman.

Mountain pine beetles are native to much of the American West and have passed through the region's pine forests for millennia, killing off old growth and making room for new. In the mid-1980s, a pine beetle epidemic in Colorado's Grand County was halted by two years of record-cold winters that killed off the insect's larvae.

As temperatures rise, however, the beetles are pushing the limits of their traditional territory, and it is becoming less likely that winter temperatures will drop low enough to curb the current outbreak.

"This implies that under continued warming trends, future outbreaks will not be terminated until they exhaust their food supply -- the pine tree hosts," said Tom Veblen, a co-author of the report and a professor of geography at CU Boulder.

Drought stresses trees, making them less able to defend themselves against parasites. Yet even after the drought of 2001 and 2002, when precipitation levels in the Rocky Mountains returned to their historical average, the pine beetles had grown to such numbers that they could maintain the momentum of their spread, Chapman said.

"Once beetle populations reach a certain number, they can overwhelm even healthy trees," she said. "After the droughts, their numbers had grown so massive that even when precipitation levels returned to normal, they continued to push their way into new areas."

A cold snap like the region experienced in the early 1980s could curb the beetles, but temperatures have remained stubbornly high for more than a decade.

While pine beetle spread can't be measured accurately until two or three years after a region has been affected, due to the time it takes host trees to die and brown, this summer's heat wave may have allowed the insects to multiply even faster than they have in the past.

The beetle is evolved to be univoltine, or to produce one generation of offspring per year. However, a different CU Boulder study found that higher summer temperatures are accelerating the beetle's life cycle, making it possible for two generations of beetles to hatch in a single summer (ClimateWire, March 19).

Because a single pine beetle can lay up to 60 eggs, a second generation could increase the number of beetles searching for hosts by up to 60 times as well, according to the earlier study.