Entomologists have long suspected climate change of pushing mountain pine beetles into new, previously hostile regions of the continent. Now, research from the University of Colorado, Boulder, suggests that higher temperatures may drive the insects to reproduce more quickly as well.

The beetles, which spend most of their lives burrowed under the bark of host trees, generally follow a one-year cycle of reproduction, emerging in July and August to attack new hosts and lay eggs before the onset of fall. Those new beetles develop over the course of the winter, emerging as adults the following spring.

However, when ecology professor Jeffry Mitton and graduate student Scott Ferrenberg tracked a generation of beetles at a research station in the Rocky Mountains, they found that higher temperatures were allowing some of the beetles to reproduce not once, but twice in the warmer summer months.

"Instead of emerging as a dribble in July and a pulse in August, the first wave of beetles were now emerging in June," Mitton said.

Because the beetles' life cycles are regulated by temperature -- that is, the higher the temperature, the faster the beetles develop -- the eggs laid by this first wave grew into fully formed adults in only two months, he said.

That put the second generation's emergence squarely in August, giving the beetles plenty of time to lay eggs of their own before the winter cold set in.

Potential of 3,600 grandchildren in a summer

"It's important that people understand the implications this has for the landscape," Mitton said. Each pine beetle can lay as many as 60 eggs, he said. If all 60 survive and grow to reproduce, laying 60 eggs of their own, it means that the original beetle will have spawned more than 3,600 grandchildren in one summer.

Mitton said that, while perfect survival rates were extremely unlikely, this kind of exponential growth potential did help explain the beetle's rapid expansion over the past decade. Beetle kill in North America has expanded more than twentyfold since 1998.

Given these findings, it is likely that this twice-yearly emergence pattern has been going on for some time in warmer, more southerly locations, said Ferrenberg. "It's a trait we've observed in other species of Dendroctonus," the genus to which mountain pine beetles belong, he said.

However, researchers are particularly concerned about the potential for beetle expansion into higher elevations and more northern latitudes because both of these regions are disproportionately affected by climate change.

The boreal forest of Canada, which has been too cold for beetle infestation for most of history, has warmed twice as much as the global average in the past half-century. And Mitton and Ferrenberg said that their study site, which is 1,000 feet higher than the beetle's historical range, has warmed 2.7 degrees Farenheit in the past two decades.

That has let the beetles into new ecosystems that cannot resist the insects as well as their native hosts, the lodgepole and ponderosa pines.

Already, many stands of high-altitude whitebark pines have been destroyed in parts of Idaho, Colorado and Wyoming. Entomologists are nervously monitoring infestations that have made their way into the jack pine forests east of the Rocky Mountains in Canada.