Beetle's Taste for Bark is Taking a Bite

On its own, the mountain pine beetle hardly seems a menace. No larger than a grain of rice, it is often the victim when it tries to burrow into a towering pine, killed by the tree’s natural defenses.

But let that beetle call over some friends, and watch out. Acres of trees can fall in their wake. In British Columbia, where pine beetle populations are exploding, the insects have already killed nearly half of lodgepole pine trees in the province’s central forest, and they’re claiming new territory each year. Authorities now say the beetle outranks wildfire or logging as the greatest threat to the Canadian forest.

What accounts for such deadly teamwork? Kenneth Raffa, an entomology professor who has studied mountain pine beetles for 30 years, says the insects use a kind of jujitsu, harnessing a tree’s natural chemical defenses to send out a welcoming beacon to other beetles. If enough respond, they can overcome their host’s resistance, turning it instead into a nursery for their eggs. As they bore deep inside, the beetles leave a trail of microorganisms that cut off the tree’s nutrient supply, choking it to death.

But the beetles’ trickery alone doesn’t explain why they are fast spreading across Canada, Raffa says. “We’ve always had outbreaks of mountain pine beetles in the western forests,” he says, but the effects of those sporadic breakouts are usually fleeting and localized. “What’s different now is that we’re seeing them expand to areas where they haven’t been before.”

Raffa is working with the Canadian Forest Service and researchers at several U.S. and Canadian universities to understand why. The researchers attribute at least some of the population boom on global warming: Average winter temperatures in Canada have risen by 4 degrees in the past 30 years, and recent winters have simply not been cold enough to kill off the beetles in large numbers.

But Raffa is also interested in the effect of forest management practices, such as decisions to thin forests or suppress wildfires. Working with three UW-Madison colleagues—forest ecologist Phil Townsend, zoologist Monica Turner and microbiologist Cameron Currie—Raffa’s lab team has begun studying the problem from several angles, including how fire affects beetle populations and the role of bacteria in helping beetles invade new habitats.

“We want to understand how all of these factors intersect, from the biological scale to the landscape scale,” says Raffa.