Call it the beetle baby boom. Climate change could be throwing common tree killers called mountain pine beetles into a reproductive frenzy. A new study suggests that some beetles living in Colorado, which normally reproduce just once annually, now churn out an extra generation of new bugs each year. And that could further devastate the region's forests.
Pine beetles (Dendroctonus ponderosae), which scuttle from New Mexico north into Canada, are trouble for trees, says study co-author Jeffry Mitton, an evolutionary geneticist at the University of Colorado, Boulder. Beginning in late summer along high altitude sites in the eastern Colorado Rocky Mountains, for instance, swarms of hundreds or even thousands of these small black bugs will single out individual lodgepole pines (Pinus contorta) or related trees, then advance on them en masse. Females dig deep burrows inside the pines' trunks and drop down their eggs. They also deposit a special type of fungus that the insects carry with them that grow inside the trees, eventually helping to kill them. Beetle larvae feed on that same fungus throughout the winter, escaping their burrows the following August.
Recently, pine beetles have inexplicably exploded across their range. In British Canada alone, the insects gutted and killed about 13 million hectares of trees in about a decade. Mitton says it's possible to fly in a small plane over pine forests here for an hour or more and see almost no living pine trees.
Four years ago, Mitton and his graduate student Scott Ferrenberg discovered a possible explanation for this epidemic—almost by accident. While hiking in mid-June to survey pines along Niwot Ridge, due east of Boulder, they saw something strange: adult beetles out and flying. Many even landed on the researchers' clothing. The insects, Mitton says, were swarming close to 2 months too early that year. It seemed so implausible that when he told colleagues about the encounter, some didn't believe him. "This would really upset the apple cart," Milton remembers thinking.
So he and Ferrenberg spent the summers of 2009 and 2010 tracking the growth of pine beetles. They even cut observation windows into the bark of dead pine trees so they could look at larvae hiding in their nests. At first, the insects seemed to be developing as normal. But then, the beetles did the unexpected—they morphed into adults and, beginning in mid-June or even earlier, escaped from their trees. The cue for this early flight seemed to be unseasonably hot weather, the team will report in an upcoming issue of The American Naturalist.
But the beetles weren't just bursting out early, Mitton adds. June-emerging bugs attacked nearby pines almost immediately, laying their own eggs. Those offspring developed speedily, becoming adults, by August or September, just in time to infest another round of pine trees—the second that season. Many Colorado beetles, then, have been able to fit a whole new generation—and an untold number of extra young—into their summers, the team found.
This reproductive explosion could be one reason why the insects have been cutting a deadly swath through North America, Mitton suggests, causing enormous losses both to mountain habitats and to the logging industry.
This study "helps to explain something that's been a big problem in the West," says David Inouye, an ecologist and specialist in Rocky Mountain ecosystems at the University of Maryland, College Park. The pine beetle is hardly the only insect to take advantage of warmer spring weather in Colorado, he adds. Inouye and a colleague previously showed that Mormon fritillary butterflies (Speyeria mormonia) flourish when snow melts later in the mountains.
"This is an important piece of work," agrees Jesse Logan, a retired ecologist with the U.S. Department of Agriculture's Forest Service who studied pine beetles extensively. But, he adds, it will be important for future studies to follow the full cycle of beetle lives. It's not clear yet whether larvae laid by the second generation of beetles survive through the winter to the next June. The extra reproduction "would all be for naught if these venerable beetles are winter-killed." Although it might put a damper on the beetle boom.
Correction: A previous version of this story incorrectly reported that butterflies benefit when snow melts earlier in the Rockies. The sentence has been corrected to say when snow melts later.