Archimedes had his bathtub, Newton his apple. Scott Ferrenberg's Eureka moment required more fieldwork to confirm, but it began when a bug the size of a grain of rice landed on his sleeve as he walked through the woods.
In late May 2008, the graduate student in ecology and evolutionary biology at the University of Colorado was strolling among limber pines at 10,000 feet along Niwot Ridge west of Boulder, home of the university's Mountain Research Station, one of the highest and oldest climate-research facilities in the world. Ferrenberg had no trouble recognizing the visitor on his arm as Dendroctonus ponderosae — the mountain pine beetle, a common sight in lodgepole and ponderosa pine forests across Colorado.
An adult beetle flying around in May on Niwot Ridge, though, isn't so common. Ferrenberg, who was accustomed to studying beetles at lower elevations in California forests, didn't immediately grasp the implications of what he was seeing. "If I'd been told that it shouldn't be there, I might have second-guessed myself," he says now.
But he did think the sighting was unusual enough to report it to Jeff Mitton, a CU evolutionary biology professor with whom Ferrenberg had just started working on a beetle genetics study. Just over a week later — June 8, 2008 — Mitton and Ferrenberg were back at the research site, preparing to dig beetle larvae out of trees for their study. Instead, they found mature beetles diligently assaulting the forest.
"It was a gorgeous day," Mitton recalls. "Shirtsleeve weather. Adults were flying in the air and landing on our shirts. They were hitting the trees and beginning to bore into them. I thought, 'Gee, this is early.'"
Mitton, who's been studying bark beetles since the 1970s, described the scene to an entomologist buddy in the U.S. Forest Service. His friend told him he must be mistaken. Among bug fighters, it was an article of faith that the beetles don't get busy in Colorado until late August. Then they carve galleries into the trees and lay their eggs, which develop from larvae into pupae, then pale yellow adolescents known as teneral — a life spent mostly beneath the bark, until they emerge as pigmented adults late the following summer. Adults flying in June? At 10,000 feet? No way.
Mitton came away from the conversation deeply annoyed. "He was so intransigent about this," he says, "that I realized I must have seen something that's decidedly different from the historic norm."
Over the next two years Mitton and Ferrenberg embarked on a carefully planned study to figure out what was going on. During the winter they identified pristine trees at two sites that hadn't yet been molested by the beetles. They set up lures loaded with pheromones to invite beetles to establish broods in those trees. They visited the sites regularly over the summers of 2009 and 2010 and documented beetles attacking the trees in June and laying eggs. Some of those eggs had developed into a second adult generation by August or September, emerging to seek out fresh hosts and start their own broods.
Early flights of mountain pine beetles have been observed elsewhere in recent years, but the research conducted by Mitton and Ferrenberg — and published in The American Naturalist earlier this year — represents the first peer-reviewed report of the insect achieving two generations in a single summer. The accelerated life cycle, the authors suggest, is a direct response to climate change.
Since the 1970s, warmer weather along the Front Range has allowed the beetle not only to operate in forests above 9,000 feet, where it's scarcely been seen before, but it's more than doubled its flight season, from 50 days to up to 120 days. And the effect on the beetle population is potentially exponential; instead of one female generating 60 offspring a year, those offspring could, in theory, generate an additional 3,600 pine-munching hordes in the same season.
The CU discovery comes in the sixteenth year of an epic mountain pine-beetle outbreak in the western United States and Canada, the worst infestation of its kind in recorded history. In Colorado, what began as a flare-up in lodgepole pine in isolated pockets on the Western Slope in 1996 has left large swaths of doomed, red-needled trees and denuded gray ones across 3.3 million acres, including thousands of ghost trees in popular tourist areas such as Vail, Dillon and Grand Lake. Although the pace of the epidemic has slowed — largely, experts say, because the beetles have already killed most of the large lodgepoles they prefer as hosts — aerial surveys indicate that the affected area expanded by 140,000 acres in 2011, mostly in ponderosa forests in the northern part of the state.
Southern Wyoming has also been hit hard. But then, so have pine forests from the San Bernardino Mountains to the Front Range, from New Mexico to Yellowstone to British Columbia. Western Canada has been particularly pummeled; by some estimates, the pine mortality there has become so extensive, releasing so many megatons of carbon dioxide from tree decomposition into the atmosphere, that it could transform the country's forests from a carbon sink to a carbon source.