A Bug in the System

Climate change is fueling the biggest outbreak ever of tree-killing bark beetles. The insects are decimating conifer forests from Alaska to Arizona—and raising concerns that they could reach the Upper Midwest.

THERE’S NO PLACE TO PULL OFF on this stretch of the serpentine road leading up Wyoming’s Signal Mountain, so Phil Townsend just stops the car in the middle of his lane. He hops out and darts across the road to get a closer look at a towering lodgepole pine. It’s a substantial tree, fatter than a telephone pole and 80 feet tall, with full, lofty branches full of needles—but Townsend has already written it off.

“This tree is dead,” says the CALS forest ecologist. “It just doesn’t know it yet.”
Townsend points to the dozens of tubes of yellow pitch sticking out from the tree’s trunk. Each one, he explains, marks the point of attack of a mountain pine beetle.

As the beetle bored in, the tree exuded resin in an attempt to trap it, but the effort failed. The pitch tubes are gritty with fresh sawdust, but otherwise empty. The beetles tunneled through the sticky wax and are now chewing into the phloem, the tree’s nutrient pipeline, creating galleries where they’ll deposit their eggs.

As [weather] conditions have gotten warmer, the outbreaks have gotten more frequent and more large-scale.

“This is a fresh attack,” he explains. “They will eat the phloem and girdle the tree. This tree is not going to succeed.”  Townsend climbs back in the car and heads to the summit to get a clear view of the sea of conifers that blanket the hillsides in this part of Grand Teton National Park. A typical sightseer at the mountaintop lookout would describe the forest as green and healthy, but Townsend points out various spots that show a subtle yellow sheen. “Next year those trees will all be red. The year after that, they’ll be gray.”

This color pattern has become all too familiar to those who live, work or play in the vast forests of the mountainous West. From Arizona to Alaska, matchhead-sized beetles are turning conifers from green to rusty red to driftwood gray. Trees in the Jackson Valley, which runs along the base of the Tetons south of Yellowstone National Park, are among the latest casualties.

Back in Madison, CALS forest entomologist Ken Raffa offers a grand assessment. And the picture is grim. “We’re talking about dozens of millions of acres across the West where it’s almost 100 percent mortality,” he says. “We’re talking about transformations of entire ecosystems.”

Changes in climate, Raffa says, are enabling this swathe of destruction. Warmer summers and milder winters have boosted beetle populations at a time when drought stress, fire suppression and other management practices have left forests ripe for attack. As a result, what used to be intermittent, isolated flare-ups of native beetles have exploded into the largest known insect outbreak in North American history.

“As conditions have gotten warmer, the outbreaks have gotten more frequent and more large-scale. The outbreaks are normal, but the size of the current outbreak is unprecedented,” Raffa says.  “The enormity is such that it has transformed lodgepole pine in British Columbia from a carbon sink to a carbon source. So it’s taking something that’s normally sequestering carbon and turned it into something that’s releasing carbon,” he says. “That has implications for global carbon cycles and global warming.”

Raffa has a good perspective on what’s normal and what’s not about the mountain pine beetle and its close relatives. He has been studying the insects since his grad student days in the 1970s, and he has continued that line of research at Wisconsin. Those studies inform his work related to some of the Badger State’s problem beetles—in particular the fir engraver, a serious pest in red pine plantations.

Most of his work focuses on the thresholds—the tipping points at which an endemic, low-level population surges to the point where it can successfully infest a stand of healthy trees, or an entire forest, or—as is now the case—a major forest ecosystem. While the infestations he’s studying can be epic in size, his approach is to think small.

“The critical features that drive whether or not these large-scale outbreaks can occur are happening at the scale of the individual beetles confronting biochemicals at the point where they enter the tree,” he explains. “Those fine-scale processes ultimately determine if outbreaks can take place.”