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.
By Bob Mitchell BS'76
Bark beetles have been killing conifers in western North America for at least 38 million years. Fossilized wood from the region shows evidence of Dendroctonus (the name means “tree killer”) beetle galleries. But the attacks usually have been few, far between and short-lived. Beetle populations would burgeon when trees were under stress, kill off the oldest, biggest trees, and then die back when that food source was gone.
Outbreaks were less common because trees that evolved with the beetles are very good at defending themselves. As a lodgepole pine oozes sticky resin to block the beetle’s path, it is also killing its own tissue at the attack site and flooding the area with heavy doses of insecticidal chemicals. This leaves the beetle trapped in a toxic environment with nothing to eat. Raffa calls it a “scorched earth strategy.”
“Trees are vicious,” he says. “That’s why an outbreak occurs in only one area maybe every 30 years. Most of the time, things are weighted very much against the beetle.”
The only way the beetles can defeat a healthy tree is through a swift mass attack. The first ones to the tree sound the charge by releasing chemical attractants that draw hundreds or thousands more, enough to deplete the resin and weaken the tree before it can bring its defenses up to speed. The bigger the beetle population, the better the odds of success.
This is where warmer weather gives the beetle a leg up. Higher temperatures boost beetle numbers by speeding up the life cycle and reducing overwinter mortality. The minus-40-degree cold snaps needed to freeze beetle larvae have become rare occurrences. Warmer temperatures have also allowed beetles to thrive in places that used to be inhospitably cold.
“What we’re seeing now is the mountain pine beetle getting into higher latitudes and higher elevations,” says Raffa. As it makes itself at home farther up in the mountains, the beetle has devastated one of the Rockies’ longest-living conifers, the white bark pine. It’s what’s known as a naïve species, explains Monica Turner, a UW-Madison landscape ecologist who studies the Yellowstone National Park ecosystem. Because it evolved at higher elevations where cold temperatures used to suppress beetle populations, the tree didn’t develop the same level of ability to defend itself.
“Now it’s being hammered by the mountain pine beetle. I think we could lose this forest type, or see it very much reduced,” Turner says. And that could bring a cascade of impacts, she adds. Grizzly bears, for example, fatten up on white bark seeds before hibernation. Losing this food source could force them to move down slope where they’re more likely to encounter people. The beetle’s move into more northern latitudes opens up what some have called the “doomsday scenario,” in which the beetle takes a right turn in Canada and make its way across the boreal forest to the Midwest. The worries began when mountain pine beetles crossed the Rocky Mountains from British Columbia into northern Alberta.
“This is frightening, because lodgepole pine is a sister species to jack pine, and the two hybridize in Alberta,” Raffa says. Mountain pine beetles have been killing hybrid lodgepole/jack pine forest in Alberta, and in 2010, for the first time, have successfully reproduced in jack pine, a dominant species in the forest that stretches across Canada. “For the first time ever, the mountain pine beetle is physically connected with Wisconsin’s forest,” he says. What are the chances of the beetle making it to Wisconsin? “The short answer is that nobody knows,” says Raffa. But he hopes to get some answers through collaborative research with UW-Madison microbiologist Cameron Currie.
Cameron Currie studies partnerships between insects and microorganisms, and he’s learned a lot about how insects employ microbes in the struggle against predators, competitors and prey. His earliest studies focused on species of ants that cultivate fungus to feed their young. The ants, Currie discovered, carry antibiotic-producing bacteria that protect their fungus crop from predators. He sees parallels between this system and that of the tree-killing beetles.
Mountain pine beetles also employ fungi, both to help break down tree tissue and to provide a food source for beetle larvae and adults. Currie suspects that bacteria also play a role in the beetle system. If so, he says, they may help the beetle adapt to a new host tree species.