National Geographic : 2015 Apr
110 national geographic • April 2015 thousands or just a few hundred beetles to kill it. The drought and warmer temperatures that have struck western forests in recent years have helped the beetles in two ways: by stressing the trees so much that they succumb more readily and by giving the beetles more time to attack them. Beetles in Montana, says Six, used to fly from one tree to another mainly during two weeks in July. But now, as that infested tree in the Big Hole Valley shows, the flying season lasts into October. That means the beetles have ad- ditional time to reproduce—even as the extreme cold snaps of fall and spring, which helped keep populations in check, have become rare. There’s an eerie wisdom in a beetle mob. Sparsely distributed beetles besiege small trees; denser groups go for larger quarry. They seem to attack a large tree only when they know there are enough of them to take it. How does the first beetle know? Carroll and his graduate student Jordan Burke suspect it’s the pheromone. A bur- rowing beetle releases it to call for help, but the amount that’s in the air already tells her whether help is available and it’s safe to lay her eggs. The beetle in Carroll’s office is part of an experiment testing that hypothesis. In any case, the cycle feeds on itself. The big- ger the tree, the more beetle babies a beetle mom produces. More beetles mean more mass attacks on bigger, healthier trees. Once there are enough beetles to blast large trees, large swaths of forest are essentially doomed. In 2013 scientists at the University of British Columbia sequenced the mountain pine beetle’s genome, making it only the second of more than 400,000 beetle species to bear that distinction. (The first was the red flour beetle, which infests stored grains.) But Joerg Bohlmann, the plant biochemist who oversaw the sequencing effort, doesn’t think a biotechnical fix to the pine beetle epidemic is imminent. “We have to be extremely careful we don’t promise things that are not realistic,” he says. Pesticides can save a few individual trees but not a forest; they’re too expensive, and they’d kill all sorts of other organisms. Breeding beetle-resistant trees would take decades, even with modern ge- netics. And even then the beetles might rapidly adapt and break through the resistance. For the moment most research, from the ge- nome work to the beetle in Carroll’s bucket, is aimed at simply improving our ability to predict insect outbreaks. “No one can give you a certain answer about where the bark beetle will be in five years,” says Bohlmann. “That is the biggest issue.” If you can get to a forest soon enough, when beetle populations are still relatively small, you may control the infestation. At least that’s the theory that’s being tested in Alberta, which has become the front line in the battle To save a forest, you have to cut a lot of trees— to ensure the beetles have nowhere else to go. to stop the beetle’s march across the continent. In many parts of British Columbia and the western United States the infestation is slowing on its own—in some places because there aren’t many trees left to kill. But in Alberta it began more recently. Alberta has about 15 million acres of pines, far fewer than British Colum- bia, but they’re in vitally important areas, at the sources of mountain rivers that feed the prairies and cities below. If those forests are devastated, the unshaded snow will melt faster, and rivers will crest earlier—before the dry season, when people and ecosystems need the water most. To save a forest, you have to cut a lot of trees; the only way to stall the beetles is to ensure that they have nowhere else to go. Alberta’s long- term strategy is to log or burn most patches of forest that are dominated by mature pine trees and thus highly susceptible to the beetles. Its short-term strategy is to fight the beetle tree by tree. In the past decade it has spent more than $320 million (U.S.) on beetle management.