By Lisa Swann

whitebark pine killed by mountain pine beetle
This pine has likely been killed by beetles. Credit: Monty VanderBilt

A new study from researchers at the University of Colorado, Boulder shows that younger trees with smoother bark are better at repelling the mountain pine beetle.

The epidemic pine beetle attack has spread across western states since 1996, affecting millions of acres of forest, including those in the Greater Yellowstone Ecosystem.

Colorado doctoral student Scott Ferrenberg, who led the study, noted that the mountain pine beetle had a harder time holding on to smooth-bark trees, which have a slippery surface. These tend to be the younger trees in a stand. The findings, published in the journal Functional Ecology, may help land managers decide which trees to cull and which trees can help protect forested areas against the pine beetle.

To test their findings, the researchers placed 22 beetles on a rough patch of bark, then on a smooth patch. After five minutes, 22 beetles held onto the rough bark, but all the beetles had fallen off the smooth bark.

“We found trees that had both textures on the same stem, and when the tree was attacked, it was on the rough surfaces,” Ferrenberg says. “We thought the beetles were either choosing to avoid the smooth surface, or they just couldn’t hang onto it.”

The results — especially when combined with the findings of a second study also recently published by the research team — provide information that may be useful to land managers who are trying to keep public parks and other relatively small forested areas healthy.

The research suggests that when culling properties to resist the mountain pine beetle, land managers should consider cutting down older trees. Fire management usually suggests cutting the small trees, but this may not be the best strategy for defending against the beetle.

Another study published in the journal Oecologia by Humboldt State University in California found that lodgepole and limber pines that survived more mountain pine beetle attacks had more resin ducts than trees that were killed. Generally, younger trees had more resin ducts.

However, another type of pine that is under attack by the mountain pine beetle has little to no ability — at any age — to use the strategy of pushing out resin to flush beetles out. It’s the whitebark pine and it’s an important foundation and keystone species in the Greater Yellowstone Ecosystem for a number of reasons.

The whitebark pine lives at high elevations where mountain pine beetles were killed by harsh winters until winters started getting warmer. Unlike lodgepole pine, whitebark pine produces little or no resin, leaving it with fewer natural defenses against the beetle. So we’re giving it a boost. American Forests has been placing pheromone patches on healthy whitebark pines to repel the beetles. You can help by donating to our Endangered Western Forests initiative or by emailing to learn about opportunities to help attach patches to trees in the Greater Yellowstone area.