Budworm attacking Park forests

Some forests on Glacier National Park’s east side have died from an infestation of spruce budworm.

The small brown caterpillar, which is a natural predator in conifer trees like spruce and Douglas fir, feeds on the needles of the trees. Usually large trees survive a spruce budworm infestation, explained Dawn LaFleur, the Park’s integrated pest management supervisory biologist.

Birds will feast on the insects, and the trees will grow back the needles, as the caterpillars generally only eat off the end of the branches or the branches at the tops of trees.

But in recent years, some trees on the east side have seen what amounts to a double infestation. The caterpillars from the previous year feed on the trees as well as the young of the year, eating all the needles on the trees and killing them, she explained. The problem is particularly acute near Rising Sun and along the Going-to-the-Sun Road.

The culprit is a warmer climate. A normal Glacier Park winter gets cold enough to kill the caterpillars in January and February, but while winters have been snowy the past few years, they haven’t been particularly cold. That’s allowed the bugs to survive, and keep munching on trees.

“We’re not getting the hard frost days in January and February,” LaFleur said.

A careful observer can see where colder air killed the budworms and where it didn’t. In areas where it got cold, the trees are relatively healthy. Where it didn’t, they’re dead or showing bare branches.

“The reality is these forests will change in the next 15 years,” LaFleur said.

Glacier Park is also seeing problems with other natural but sometimes devastating pests. In recent years on the west side, Douglas fir beetle has killed many older Douglas fir trees, particularly in the North Fork.

In order to preserve big Douglas fir trees in campgrounds, the Park has used a pheromone treatment that makes beetles believe a tree is already infected so they leave it alone and look for another host. But in the backcountry, the beetles have killed many Douglas firs, many of which were hundreds of years old.

Some forests are also seeing infestations of mountain pine beetles. They’re also natural predators, but they can also cause high tree mortality. For the east-side forests, it’s not clear what will return as the mature trees die, LaFleur noted.