Assault by Bark Beetles Transforming Forests

Drive along the picturesque North Fork of the Shoshone River east of Yellowstone National Park and it’s tough to miss the changing forest along the rocky valley.

The vast tracts of Douglas fir that stood green and venerable for generations are peppered and painted with swaths of rusty red and gray.

For Douglas fir, those are the colors of death.

The industrious architect reshaping this forest and many others in the West is barely the size of pencil eraser.

Along the North Fork valley connecting Cody and Yellowstone National Park, the Douglas fir bark beetle has killed 50,000 trees in the last two years. Tens of thousands more have been killed since the epidemic started more than a decade ago.

Although most bark-beetle infestations only last a few years, there’s no end in sight here.

“For the next few years, you’re going to see a lot of red and gray trees and basically dead forest,” said Kurt Allen, a bug expert for the U.S. Forest Service in Rapid City, S.D., who has studied the North Fork beetles. “You’re probably going to see whole hillsides that are going to be red by the end of the summer.”

Bark beetles, including the spruce beetle and mountain pine beetle, have recently reached epic proportions elsewhere in the West, including western Montana, the Idaho panhandle, and parts of California, Colorado and Utah.

The beetle outbreak appears to be the result of a number of circumstances:

* A general warming trend during the past few years.

* Drought.

* Years of fire suppression.

* Less logging than normal.

* Densely packed stands of trees that are growing old and weak.

“It’s kind of an ecosystem out of balance. There’s more food than they’re used to, and bark beetles are opportunists,” said Ken Gibson, a Forest Service entomologist in Missoula who’s been studying the record outbreak of bark beetles in western Montana that started in 1997.

Others say the flourishing beetle population might also be a sign of global warming. Because insects tend to thrive in warmer temperatures, some scientists say warming can be seen in the mountain pine beetle’s spread to higher elevations, where it bores into whitebark pine, and in the faster reproduction cycles of the spruce beetle.

The bark beetle is part of a family of similar insects known as Scoytidaes.

“They are very effective tree killers,” Gibson said. “Many bugs feed on leaves or needles, but the trees don’t die. With the bark beetle, they attack in such numbers that the tree dies within a few weeks.”

The Douglas fir bark beetle leads a short but vigorously productive life.

In its one year of existence, the typical beetle is born in a tree, feeds on its inner bark along with thousands of others until the tree is dead, then flies to the next tree, where the beetle reproduces and dies.

The beetles don’t generally threaten healthy trees, which emit a chemical that the beetles avoid. Instead, they look for trees that are old, weakened, recently burned or blown down. Drought stresses the trees, making them susceptible to the insects’ onslaught.

The beetles drill through the outer bark to get to the cambial layer, between the wood and outer bark, where sugar from the leaves and needles are transported to the roots. The beetles lay eggs in the tree and, eventually, thousands of beetles form a sort of girdle around the tree, cutting off the flow of nutrients that keep the tree alive.

The beetles also carry a fungus that clogs the sapwood and blocks water from going to the roots and crown.

The trees’ main defense is to “pitch out” the beetles by secreting enough sap to flood the beetle from the inner layer of bark. But in drought years, many trees lack enough water to produce sufficient sap to defend themselves.

Although it takes just weeks for beetles to kill a tree, it can take up to a year for the tree to show outward signs of damage.

Aside from culling older, weaker trees out of the forest, the beetles play another important ecological role.

When the tree dies and falls to the forest floor, it provides nutrients to the soil, homes for other animals and other natural benefits. With fewer trees, more sunlight is able to reach the forest floor, promoting growth of other plants and adding diversity.

“They are recycling organisms. When they kill a tree, it becomes organic matter in the forest,” Bentz said.

Although it’s happening at a more prolific rate than usual, the deadly relationship between the beetles and the trees is natural, Bentz said.

“These insects have been around for just as long as Douglas fir; it’s a native part of the system that has evolved with the system,” Bentz said. “It’s us that think they’re a problem.”

The latest beetle outbreak west of Cody started with the 1988 fires around Yellowstone National Park. Bark beetles are attracted to trees that have been injured but not killed by fires.

Shortly after the fires, the beetles started occupying acres and acres of forest. Allen said the beetles found such prime conditions that, after they finished with the burned trees, they moved to neighboring green trees.

“It’s almost all Douglas fir, basically the same age and same size, which just makes it perfect habitat for the beetles to keep going and going,” Allen said. “In the last three years, they’ve been growing exponentially. The populations are just growing and growing.”

Aggressive fire suppression and relatively little logging along the North Fork have created long stretches where trees are the same species and age. Most of the trees are dry from drought and 80 years old and older -- perfect targets for the bark beetle.

Forest managers in Montana, Wyoming, Idaho and elsewhere have been trying to figure out what to do about the bark beetles.

Some treatments have shown limited success.

In some places, the Forest Service has attached capsules to certain trees. The capsules emit a pheromone that tells the bark beetle to stay away, protecting one tree at a time.

So far, fears that the capsules just drive the beetles farther down the road have been unfounded, Gibson said.

“We think it keeps the beetles confused and they fly until they just run out of gas,” he said. “So it actually seems to increase their mortality.”

Traps are available to snare the beetle, but they are really only effective in smaller, isolated areas where the beetles aren’t fully established.

In a heavily infested area such as the North Fork, Allen said, “even if you put out 100 traps, you wouldn’t catch 1% of them.”

The best option, Gibson said, is to keep the beetles at bay in the first place. That will mean changing the composition of many forests in the West, primarily by logging to reduce tree density.

“We just don’t remove as many trees as we once did,” he said. “In western Montana, in many places you have 400 or 500 trees per acre where there used to be barely 100 trees per acre. Maybe it’s because we’re more reluctant to harvest.”

Gibson isn’t recommending heavy logging, but thinning in areas where trees are tightly packed.

“That will increase the vigor of the remaining trees, plus the beetles don’t like a lot of sunlight,” he said.

Still, don’t expect the beetles to go away.

“Bark beetles are opportunists,” Gibson said. “They take advantage of situations that Mother Nature or man provides them.”