Insects infesting forests in western Montana, killing vast stands

Posted: August 1, 2011

Source – Missoulian

Insects killing large areas of MontanaHAMILTON – Amy Gannon was rained on the other day just west ofHamilton.

There wasn’t a cloud in the sky.

The moisture falling to the ground was excrement from what the state entomologist described as “massive, massive numbers of insects.”

The ponderosa pine trees on the west side of the Bitterroot Valley were filled with pine butterfly caterpillars busily snacking on the tree’s needles.

It was an experience unlike anything Gannon had ever seen.

“It was alarming,” said Gannon, an entomologist with the state Department of Natural Resources and Conservation. “All of the neighbors had been talking about all of this raining insect excrement and all the silk in the trees. I have never seen anything quite like it.”

No one is quite sure why, but this spring and summer have been good ones for the insects that like pine, spruce and fir.

Pine butterfly, western spruce budworm and the Douglas fir tussock moth are the main players in this year’s outbreak.

While a cool wet spring would typically set back many insects, that hasn’t been the case.

Gannon expects people will soon see the clouds of white butterflies that filled ponderosa pine stands on the west side of the Bitterroot Valley last summer.

“The pine butterfly was pretty active in the Bitterroot Valley last summer,” she said. “We saw it in the Garnett Mountains, the Bitterroot and, to some extent, around Missoula, too.”

This year could really turn out to be something.

Gannon has heard reports about others being rained on by the caterpillars’ excrement all along the west side.

“The last time there was a pine butterfly outbreak like this was in the 1970s,” she said. “There’s not been anything like it since. … It’s not uncommon for insects to cycle. Every so often, an event triggers an outbreak.”

No one is sure what that event might have been, though.


In large enough numbers, the pine butterfly can be a tree killer. There is no forest management technique to repel the pests.

“All you can do is make your trees as healthy as possible,” she said. “They are commonly not a major pest.”

This year, the western spruce budworm and Douglas fir tussock moth have been turning trees brown after their respective caterpillars have done their work.

Both are defoliators. They chew on the needles, which places stress on the tree. If the tree is large enough, with plenty of reserves stored away in its roots, it can survive an attack. Smaller trees or drought-stressed larger ones sometimes will succumb.

“There has been a tremendous outbreak of spruce budworm in the Helena area for a number of years now,” Gannon said. “It’s just starting to get active here.”

Last year there were about 300,000 acres statewide that showed signs of attack by the insect.

Both bugs show up at the end of branches in the tree’s new growth in a spidery-like web of silk that encompasses the tips of spruce, Douglas fir, subalpine and grand fir.

“The trees under attack will look kind of scorched,” Gannon said.

The caterpillars of the two species are quite different in appearance.

The spruce budworm starts off green and turns brown with parallel dots down its back. The Douglas fir tussock moth caterpillar has small broom-like patches of hair on its back.

Good forest management helps disrupt the insect’s life cyle.

Thinning a stand provides trees with more of the sunlight, water and nutrients they need to thrive. When an outbreak subsides, they will also rebound quicker, Gannon said.

In some cases, landowners can use bacterial or chemical treatments to control the insects. To be effective, those control methods need to happen in the spring.

The male tussock moth can also be tricked into thinking there’s a female nearby through the use of synthetic sex pheromones.

“The females don’t fly,” Gannon said. “The sex pheromones attract the males and they fly around looking for the females until they peter out with exhaustion and die.”

All three insects are native to western Montana forests.


The balsam wooly adelgid is not.

“It came from Europe, likely on infested nursery stock,” Gannon said. “It’s already caused quite a bit of damage in eastern forests.”

And now it’s here and starting to spread.

The white and wooly insect focuses on subalpine fir. One population has been found in the Sapphire Mountains and all the rest are along the Montana-Idaho border.

“We’re still getting a handle on just how far it’s spread,” Gannon said.

Infected trees may appear to be covered in white wool, which is sometimes hard to see. There can be tiny swellings along the branches that look similar to knuckles. That symptom is called gouting.

The insect impacts the tree by sucking out nutrients and injecting a chemical that creates abnormal cell division.

“We’re not really sure how well it will survive here,” Gannon said. “It’s not as widespread as the other defoliators. The average hiker probably won’t notice it, but it’s nice to have it on people’s radar.”

“We can use their help to track it,” she said.

For more information regarding forest insects and diseases, please refer to the DNRC website,