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Spruce Budworm leaves area trees ‘singed’ after wet, cool summer

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An airplane spraying insecticide in 1955 for the Western spruce budworm control project. PHOTO COURTESY OF WIKIPEDIA

By Bay Stephens EBS LOCAL EDITOR

BIG SKY – In July of 2018, Scott Orazem, a member of Big Sky’s Porcupine Park HOA, noticed a funny thing: many of the fir trees around the development had taken on a rusted hue, their green tarnished by a perimeter of dead red needles as the youngest needles on the trees, which should have been the healthiest, most photosynthetic portions of each branch, had been munched by some evasive culprit.

“Nobody wants their trees to look like that,” Orazem said.

Forest managers call these trees “singed” as if just their outer needles were breathed on by fire; it’s the telltale sign of the native spruce budworm, which feeds on the buds and new growth of Douglas and Subalpine firs.

Spruce budworm moths lay eggs in the treetops in the fall, where the eggs overwinter until larvae, which look like caterpillars, become active in the spring. Though they begin feeding in late May and early June, the caterpillars don’t cause appreciable damage to trees until around the beginning of July when they are at their biggest—from a half inch to an inch long—just before they go into cocoons.

This year, 11 Big Sky HOAs, including Porcupine Park, paid to have their forests sprayed by helicopter to kill the insects and save their trees, but the spraying was less effective than past years due to the cool and moist summer weather.

A branch that shows a season’s work of the spruce budworm, which feeds on the new growth of firs, but won’t eat needles that budded in previous years. PHOTOS BY BAY STEPHENS

“It’s been a very, very odd year for the spruce budworm,” said Chuck Gesme, a forester for Northwest Management Inc., a private consulting firm that specializes in forest management and orchestrated the spraying.

According to Gesme, the cooler summer led to more variability in the stages of larval development within the spruce budworm population. So, while most of the insects were at their largest, chomping on this year’s new growth, he estimated a fifth of the population were still the size of a grain of rice, nibbling on the insides of buds, and another fifth had ceased eating as they moved toward pupating.

This poses a problem for spraying because the organic biological insecticide uses to spray the trees, called BT, is only effective for a very specific window when the caterpillars are big and eating on the exterior of new growth. As an insecticide that can cover entire swaths of forest without harming any insects other than the budworm, it’s a nifty substance.

“It’s a bacterium that is only harmful to those caterpillars,” Gesme said. “I could spray it on an apple and immediately eat the apple and I would be fine.”

BT lands on the limbs of trees and is ingested by the spruce budworm larva, which causes the caterpillars die and fall off the trees. But timing is critical, according to Gesme, because BT is only active for 3-5 days before sunlight breaks it down.

After monitoring Big Sky’s forests twice a week to ensure optimal timing for BT, Gesme gave the go-ahead in the second week of July and a helicopter sprayed more than 16,077 acres of land in 11 different Big Sky developments with which Northwest had been working.

Despite hitting the population at the point where the most budworms would be affected by BT—at the peak of the population’ bell curve—Gesme thinks they had about a 60 percent mortality rate for the bugs, instead of the target range of 75-80 percent.

While checking the effectiveness of the treatment on July 20, Gesme was astounded to find a larva and a cocoon on the same tree weeks after any larva would normally be left.

“It was just ridiculous,” he said. “And that’s just a measure of the strange weather we’ve had.”

Spraying isn’t the only way to mitigate for spruce budworm, and property owners seeking to protect their trees can do other mitigation work in tandem with spraying.

According to Nancy Sturdevant, forest health specialist with the Missoula field office of the U.S. Forest Service, property owners should thin their forests, removing trees that have been significantly defoliated by spruce budworm and plan ahead to plant different species of young trees.

“I understand people want to hold onto their large, old trees … [but] trying to keep trees that are significantly impacted by budworm or have very small crowns, that is not sustainable without a lot of input,” Sturdevant said.

She also mentioned that homeowners could site-spray carbaryl, a pesticide that is toxic to all insects, on treasured fir trees, especially if they are not already too defoliated.

Gesme seconded that managing one’s forest is important to managing spruce budworm, especially thinning, which creates less competition between trees, and therefore less stress, and impedes larvae from parachuting on silk threads carried by the wind from one munched tree to the next food source.

“It’s kind of funny and I try to bring it up, but you can actually vastly improve the health of your forest by killing [a good percentage] of your trees,” Gesme said.

Years of repeated defoliation by the budworm can leave trees haggard, stressed for nutrients and prone to other insects, like the Douglas-fir bark beetle, which will finish the trees off.

Although the spruce budworm and bark beetle may be pests to property owners who adore their trees, Sturdevant stressed the insects’ roles in forest regeneration, especially in the largely fir forests of Gallatin County.

“The spruce budworm is a native insect that’s coevolved with these forests for forever,” she said. “So is the Douglas-fir bark beetle. And the Douglas-fir bark beetle is called ‘the recycler of mature forests.’ That’s its job: When we have too many mature trees in an area, they hone in on that and kill those trees and reset succession.”

“When you see those two working in tandem, that’s when you’ll see these large stand replacement events of a sweeping hillside of all dead timber,” Gesme said. “There are places in Big Sky that I see that happen currently,” such as in Jack Creek and on the slopes of Fan Mountain, he added.

Once a forest gets to that point, it’s time to start over, Gesme said, a process historically carried out by wildfire.

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