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An outbreak of tussock moth caterpillars has defoliated a swath of trees on a ridge on the south side of Elkhorn Road, east of the stoplight on state Highway 75.

A swath of defoliated fir trees on a ridge south of Ketchum indicates that a tussock moth outbreak affecting south-central Idaho for the past few years has spread to the Wood River Valley.

According to the Boise National Forest, the outbreak began near Sage Hen Reservoir, southwest of Cascade Reservoir, in 2016. Outbreaks typically last for four years, the Forest Service stated.

“This is the year that the outbreak is starting to move into Blaine County,” Sawtooth National Forest forester Nelson Mills said. “It’s going to change the viewshed for years to come.”

Mills said the local defoliation was detected about three weeks ago on a ridge on the south side of Elkhorn Road, just west of Sunrise subdivision. Since then it has spread rapidly.

“It’s just coming down the mountain like a freight train,” nearby resident Tom Allen said last week. “Two weeks ago it was just green trees and beautiful.”

Another, smaller area of defoliated fir trees has appeared on a north-facing slope across state Highway 75 from The Meadows mobile-home park. Mills said he just noticed that one last week, but has not yet determined whether it is caused by tussock-moth caterpillars or by fir bark beetles. He said a tussock moth infestation has also occurred at Craters of the Moon National Monument.

Mills said he suspects the caterpillars have been transported by firewood cut in an infested area.

“Burn your firewood where you buy it,” he advises campers.

The caterpillars eat the needles of Douglas fir, grand firs and to a lesser degree subalpine firs. The Boise National Forest stated in a 2018 report that the impacts of outbreaks can vary. Trees defoliated more than 90 percent may die, especially those growing on the upper third of slopes. Those trees defoliated less heavily will have reduced growth and may be subsequently attacked by bark beetles.

Outbreaks often occur in the same general area about once every 10 years. Caterpillar populations increase rapidly from year two to year three, so that in year three, trees appear to be so completely defoliated that they look dead. Outbreak populations crash in year four, largely due to the effects of a native, tussock moth-specific virus that attacks the caterpillars.

The last two outbreaks in south-central Idaho occurred from 1991 to 1994 and from 2005 to 2008. The 1990s outbreak occurred over 400,000 acres across the Boise, Sawtooth and Payette national forests. The 2000s outbreak was recorded only on the Sawtooth National Forest.

According to the Boise National Forest report, in 2015 and 2016, high trap catches of adult moths on the Salmon-Challis, Sawtooth and Boise national forests indicated that an outbreak was developing across historically defoliated areas.

The caterpillars were detected on Bald Mountain in the summer of 2017, though no defoliation has yet occurred there, Mills said.

“They live there—they’re just not at outbreak levels,” he said.

Forest Service scientists have created a treatment to kill the caterpillars using the natural virus that targets them. In an interview with the Idaho Mountain Express two years ago, Mills said the Ketchum Ranger District would apply for funds to spray infested areas of Bald Mountain with the virus by helicopter in the summer of 2018. However, he said last week that that effort was suspended since there is not yet an outbreak there.

However, he said it is probably too late to treat the area along Elkhorn Road where defoliation has already occurred.

“Right now we just have to ride it out, unfortunately, and let nature do its thing there,” he said.

This year is expected to be the last year of the outbreak where it began on the Boise National Forest, and the Forest Service stated that caterpillar populations should crash there by the end of this summer.

Mills said he will do a plane flight over the region—from Featherville, along the South Fork of the Boise River, on the west to the Pioneer Mountains on the east—to evaluate the extent of the outbreak.

Email the writer: gmoore@mtexpress.com

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