Tussock moths’ impact on fir trees visible

Forest entomologists say it’s been a banner year for tussock moths, a native pest that strips green foliage from fir trees.

The moths are in their caterpillar stage, actively feeding on Douglas fir and grand fir trees in Spokane and Kootenai counties. They leave behind red-colored, damaged-looking foliage.

Though the trees appear dead, they usually recover, with new buds appearing in the spring, said Tom Eckberg, a forest health specialist with the Idaho Department of Lands.

Tussock moth outbreaks typically peak every 10 years. Last year, state agencies reported 140,000 affected acres in North Idaho and 570 affected acres in Spokane County. Acreage estimates for this year’s outbreak aren’t yet available.

In Idaho, reddish trees can be spotted at Moscow Mountain, near Plummer and Worley, and in the Signal Point area south of Post Falls, which is visible from Interstate 90. In Washington, the outbreak has affected Mica Peak and Tekoa Mountain.

In addition to Douglas fir and grand fir, the caterpillars sometimes feed on ornamental spruce trees in people’s yards, Eckberg said. Insecticides targeting tussock moths are usually effective for treating individual trees, he said.

The outbreaks usually subside within two to three years. As the tussock moth population builds up, the insects become susceptible to a naturally occurring virus that reduces their numbers, Eckberg said. Birds and parasitic wasps also help keep tussock moth populations in check.

Tussock moth outbreaks may be larger than they were historically, as a result of changes in tree species, said Chris Schnepf, a University of Idaho extension educator in forestry. Past logging practices and fire suppression have reduced conditions favorable for growing Ponderosa pines and Western larch – which the moths don’t target – and allowed more firs on the landscape, he said.