Tussock moth 'can clean all of the needles off of a tree'

By MATTHEW WEAVER

Capital Press

A forest pest that can kill evergreen trees is spreading across parts of Eastern Washington and Northern Idaho.

The Douglas-fir tussock moth has been found in the Mica Peak, Tekoa Mountain and Gelbert Mountain areas in Washington and across the border in Idaho, according to the Washington State Department of Natural Resources.

The insects attacked roughly 8,500 acres in Idaho's Kootenai and Benewah counties last year.

Forest entomologist Glenn Kohler said the defoliation caused by the pest is spreading and will be more severe this year.

The caterpillars leave behind red damaged needles on conifers, particularly Douglas fir, grand fir, Engelmann spruce and blue spruce ornamental trees.

Kohler said the moth is unique in that it feeds on all ages of needles.

"The tussock moth in a single year can clean all of the needles off of a tree," he said. "Once that happens, the tree is likely to die.

"If the tree can retain even 5 percent of its needles, it may survive after the outbreak," he said.

The native insects are typically responding to forest conditions, such as more hosts than normal in a dense stand. Outbreaks tend to have epicenters, where there's more pine and fir intermingled with lower-elevation scrublands where the fir is already stressed.

The department recommends careful tree thinning to protect pine and larch, which are not eaten by the caterpillars, and removing Douglas fir and grand fir, the main hosts.

Even in a severe outbreak, Kohler said, mortality of fir trees doesn't reach much higher than 30 to 40 percent.

Insecticides can be applied, but are less effective when caterpillars have matured into pupae and stop eating, usually around mid-August.

Tussock moth outbreaks are cyclical, occurring every seven to 10 years.

"It comes along every once a while, and it is a real pain when it does," Kohler said.

A previous outbreak occurred in Okanogan County, Wash., in 2008 and 2009.

The department recommends not handling the caterpillars because their hairs irritate the skin of many people.

Halli Hemingway, forestry and GIS analyst for Bennett Lumber Co. in Princeton, Idaho, said some loggers and foresters have noticed the caterpillars.

The company is monitoring the situation, Hemingway said, noting the last time the cycle occurred, the trees were able to bounce back within several years.