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Tussock moth outbreak right on time

Published on January 28, 2011 3:01AM

Last changed on February 25, 2011 7:19AM

Caterpillars invade trees in Spokane, N. Idaho region


Capital Press

Unfortunately for some forest owners, the Douglas-fir tussock moth is right on time.

The Washington State Department of Natural Resources, the Idaho State Department of Lands and the U.S. Forest Service recently reported new areas of defoliation by the caterpillar.

During the summer of 2010, the insect affected roughly 8,500 acres in Idaho's Kootenai and Benewah counties and 570 acres in eastern Spokane County in Washington.

Tom Eckberg, forest health specialist with the Idaho Department of Lands, said outbreaks tend to develop about once a decade.

"It's always been here, but it's not always a problem," Eckberg told the Capital Press. "We're seeing it now because it has been about 10 years, it's time for another tussock moth outbreak."

The last outbreaks in the border area between northern Idaho and Washington occurred in from 2000 through 2002 and 1986 before that. In Southern Idaho, an outbreak occurred from 1990 through 1992.

Monitoring programs with pheromone traps set out each year usually catch moths each year, Eckeberg said

Populations are usually kept in check by natural controls, such as a virus specific to the moth or parasitoids that feed on eggs, larvae or pupae.

"It will build up, we will see two to four years of defoliation and then the populations will crash," Eckberg said.

Male moths have wings, but females are flightless, with very small wings, and lay thousands of eggs through the winter. In the spring, the wind blows young caterpillars from tree to tree, Eckberg said.

Officials are asking forest and woodlot owners to look for signs of the pests. Cocoons and egg masses can be found on the underside of tree branches, building overhangs or fences. Damage consists of reddish, half-chewed needles.

Caterpillars feed on the needles of Douglas fir or grand fir, usually beginning at the top of the tree, Eckberg said. Several years or more of this can cause topkill or kill the whole tree if heavily defoliated. Sometimes the insects will eat only a portion of the needle, which eventually turns brown.

The caterpillars prefer to eat new growth.

Arborists or tree-care companies can apply labeled insecticides to smaller, ornamental trees.

Forest owners don't have as many options. Some treatments can be applied, but it is difficult to treat larger trees or large acreages from the ground, Eckberg said.

In the past, the department has offered spray programs targeting the tussock moth, but there are no plans for that in 2011, he said.

The department recommends landowners with larger acreages manage their stands with more pines and larch, species that do not host the tussock moth.


For more information or to report damage, contact Tom Eckberg at 208-666-8625 or teckberg@idl.idaho.gov and Washington Forest Entomologist Glenn Kohler at 360-902-1342 and glenn.kohler@dnr.wa.gov


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