FORESTS -- The Washington Department of Natural Resources has discovered a new infestation of Douglas-fir tussock moths that occurred last summer in the Blue Mountains of Washington and Oregon.

Light defoliation caused by the moths was mapped across 9,000 acres of the Umatilla National Forest, with Washington accounting for 7,800 acres, according to a DNR press release and following report from the Associated Press.

Most of the defoliation occurred in the Wenaha-Tucannon Wilderness Area, but it may spread and increase in severity this year, the state Department of Natural Resources said Monday.

Officials say another tussock moth outbreak that affected 1,600 acres in eastern Spokane County in 2011 will likely end this year.

In nearby northern Idaho, approximately 68,000 acres with tussock moth defoliation were recorded in 2011 and that outbreak may spread this year, the DNR said.

The defoliation can reduce growth, cause top-kill, and may make some trees vulnerable to attack by bark beetles. An outbreak typically kills up to 40 percent of the trees in an area.

The outbreak in the Blue Mountains primarily affects grand fir, subalpine fir, Douglas-fir, and some spruce.

Recreation can be affected in areas with tussock moth present because the hairs found on caterpillars, cocoons, and egg masses are a skin irritant to many people, the DNR said.

The last outbreak in the Blue Mountains occurred from 2000-2002.

Outbreaks typically collapse within two to four years due to a buildup of natural enemies, such as disease and parasites.

The Washington DNR said new damage becomes most noticeable in July and is often worst in the tops of trees.