Douglas-fir Tussock Moth Defoliation Increases in the Blue Mountains

By: Paul Oester, Extension Forester and David Shaw, Forest Health Specialist, Oregon State University Extension Service


Douglas-fir Tussock moth distributionThis article was originally written by Glenn Kohler, Entomologist, Washington Department of Natural Resources and has been revised for the MWM Gazette.

In the summer of 2011, forested areas with new defoliation caused by Douglas-fir tussock moths were detected in the Blue Mountains by the U.S. Forest Service, the Washington State Department of Natural Resources (DNR), and the Oregon Department of Forestry (ODF).

Through the agencies’ aerial surveys, light defoliation was mapped across 9,000 acres of the Umatilla National Forest in Washington (7,800 acres) and Oregon (1,200 acres). Most of the defoliation occurred in the Wenaha-Tucannon Wilderness Area but may spread and increase in severity in 2012. The damage was typically light, with the top third of the crown most heavily defoliated.

Another tussock moth outbreak that affected 1,600 acres in eastern Spokane County in 2011 will likely collapse in 2012. In northern Idaho, approximately 68,000 acres with tussock moth defoliation were recorded in 2011. In parts of northern Idaho, the outbreak may spread and increase in severity in 2012.

The damage primarily affects grand fir, subalpine fir, Douglas-fir, and some spruce. Douglas-fir tussock moth has a few characteristics to remember:

  • Caterpillars feed on both new and old foliage.
  • Defoliation damage can reduce growth, cause top-kill, and may predispose some trees to attack by bark beetles.
  • Repeated defoliation is most damaging to trees.
  • It is important to remember that defoliated trees observed in the summer and fall are not necessarily dead.
  • If a tree is able to form buds that survive the winter, needles will develop in the spring.
  • Recreation can also 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.

Mature Larvae. Photo by Glenn KohlerThe 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 a viral disease and parasites.

To evaluate management options, the Oregon Department of Forestry (ODF) and Washington Department of Natural Resources (WDNR) can assist forest and woodlot property owners in the affected areas who observe Douglas-fir tussock moth egg masses or tree damage. New defoliation damage becomes most noticeable in July and is often worst in the tops of trees. To report tussock moth damage or for more information, please contact your state’s Forest Entomologist: Glenn Kohler (Washington DNR), 360-902-1342, glenn.kohler@dnr.wa.gov or Rob Flowers (ODF), 503-945-7396, rflowers@odf.state.or.us.

To reduce potential tree damage in future outbreaks, susceptible stands can be thinned to favor non-host species and increase spacing between host trees that are retained. Because severely defoliated trees may recover, thinning and salvage is best done following the outbreak. This will also reduce worker’s exposure to irritating caterpillar hairs. Early in the outbreak, the application of labeled insecticides on high-value stands can reduce tree damage and may prevent an outbreak’s spread.

Detailed information on how to recognize Douglas-fir tussock moth damage, maps and images are available at ODF or DNR websites: http://www.oregon.gov/ODF/privateforests/fh.shtml and http://1.usa.gov/2z8nkG

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