Local News

New tussock moth outbreak causes tree defoliation in Okanogan County


May 08, 2020

Forest health scientists with the Washington State Department of Natural Resources (DNR) say owners of wooded property in Okanogan County should be on the lookout for the Douglas-fir tussock moth, a native insect that has munched the needles off Douglas-fir trees on at least 600 acres of forest in the county so far.

The Douglas-fir tussock moth is a conifer defoliating caterpillar that damages trees in cyclical outbreaks. Severe outbreaks can kill up to 40 percent of affected trees and cause growth loss, top-kill, and increased vulnerability in surviving trees. In Okanogan County, the damage has been observed east of the Okanogan River between Oroville and Chesaw.

“These moths are normally kept in check by diseases, predators, weather, and food supply, but sometimes those factors are reduced. That’s when we see a burst in the tussock moth population and the real damage these insects can cause in our forests,” said Glenn Kohler, a forest entomologist with DNR. “If you live in an area experiencing a tussock moth outbreak and are concerned for the health of your trees, there are warning signs and steps you can take to reduce damage.”

The first signs of damage typically appear in late spring, when newly hatched larvae feed on newly emerged foliage on the outermost branches and tops of trees. By mid-July, the larvae are larger and more visible. This is when they begin to feed on older foliage as well. By August, the larvae reach pupation and have now caused brown, bare, and damaged firs. The moths hatch from their cocoons as early as late July and even through October.

There are several signs to look for:

  • Egg masses on the underside of branches, bunched on the trunk, or even some distance away from the tree on buildings and fence posts. Egg masses are about an inch across, round, hairy, gray, and covered with white eggs. Masses may contain just a few spherical, white eggs or up 350 depending on location.
  • Webbing can sometimes be visible in the tops of infested trees.
  • An adult male tussock moth is grayish or blackish-brown, with feathery antennae and two vague white dots on their front wings.
  • Females are flightless and have tiny wings, small thread-like antennae, and large abdomens with dark hairs.

DNR and the U.S. Department of Agriculture’s Forest Service closely monitor tussock moth activity using aerial surveys, ground surveys, and pheromone-baited traps at around 250 locations across the state.

In addition to the new outbreak in Okanogan County, forests around Blewett Pass in Kittitas and Chelan counties experienced their second year of a tussock moth outbreak in 2019 that has killed or damaged trees on approximately 5,600 acres.

Outbreaks usually collapse within two to four years from natural controls, but there are management options that can reduce damage. Reducing the number of host trees, increasing spacing between them, thinning out Douglas-fir in the understory, and favoring non-hosts such as pines and larch will make a forest much more resilient. Insecticides can be used to reduce damage, but landowners should consult with a forest health professional on best practices for pesticide use. Residents in affected areas can report Douglas-fir tussock moth activity and discuss mitigation options by contacting Kohler at glenn.kohler@dnr.wa.gov.

More information about tussock moth identification and treatment is available here.

 


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