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Forest Health Notes:
A Series for the Non-Industrial Private Forest Landowner

(Orgyia pseudotsugata)

Hosts: Douglas-fir, grand fir, subalpine fir; rarely pine, western hemlock, western larch,
Engelmann spruce

Quick ID:

  • Foliage appears scorched or off-color
  • Defoliation from top down and from outside in
  • Thin crowns and topkill
  • Defoliation from hairy caterpillar up to 1" in length
  • Grey cocoons and webbing present on branches

Douglas MothField Identification

Tree: From a distance, trees appear scorched or off-color, often with a noticeably thin crown. Tussock moth larvae prefer new foliage, especially when they are young, hence the defoliation
occurs from the top down and from the tips of the branches inwards. As the top of the crown has a high percentage of new foliage each year, it is severely defoliated and often killed. Larvae spin webs enclosing leader and branch tips. When the population density is normal, grey cocoons may be found on the underside of old foliage. When the population density is high, additional cocoons may be found on branches, on the trunk, or on understory vegetation.

Insect: Adult insects are charcoal-brown moths. The male has a wingspan of about 1-1/4" (31 mm), and the female has stubby, vestigial wings. Egg masses are greyish as they are mixed with black hairs from the female's abdomen; they are laid on the female's cocoon. Larvae go through several stages of development. They hatch as tiny, black-headed caterpillars and finish as larvae about 1" in length, with four distinct tufts of white hair with red tips in the middle of the back, two "horns" of hair in front, and two tufts at the end of the abdomen, one of which may be reddish. All stages of development are hairy. If you have a defoliator which has no hair, it's not tussock moth.

May be confused with: Western spruce budworm, sawflies

Life cycle: The female, who does not fly, lays eggs on her cocoon anywhere from August through October. These eggs are mixed with hairs from her abdomen, held together with a gelatinous substance. Eggs overwinter in a mass and hatch from late May to late June. Larvae initially feed on the underside of new needles, then switch to older needles and next year's buds. Young larvae may be dispersed to other trees by a "ballooning" effect, where the wind carries them on a strand of webbing for some distance. Fully developed larvae spin a cocoon and pupate from mid-August through September, emerging to mate and die soon thereafter. Only one generation occurs each year.

Predisposing factors: The probability of defoliation for a given stand in Eastern Washington may be related to certain factors. Geographically, those stands which are lower in elevation, on east-facing slopes, or on ridgetops are more likely to be defoliated. Tussock moths are favored by stands of high density, a high percentage of true fir and Douglas-fir, or many trees with a large crown diameter. Naturally, the probability of defoliation increases when adjacent or nearby stands are defoliated, or have a high probability of defoliation. Water deficiency also predisposes trees to attack. Defoliation by tussock moths is itself a predisposing factor for bark beetle infestation, particularly fir engraver and Douglas-fir beetle. Bark beetle outbreaks often occur shortly after tussock moth outbreaks, once trees have been weakened.

Impact: The magnitude of outbreaks in terms of infested acreage is known to be increasing. Smaller trees suffer more mortality from the effects of defoliation, and larger trees suffer more mortality from subsequent bark beetle attack. In a major outbreak in the Blue Mountains, 72% of the trees heavily defoliated were killed. Growth loss after defoliation can be significant, with percentage lost varying from 58% to 40%, depending on severity of defoliation.

Management: Both direct and indirect methods of control have been used against the Douglas-fir tussock moth. Direct methods include chemical, viral and bacteriological sprays, usually applied aerially. Chemical sprays have been effective in knocking back the population, but do not address the underlying cause of outbreaks and hence will not prevent recurrence. They have fallen out of favor as questions have arisen about their toxicity in the environment.

Attention has shifted recently to viral and bacterial controls, as these naturally-occurring agents have historically caused populations to crash. These can be highly effective but are somewhat more expensive than chemical sprays. However, they are specific to butterflies and moths and hence do not disturb other in sects, nor other members of the ecosystem. Preparations of a bacteria, Bacillus thuringiensis, are available commercially. Again, like chemical sprays biological agents knock populations back but do not treat the underlying cause of the outbreak.

The US Forest Service and others have made the decision to concentrate on silvicultural measures to control insect pests. Some recommendations for reducing the hazard of serious defoliation include:

  • Thin to a generate a low-hazard stand. An appropriate stocking level will reduce stress from intertree competition, particularly water stress. Stands less than 50 years of age and without a high proportion of grand fir will be more resistant to defoliation.
  • Do not use equipment that causes soil compaction or erosion. Match equipment to site to minimize damage on trees that would favor insect and disease attacks.
  • Lop and scatter slash, pile and burn largest pieces (greater than 6" in diameter) only if fuel load is unsatisfactory. The idea here is to encourage nutrients to cycle rather than to remove them (by removing the slash) or to permit them to be leached (by burning everything).
  • Under even-aged management systems, schedule harvest cuts to minimize adjacent stands' exposure to heat and wind and to improve snow retention.
  • Favor establishment of species adapted to drought, such as ponderosa pine in place of Douglas-fir on dry sites, and ponderosa pine, lodgepole pine, Douglas-fir and larch on sites capable of supporting true fir species.

Adapted from The Douglas-fir Tussock Moth: A Synthesis, USDA Forest Service Technical Bulletin 1585, M. Brookes, R.W. Stark, and R. Campbell, eds.

Note: Use pesticides with care. Apply them only to plants, animals, or sites listed on label. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets and livestock.

Forest Health Notes were prepared by Donna Dekker-Robertson, Peter Griessmann, Dave Baumgartner, and Don Hanley, Washington State University Cooperative Extension. The assistance of Robert L. Edmonds and Robert I. Gara, University of Washington College of Forest Resources, is gratefully acknowledged.

Department of Natural Resource Sciences
Washington State University Cooperative Extension
Pullman, Washington 99164-6410

Insect or disease treatments should always be part of an overall forest management planning process. It's a good idea to have a forest management plan for your property. This tool will help you prevent problems and keep your forest healthy. For information on forest stewardship educational programs, contact your local WSU Cooperative Extension office. For information on technical assistance and financial incentive programs, contact the Washington Department of Natural Resources. The Natural Resources Conservation Service will assist landowners in developing conservation plans.

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