Douglas-fir tussock moth

Written by V.G. Nealis Revision 2025

• French common name: Chenille à houppes du Douglas
• Scientific name: Orgyia pseudotsugata (McDunnough)
• Kingdom: Animalia
• Phylum: Arthropoda
• Class: Insecta
• Order: Lepidoptera
• Family: Erebidae
• Partial list of synonyms:
  • Hemerocampa pseudotsugata McD.
  • Hemerocampa vetusta (Bdv.)

General information and importance

Douglas-fir tussock moth is a native defoliator of conifers in North America west of the Rocky Mountains. Periodic outbreaks occur at about 10-year intervals and last up to four years. Outbreaks are most pronounced in the semi-arid interior forests of British Columbia and the western United States, particularly at lower elevations (less than 800 metres). In British Columbia, 11 outbreaks have been recorded since 1916, although severe defoliation did not occur during all outbreaks. The largest outbreak on record defoliated nearly 26,000 hectares in 1983. The most recent outbreak began in 2018, with about 2,700 hectares defoliated in 2019.

Douglas-fir tussock moth outbreaks tend to be localized but very intense with clusters of trees severely defoliated in one year and nearby trees in subsequent years. Open grown trees in urban and semi-rural settings seem especially susceptible. During these events, associated tree species as well as understory shrubs may be damaged. Larvae of Douglas-fir tussock moth feed on both new and old foliage and can completely defoliate a tree, causing top-kill and tree mortality after only one year of defoliation. Outbreaks end as quickly as they begin, associated with mortality caused by several natural enemies, most notably a specific virus, which becomes epidemic as tussock moth populations become very dense.

Douglas-fir tussock moth is sometimes confused with two forest insects: one native, whitemarked tussock moth (Orgyia leucostigma) and one introduced, rusty tussock moth (O. antiqua). These three species are closely related and share many characteristics including a spectacularly coloured, hairy caterpillar stage and a flightless female moth.

A significant, non-forest impact of Douglas-fir tussock moth outbreaks is the occurrence of tussockosis, an allergic reaction among many people exposed to the urticating hairs on the larvae. Itching is the most common complaint, but symptoms vary with individual sensitivity and repeated exposure.

Distribution and hosts

Douglas-fir tussock moth is found west of the Rocky Mountains from the central interior of British Columbia to northern Mexico and in coastal forests from Vancouver Island to northern California. Preferred host trees are associated with different forest ecosystems across this wide range. In British Columbia and the Pacific Northwest of the United States, primary hosts are Douglas-fir (Pseudotsuga menziesii var. menziesii and P. menziesii var. glauca) and grand fir (Abies grandis). Further south, white fir (A. concolor) is preferred with occasional attacks on red fir (A. magnifica) and subalpine fir (A. lasiocarpa) at higher elevations. During outbreaks, associated trees such as Ponderosa pine (Pinus ponderosa) and western larch (Larix occidentalis) as well as understory plants may be damaged.

Host parts affected

New and old foliage of preferred host trees

Symptoms and signs

The first sign of attack occurs in late spring when feeding by small tussock moth larvae partially consume new foliage, causing the foliage to turn reddish-brown. Small larvae are less than 6 millimetres long, light tan to dark grey and covered with long hairs. As they grow, larvae feed on all needles with the most intense colour changes beginning at the top of the tree crown and branch tips. Mature larvae are up to 30 millimetres in length. They have four thick, reddish-brown tufts, or tussocks, on the middle of their backs, and two black pencil-shaped tufts projecting forward from behind the head plus a longer tuft projecting backwards from the posterior of the abdomen. White hairs radiating from red tubercles cover the rest of the body.

Larvae pupate inside a grey-brown cocoon about 20 millimetres long and constructed of silk and hairs. Female moths have greatly reduced, vestigial wings and are flightless. The brownish-grey males have a wingspan up to 32 millimetres and elaborate, plumose antennae to detect the mating pheromone. Egg masses are about 13 millimetres in diameter. These masses are formed on the cocoons of the females, and each consists of about 200 white round eggs in one to three layers covered with a frothy secretion intermixed with body hairs that hardens to protect the eggs.

Under severe outbreak conditions, the entire tree may be defoliated and appear a skeleton of bare branches in profile. Damage by Douglas-fir tussock moth may be confused with that caused by the western spruce budworm (Choristoneura occidentalis occidentalis) because both defoliators occur on Douglas-fir in similar forest types. However, the two insects are distinct in appearance as is their characteristic damage. Western spruce budworm restricts its feeding to new growth on the branch tips whereas Douglas-fir tussock moth consumes both new and old foliage.

Life cycle

Douglas-fir tussock moth has one generation per year. It overwinters in the egg stage. Eggs hatch in late spring coincident with new growth on host trees. They remain on the egg mass for a few days, then disperse to branch tips. From there, they balloon on silk threads, sometimes for several hundred metres, facilitated by their long hairs, which help keep them airborne. Because the adult female tussock moth does not fly, this is the main mechanism of dispersal for the species. It also accounts, in part, for the characteristic intensity of populations on individual trees. Small larvae feed on the freshest, most nutritious foliage.

Typically, there are five larval instars. As fresh foliage is depleted, larger larvae complete their development by consuming old foliage.

Pupation occurs in late summer and early fall. Most larvae pupate on branches where they were feeding, but some may migrate to non-host structures, including buildings. Adults emerge in one to two weeks. Flightless female moths emit a pheromone that attracts mobile males. Once mated, the female moth lays all her spherical, white eggs (around 200 eggs in British Columbia) in a single mass.

Damage

The intensity of Douglas-fir tussock moth outbreaks and the ability of larvae to consume all needles on a tree can result in severe damage and death of trees in one year. Studies examining impacts during the most severe outbreak in the early 1980s in British Columbia estimated that cumulative mortality and top-kill of Douglas-fir resulted in nearly 50% loss in affected forests. Damaged trees may become more susceptible to Douglas-fir beetle (Dendroctonus pseudotsugae). In general, overall losses are far less and more restricted in area compared with other forest pest impacts.

Prevention and management

Pest management strategies for a particular pest vary depending on several factors. These include:

• the population level of the pest (i.e., how numerous the pest is on the affected host[s]);
• the expected damage or other negative consequences of the pest’s activity and population level (either to the host, property, or the environment);
• an understanding of the pest’s life cycle, its various life stages, and the various natural or abiotic agents that affect population levels;
• how many individual host specimens are affected (an individual tree, small groups of trees, plantations, forests);
• the value of the host(s) versus the costs of pest management approaches; and
• consideration of the various silvicultural, mechanical, chemical, biological, and natural control approaches available and their various advantages and disadvantages.

Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.

The explosive appearance of high-density populations of Douglas-fir tussock moth and the possibility of tree mortality in the first year of an outbreak present a challenge to conventional forest protection programs. Systematic deployment of pheromone traps in areas of recognized hazard with follow-up egg mass sampling are key tools in forecasting outbreaks and supporting decisions for management intervention.

Aerial application of chemical insecticides was used until the 1970s when improvements using the biological insecticide, Bacillus thuringiensis var. kurstaki (Btk), lead to its replacement of chemical control products. Foliage protection was not always satisfactory with Btk because the bacterium in the product must be eaten by the insect and may take several days to weeks before toxicity is expressed. By this time, the severe defoliation may have already happened.

A naturally occurring nucleopolyhedrovirus (NPV) was discovered to be effective in reducing high-density populations of Douglas-fir tussock moth. A specific isolate was registered first in the United States and then in Canada as Virtuss. Emphasis shifted to manipulation of natural viral epizootics in combination with Btk to reduce outbreak populations. The NPV virus has many advantages, including specificity to Douglas-fir tussock moth and only a few closely related insect species. Once applied, infected tussock moths produce more inoculum, which perpetuate and spread the virus in the year of treatment as well as contribute to a background reservoir of virus that can persist for more than one year. By applying the virus early in an outbreak, a natural epizootic can be propagated sooner and at lower insect densities than would occur naturally with substantial foliage protection and early collapse of populations.

Pheromones and insecticides are defined as pest control products and are regulated in Canada. Products registered for use against Douglas-fir tussock moth under specific situations may change from year to year. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered products and product information for use against this insect. The application of any registered pesticide should be based on population size and applied only when necessary and against the approved life stage. It is also recommended to consult a local tree care professional. Pesticides may be toxic to humans, animals, birds, fish, and other beneficial insects. Apply registered products only as necessary and follow all directions and precautions noted on the manufacturer’s label. In some jurisdictions and situations, only a licensed professional can apply pesticides. Consulting relevant local authorities to determine local regulations that are in place is recommended.

Selected references

Alfaro, R.I.; Wegwitz, E.; Brown, R.G; Taylor, S.P. 1987. Douglas-fir tussock moth damage in British Columbia. The Forestry Chronicle 63(5): 351–355. https://doi.org/10.5558/tfc63351-5

British Columbia Ministry of Forests and Range. 2009. The Douglas-fir tussock moth and tussockosis. Ministry of Forests, Lands & Natural Resource Operations, Thompson Okanagan Region. Kamloops, British Columbia. Douglas-fir Tussock Moth Public Information Sheet #3. 1 p.

Erickson, R.D. 1995. Douglas-fir tussock moth. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. Forest Pest Leaflet 9. 4 p. https://ostrnrcan-dostrncan.canada.ca/entities/publication/d645b678-628e-43d1-9843-b8df605ef743?fromSearchPage=true

Harris, J.W.E.; Dawson, A.F.; Brown, R.G. 1985. The Douglas-fir tussock moth in British Columbia 1916-1984. Canadian Forestry Service, Pacific Forestry Centre. Victoria, British Columbia. Information Report BC-X-268. 16 p. https://ostrnrcan-dostrncan.canada.ca/entities/publication/c9878020-1e8a-4a29-b21b-c22ab756a99e?fromSearchPage=true

Maclauchlan, L.; Buxton, K.; Stock, A.; Rankin, L.; Cleary, M.; Murray, M. 2009. 2009 overview of forest health in the Southern Interior Forest Region. British Columbia Ministry of Forests and Range, Southern Interior Forest Region, Kamloops, British Columbia. https://www2.gov.bc.ca/assets/gov/environment/research-monitoring-and-reporting/monitoring/aerial-overview-survey-documents/2009_sifr_fh_overview.pdf [Accessed April 2024]

Otvos, I.S.; Maclauchlan, L.E.; Hall, P.M.; Condor, N. 1998. A management system to control Douglas-fir tussock moth, Orgyia pseudotsugata, using OpNPV. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. Technology Transfer Note No. 11. 4 p. https://ostrnrcan-dostrncan.canada.ca/entities/publication/530ad05f-5057-4b28-95f7-c85608eef815?fromSearchPage=true

Shepherd, R.F.; Otvos, I.S.; Chorney, R.J.; Cunningham, J.C. 1984. Pest management of Douglas-fir tussock moth (Lepidoptera: Lymantriidae): Prevention of an outbreak through early treatment with a nuclear polyhedrosis virus by ground and aerial applications. The Canadian Entomologist 116(11): 1533–1542. https://doi.org/10.4039/Ent1161533-11

Cite this fact sheet

Photos

Dorsal view of a mature larva of Douglas-fir tussock moth on Douglas-fir.

Dorso-lateral view of a mature larva of Douglas-fir tussock moth on Douglas-fir.

Lateral view of a mature larva of Douglas-fir tussock moth feeding on the needles of Douglas-fir.

Dion Manastyrski Centre de foresterie du Pacifique, Victoria (Colombie-Britannique) / Pacific Forestry Centre, Victoria, British Columbia

Dorsal view of a mature larva of Douglas-fir tussock moth feeding on the needles of Douglas-fir.

Dion Manastyrski Centre de foresterie du Pacifique, Victoria (Colombie-Britannique) / Pacific Forestry Centre, Victoria, British Columbia

Mature larvae of Douglas-fir tussock moth feeding on the needles of Engelmann spruce.

Dion Manastyrski Centre de foresterie du Pacifique, Victoria (Colombie-Britannique) / Pacific Forestry Centre, Victoria, British Columbia