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This defoliating insect is native to North America and occurs in southern British Columbia. Its preferred host is Douglas-fir, although it will attack adjacent ponderosa pine, western larch, and Engelmann spruce during severe infestations. Most of the outbreaks of this insect have occurred in the Kamloops Forest Region. The young larvae feed only on new foliage while older larvae will consume both old and new foliage. They start feeding in the upper crown and branch tips and continue feeding down the crown resulting in a scorched appearance by July. As defoliation continues, the tree will eventually be completely stripped. In moderate defoliations, top-killing and dieback are common, but 1 or 2 years of severe defoliation may result in tree mortality. Defoliation of larger trees increases the risk of a secondary attack by the Douglas-fir beetle, Dendroctonus pseudotsugae Hopk.

Points of interest

• one generation per year
• first reported outbreak in Canada occurred in 1916
• females are wingless
• larvae produce long silken threads facilitating their dispersion via wind currents
• the hairs from caterpillars can cause irritation or an allergic reaction
• average of mature larval or full-grown length is 30 mm

References

Erickson, R.D. 1995. Douglas-fir tussock moth. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre, Victoria, B.C. Forest Pest Leaflet 09. 4 p.

Prebble, M.L. 1975. Aerial Control of Forest Insects in Canada. Department of the Environment, Ottawa. 330 p. + xii.