Douglas-fir tussock moth defoliates Douglas-fir, spruce and true fir tree species, including white fir.
Douglas-Fir Tussock Moth
Douglas-fir tussock moth (Orgyia pseudotsugata) is a native defoliator of spruce, Douglas-fir (Pseudotsuga menziesii) and true firs (Abies spp.), though will rarely feed on planted Colorado blue spruce in urban areas. The moth is a native species found throughout mixed-conifer forests in the western United States and southern British Columbia. Outbreaks typically develop rapidly and subside abruptly, usually after one to three years due to a virus disease that spreads throughout each isolated population. Widespread outbreaks have occurred in the Pacific Northwest, northern Rocky Mountains, California and the Southwest. In Colorado, isolated outbreaks tend to be cyclic in nature and may occur in intervals of seven to 10 years across the state.
Signs & Symptoms
Tree defoliation is the most conspicuous symptom of Douglas-fir tussock moth infestation. During outbreaks, trees can be stripped of all of their foliage in a single season. Defoliation typically begins at the top of infested trees and progresses downward. Early stages of infestation may lead to partial tree defoliation, which typically does not cause tree mortality. However, subsequent attacks of Douglas-fir tussock moth over multiple seasons may weaken a tree and predispose it to bark beetle attacks which can lead to tree death.
An outbreak that began in 2014 in three areas along the Front Range mountains expanded significantly in 2015 and collapsed in 2016. Isolated areas within the outbreak perimeter reported seeing larvae in 2016 though damage was minimal from larval feeding. All three outbreak populations of this pest within Colorado’s Front Range mountains are thought to have declined from the build-up and dispersal of a viral “wilt”disease.
No new acres of Douglas-fir, true fir and spruce were aerially detected statewide in 2017.
Ongoing management of Douglas-fir forests is the most effective long-term strategy for reducing tree losses from Douglas-fir tussock moth. Thinning overly dense stands of trees to reduce the competition between trees is the most successful forest management strategy to promote tree vigor. Improving tree stand condition, achieved by creating tree age and species diversity, will maintain and support tree health and reduce the potential impact of future outbreaks.
Options for management include:
- Reducing an over-abundance of host trees (Douglas-fir and true firs) and favoring non-host species where possible, for example, if ponderosa pine and quaking aspen are naturally present.
- Deployment of traps baited with the Douglas-fir tussock moth female sex pheromone in areas suspected of infestation, or areas which have had a history of outbreak populations, to predict looming outbreaks. Trap catches of 25 or more male moths provide an early warning that an outbreak may be imminent.
- Individual, high-value landscape trees also can be treated with one of several insecticides, provided the entire tree is treated – especially the upper crown. The best time to apply insecticides is early spring, when new needles emerge, as this will prevent feeding on the new needles. The Colorado State Forest Service (CSFS) recommends treating only high-value trees, such as those near homes, businesses or recreation sites. Follow the label of any chosen product.
Douglas-fir Tussock Moth (5.4 MB PDF)
Treatment Options for Douglas-fir Tussock Moth (8.9 MB PDF)