Douglas-fir tussock moth, Orgyia pseudotsugata
Order: Lepidoptera
Family: Erebidae
Description
Young larvae of the Douglas-fir tussock moth are black with long body hairs. Mature larvae are around 3.5 cm (1.4 inch) in length and develop brightly colored tufts of hair. Female moths have thick bodies and small, non-functional wings, while male moths have a wingspan of 2.5-5 cm (1-2 inches) with rusty-colored forewings and gray or brown hindwings. The cocoons are brown and covered with hairs. The white spherical eggs are laid in masses of 300 and covered with a frothy substance and gray body hairs from the female moth.
Quick Facts
- The Douglas-fir tussock moth is native to Colorado and is widely distributed throughout much of North America.
- Caterpillars of Douglas-fir tussock moth feed on the needles of spruces, Douglas-firs, and true firs. Feeding is initially concentrated on new growth at the top trees, and entire trees can be defoliated and killed when caterpillar densities are high.
- There are many natural defenses that prevent outbreaks of Douglas-fir tussock moths, such as the virus that attacks caterpillars and causes “wilt disease”. Parasitoid wasps, birds, and spiders also prey on caterpillars.
- Several insecticides can prevent injury to susceptible trees during Douglas-fir tussock moth outbreaks. These treatments tend to be the most effective when applied after egg hatch.
Caterpillar of Douglas-fir tussock moth. Note the tufts of hair, some of which are dense or brightly colored. Image credit: Donald Owen, California Department of Forestry and Fire Protection, Bugwood.org
An adult female (left) and an adult male (right). Note the absence of wings on the female. Image credit: Tom Gray, Canadian Forest Service, Bugwood.org
Cocoon of the Douglas-fir tussock moth. Image credit: William M. Ciesla, Forest Health Management International, Bugwood.org
Feeding injury on a Douglas-fir tree caused by caterpillars of the Douglas-fir tussock moth. Note that the injury is concentrated at the top of the tree. Image credit: USDA Forest Service – Coeur d’Alene Field Office, Bugwood.org
Feeding injury on Douglas-fir tree caused by caterpillars of the Douglas-fir tussock moth. Note the wilting and browning of needles. Image credit: William M. Ciesla, Forest Health Management International, Bugwood.org
Egg mass of the Douglas-fir tussock moth. Image credit: Kenneth E. Gibson, USDA Forest Service, Bugwood.org
Life history and habits
Douglas-fir tussock moths have four life stages: egg, larvae, pupa, and adult. Eggs overwinter in the foliage of host trees and typically hatch in late May. A single egg mass can contain as many as 200 eggs. Newly emerged larvae migrate upward and feed on new foliage, which can cause needles to wilt and turn from green to brown. During this stage of development, the caterpillars may be dispersed within 200 m (656 feet) of an infested tree through wind currents, which is necessary for establishing new infestations because female moths do not fly. By mid-July or August, the larvae become fully grown and will migrate to a nearby tree that is not infested. Here, they form brown, spindle-shaped cocoons until they emerge as adults in late July to mid-August. Adults mate near the pupal case of females. One generation is produced annually.
Outbreaks of this pest usually subside after one to three years and pose a significant risk to millions of acres of western coniferous forests. Populations can increase rapidly, and outbreaks of Douglas-fir tussock moth tend to be abrupt and unpredictable, with the most recent outbreak in Colorado occurring in 2014 along the Rampart Range. Along the Front Range of Colorado, this pest most frequently causes problems in urban environments rather than natural forests. The greatest predictors of defoliation include stand age, elevation, and the proportion of host species. Research suggests that host-dominated stands on ridges at lower elevations or occupying the high slope are at the greatest risk of infestations. Hosts of Douglas-fir tussock moth include spruce (Picea spp.), Douglas-fir (Pseudotsuga menziesii), and true firs (Abies spp.).
Injury
Initial signs of feeding injury tend to be concentrated on newer foliage around the top of the tree. Feeding on young needles causes them to turn brown and wilt, and caterpillars begin to feed on older needles after the new foliage is consumed. Observations in the Sierra Nevada Mountains in northern California indicate that trees with a 2.5-15 cm (1-5.5 inches) diameter at breast height are most susceptible to Douglas-fir tussock moth infestations. Tree mortality often results when caterpillar densities are high and over 90% of needles are destroyed, while top-kill results from 50-90% defoliation at the crown of infested trees. Lower levels of defoliation increase the susceptibility of trees to other pests such as the Douglas-fir beetle (Dendroctonus pseudotsugae). Furthermore, the death of large numbers of trees can increase the risk of wildfires, and the hairs of larvae can also trigger allergic reactions in some individuals and domestic animals, a condition known as “tussockosis”. This can be especially problematic in forested recreational areas, such as campgrounds.
Monitoring
Monitoring protocols provide forest managers with an early warning system for Douglas-fir tussock moth one to three years prior to the occurrence of outbreaks. Based on research, a minimum of one early warning system plot per 3,000 acres is recommended. As of 2004, there were about 800 early warning system plots throughout susceptible forests of Arizona, California, Colorado, Idaho, Montana, Oregon, Utah, and Washington. Forests containing high densities of Douglas-fir or true firs are the most at-risk. The early warning system consists of five traps placed in a line at 75-ft intervals and a minimum of 75 feet away from roads. Traps are deployed about 6 ft above the ground on the ends of branches of host trees.
Each trap is made of a half-gallon milk carton cut into a triangular shape. The interior walls are lined with an adhesive, and the interior is baited with a synthetic pheromone suspended on a pin and positioned above the adhesive. Traps are deployed from late July to mid-August and collected from mid-October to early November annually. Average trap captures of more than 25 moths indicate a potential outbreak within one to two years. When this threshold is reached, it is recommended that larvae or egg masses be sampled around the monitoring plot to pinpoint and evaluate the severity of an infestation. To do this, trees should be inspected for empty eggs in the egg mass and small holes in foliage created by newly emerged caterpillars. It is recommended that ground sampling be conducted near campgrounds, parks, and administrative sites where wildfires are more likely to occur due to human activity. It is important to note that rising trap captures should be evaluated in the context of the time elapsed since the previous outbreak and the spatial distribution of plots in which traps exceed the capture threshold.
To confirm that an outbreak of moths has ended due to NPV, the infested tree should be surveyed in winter and early spring for egg masses in the year following the viral outbreak. If there are few egg masses present by August, it is unlikely an outbreak of Douglas-fir tussock moths will occur. If unhatched egg masses are easily visible on the tree or in the vicinity of host trees, further injury can occur. Before applying treatments, it is recommended to inspect for feeding larvae shortly after bud break.
Biological control
There are many natural enemies of Douglas-fir tussock moths that can suppress outbreaks, Two native nucleopolyhedroviruses (NPVs), Orgyia pseudotsugata multiple nucleopolyhedrovirus (OpMNPV) and O. pseudotsugata single NPV (OpSNPV), are important pathogens that suppress populations of Douglas-fir tussock moth. High infection rates are observed during pest outbreaks, and the viruses tend to peak when the host population begins to decline. Applications of OpMNPV are made early in the outbreak cycle based on monitoring observations. These viruses cause lethal “wilt disease”, in which infected larvae hang from needles or stems. The infected caterpillar eventually ruptures, releasing millions of virus particles that can persist and suppress populations of Douglas-fir tussock moth for years following an outbreak.
Several species of parasitoid wasps and tachinid fly can attack developing larvae and pupae. Young caterpillars are susceptible to predators such as spiders and birds, but as the caterpillars mature, the dense tufts of hair provide more protection against predators.
Chemical control
There are several insecticides that can control Douglas-fir tussock moth populations in landscapes. Sprays should be applied thoroughly to cover new growth at the top of the tree where feeding of caterpillars is concentrated. It is most effective to spray shortly after eggs have hatched, usually around bud break between mid-May and mid-June. Therefore, monitoring for egg hatch is recommended for optimal timing of the insecticide application. For more information on chemical control of Douglas-fir tussock moth, the full factsheet is available here.
CSU Extension Fact Sheet
Download or view the CSU Extension’s PDF fact sheet for your reference.
References
Negron, J., A. Lynch, W. Schaupp, and J. Mercado. 2014. Douglas-Fir Tussock Moth- and Douglas-Fir Beetle-Caused Mortality in a Ponderosa Pine/Douglas-Fir Forest in the Colorado Front Range, USA. Forests. 5: 3131-3146. Available https://mdpi-res.com/forests/forests-05-03131/article_deploy/forests-05-03131.pdf?version=1418382737
Sheehan, K., G. Daterman, and J. Wenz. 2004. Early Warning System for Douglas-Fir Tussock Moth Outbreaks in the Western United States. Western Journal of Applied Forestry. 19(4): 232-241. Available https://academic.oup.com/wjaf/article/19/4/232/4717693
Thorne, C., I. Otvos, N. Conder, and D. Levin. 2007. Development and Evaluation of Methods to Detect Nucleopolyhedroviruses in Larvae of the Douglas-Fir Tussock Moth, Orgyia pseudotsugata (McDunnough). Applied and Environmental Biology. 73(4): 1101-1106. Available https://journals.asm.org/doi/epub/10.1128/aem.01958-06
Utah State University. (n.d.). Douglas-fir Tussock Moth. Utah State University – Extension. Available https://extension.usu.edu/pests/ipm/ornamental-pest-guide/arthopods/moths/douglas-fir-tussock-moth