Douglas-fir tussock moth, Orgyia pseudotsugata
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.
- 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 are the overwintering stage, and typically hatch in late May. 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 to new locations through wind currents, which is necessary for establishing new infestations because female adult 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, and this species produces one generation per year.
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. The entire tree can be defoliated and killed when caterpillar densities are high.
Tree defoliation is a clear indicator of an infestation. Pheromone traps can be deployed in areas with a suspected infestation or a history of outbreaks. To monitor for egg hatch, which can help inform decisions related to chemical control, trees should be inspected for empty eggs in the egg mass and small holes created by newly emerged caterpillars.
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.
There are many natural enemies of Douglas-fir tussock moths that can suppress outbreaks. Most notably, the Douglas-fir tussock moth nuclear polyhedrosis virus (NPV), causes 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 the disease 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.
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.
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