Rusty Tussock Moth

Orgyia antiqua (L.)

Host(s) in the Pacific Northwest:

Birch (Betula spp.), willow (Salix spp.), Prunus, Sitka Spruce (Picea sitchensis), and many other conifers, hardwoods, and shrub.

Damage: Defoliation. Major outbreaks can result in tree mortality, and during less severe outbreaks, top-kill can occur.

Warning: The hairs on the caterpillar can cause irritation to human skin, resulting in dermatitis. Do not handle caterpillars unless wearing appropriate protective clothing.

Current Status & Distribution in Alaska (2022 Update)

The rusty tussock moth outbreak in the Matanuska-Susitna Borough since 2020 appears to have collapsed. While 44,000 acres of damage caused by this generalist defoliator were mapped during ADS in the Susitna River valley in 2021, no damage was mapped in 2022 and few caterpillars were observed during fieldwork efforts. Statewide, there were scattered reports of rusty tussock moth larvae, including 13 research grade observations on iNaturalist, but no reports of substantial damage. 

Last year, rusty tussock moth egg masses collected from the outbreak area were provided to the University of Idaho for a research project. Those egg masses had been heavily parasitized by a single species of Telenomus parasitoid. In 2022, that parasitoid was confirmed as Telenomus dalmani, a documented parasitoid of rusty tussock moth and a presumed new record for Alaska. Specimens of T. dalmani were sent to the University of Alaska Museum of the North to be accessioned into its Insect Collection. 

 View the most recent observations on iNaturalist.org.

Historic Activity in Alaska

Rusty tussock moth was reported at numerous locations across the state in 2020. Notably high populations occurred in Southcentral Alaska, including 35 acres of low-level defoliation recorded near Hatcher Pass. Rusty tussock moth caterpillars were prevalent along the road system within the Matanuska-Susitna Borough, though defoliation was generally minimal; however, substantial defoliation was reported in some areas at or above treeline. Concern and reports were especially high among berry pickers who reported the caterpillars feeding on blueberries, cloudberries, currants and highbush cranberries, among others. This generalist defoliator was also reported feeding on alder, willow, birch, cottonwood, spruce, and numerous garden plants.  

With the cancellation of aerial surveys in 2020, staff were limited in their survey coverage for this insect. To supplement the survey effort, a request for reports of defoliation from this insect was distributed through social media. While most observations were from Southcentral, reports were also received from the Bethel area and near Fairbanks, as well as on the Seward Peninsula, where an outbreak of rusty tussock moth occurred in 2019. Thirty-two research grade observations of the rusty tussock moth were recorded in iNaturalist.  

Previous rusty tussock outbreaks in Alaska have occurred in 1997 and 2003, including an outbreak in 2010 in the Matanuska-Susitna Valley that led to defoliation across 55,000 acres. 

Description, Biology and Impact

The moth is so-named because of their coloration and the “tussocks” or tufts of hair running along the length of the caterpillars. Early instar caterpillars are more or less uniformly covered with hairs, whereas the hairs on later instars form into tussocks: two darkly colored tussocks near the head, one darkly colored tussock at the rear, and four yellow tussocks on the dorsal surface of the first four abdominal segments. The hairs on the caterpillar can cause irritation to human skin, resulting in dermatitis, so it is strongly recommended to not handle caterpillars unless wearing appropriate protective clothing, such as gloves and long sleeves.

Rusty tussock caterpillars hatch from overwintering eggs in early spring. Caterpillars pupate in silk cocoons spun onto the undersides of branches in August. Several weeks following pupation, adult moths emerge. Male moths have wings that are rusty brown in color with each forewing bearing a small white marking. Male moths also possess plumose antennae, which are covered with microscopic chemosensors that enable the male moths to detect the pheromones of their female counterparts. The adult female moths are wingless, sedentary, and generally 12mm long. Adult females emit pheromones to attract the flighted males in order to mate. Females deposit eggs on the host plant on which she is currently residing, which is where they will overwinter until they hatch the following spring. There is only one generation per year.

Additional Resources and News Articles

Holsten, E., Hennon, P., Trummer, L., Kruse, J., Shultz, M. Lundquist, J. (2009). Insects and Diseases of Alaskan Forests. USDA Publication Number R10-TP-140, available here.

Rusty tussock moths invade Seward Peninsula, by James Mason. The Nome Nugget 7/12/2019

Content prepared by Dr. Sydney Brannoch, Entomologist, Forest Health Protection, Sydney.Brannoch@usda.gov.

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