Small Forest Landowner News

Douglas-fir Tussock Outbreaks Return to Eastern , Starting in Unexpected Places

By Glenn Kohler, DNR forest entomologist, [email protected] (mailto:[email protected])

The Douglas-fir tussock moth caterpillar is arguably a photogenic , but when populations boom every seven to 10 years, there can be an ugly side: Hungry tussock moth caterpillars feed on both new and old foliage of their preferred hosts (Douglas-fir, true firs, and spruce).

Chewed foliage dries and turns brown, giving affected trees a scorched appearance starting in early July. If most of the foliage is removed, this can result in tree mortality or top-kill in a single season, potentially killing up to 40 percent of host trees in an infested stand. The trees most vulnerable to mortality are those with the least foliage, such as young trees and those growing in dense stands.

Fortunately, tussock moth outbreaks are short-lived, due to a number of natural enemies and disease that collapse local outbreak populations after a year or two of defoliation damage. Also, mostly because the adult females are wingless and incapable of flight, the damage footprint of tussock moth outbreaks tends to be smaller and more scattered than the western spruce budworm, another notorious Eastern Washington forest pest that prefers similar hosts.

Douglas-fir tussock moth ( pseudotsugata) is in the “tussock moth” subfamily , which includes the non-native and much more destructive gypsy moth. Unlike gypsy , the tussock moth is native to western North America and its outbreaks are a natural part of dry forest ecosystems in the West.

A Douglas-fir tussock moth infestation defoliated the crowns of many of the trees in this site near Ellensburg. (Photo by Glenn Kohler, DNR)

However, for a forest landowner who happens to have property in the epicenter of an outbreak, tussock moths can be very destructive to trees and quite literally irritating to humans. Those photogenic and colorful hairs on the caterpillars are covered with microscopic barbs that are like fishhooks. When these get under your skin, itching, swelling, and redness are not far behind (unless you are one of the lucky few who seem immune). This condition is known as tussockosis and should motivate anyone working or playing in a defoliated area to cover their skin and avoid handling caterpillars or cocoons. The first visible defoliation of the current outbreak was recorded on approximately 1,900 acres in Kittitas and Chelan counties in 2018. The damage in some areas is quite severe and can easily be seen from U.S. Highway 97 over Blewett Pass between Cle Elum and Leavenworth.

This is the first observation of tussock moth defoliation in Washington since 2012 and the first in Kittitas County since aerial surveys began in 1947. Many Douglas-fir and grand fir in this area were already damaged during a decade-long budworm outbreak that just came to an end, meaning stressed host trees may be more vulnerable to tussock moth damage.

On the positive side, generalist predator and parasite populations that built up during the budworm outbreak may have an impact on the tussock moth population.

The pheromone that flightless tussock moth females use to attract males was synthesized as a lure for sticky traps and is used to monitor changes in population. This can provide an early warning to land managers when an outbreak might be expected.

Every year since 1980, an interagency network of these “Early Warning System” pheromone traps has been deployed in Eastern Washington, with 250 locations trapped in 2018 (see map). Despite high trap catches and reports of caterpillars in Kittitas County, heavy defoliation in this area was unexpected because tussock moth outbreaks historically tend to recur in the same areas of Washington – most commonly in Okanogan County, Spokane County, and in the Blue Mountains south of Dayton.

Trap catches remain elevated in the current outbreak area and have increased in some areas of Okanogan County, which indicate higher likelihood of more tussock moth defoliation developing in 2019. Unfortunately, high trap catches do not always correlate with the location of future defoliation.

Ornamental blue spruce trees that are defoliated by tussock moth are known as “sentinel trees” because their damage often precedes forest defoliation by a year or two. There is no direct, predictable relationship between the location of sentinel trees and locations where forest areas will be defoliated.

Tussock moth populations are normally kept low by natural controls that include disease, predators, food supply, and weather. Periodically, tussock moth populations are able to escape these controls and outbreaks occur. Outbreaks typically collapse within two to four years due to a combination of build-up in natural enemies and/or starvation (see chart).

Caterpillars are highly susceptible to a nucleopolyhedrosis virus (NPV) that can rapidly spread through the population and is a frequent cause of collapse. Infected caterpillars tend to hang from branches when dead, where their virus-liquefied bodies swell and burst in the heat, spreading virus onto foliage that will be eaten by nearby caterpillars.

If they survive to adulthood and mate, female tussock moths lay masses of in late summer on top of their own cocoons. Virus can get onto eggshells through the mother, nearby “exploding” caterpillars, or rain and snow washing down over winter. When caterpillars hatch in spring, their first meal is their own eggshell, which may seal their fate if it’s covered with virus. Early in an outbreak, masses can be analyzed to determine the A Douglas-fir tussock moth level of virus and egg parasitism by tiny wasps. This information egg mass and cocoons hang can be used to predict the duration and possibly predict severity of off the needles of a Douglas- the outbreak. The U.S. Forest Service is analyzing eggs collected fir tree. (Photo by Glenn from state and federal land during the current outbreak. Kohler, DNR) If no actions are taken to manage or prevent tussock moth damage, the outbreak will subside in two to four years due to a build-up of natural controls. Management options are available to protect host trees valued for timber, recreation, or aesthetics.

Tussock moths may damage ponderosa pines and western larch, but is unlikely to kill them, so a higher proportion of non-hosts will lessen the overall impact. If timed before an outbreak, thinning the forest to reduce the proportion of Douglas-fir and true fir can reduce tussock moth damage.

Dense understory trees are more vulnerable to damage because they have fewer needles and tussock moth caterpillars may drop on them from taller trees. In an even-aged stand with widely spaced trees, caterpillars are less likely to land on host trees.

After an outbreak, killed trees can be salvaged as timber or left standing for wildlife. Selection of trees for removal should be done in spring following bud break because some trees that appear dead may produce new growth. It’s possible a tree with up to 95 percent defoliation can survive.

For immediate reduction of defoliation, insecticides can be sprayed on host trees soon after most caterpillars have emerged and begun feeding. If this option is considered, please consult a forest health specialist to assure the most effective timing, appropriate product, and application rates.

Both conventional and biological insecticides are effective against tussock moths. Biological insecticides affect fewer non-target organisms and include a commercially available bacterial insecticide, Foray or “Btk” (Bacillus thuringiensis var. kurstaki), and the naturally occurring virus “TM Biocontrol” (NPV), which is only available through the Forest Service.

Please contact the Washington State Department of Agriculture for current pesticide registration information. When using pesticides, always read and follow the label.

The Forest Service’s Forest Health Protection website has detailed information on tussock moth biology, management, and monitoring, which can be found at www.fs.fed.us/r6/nr/fid/dftmweb/index.shtml (http://www.fs.fed.us/r6/nr/fid/dftmweb/index.shtml).

Posted on April 1, 2019 by Forest Stewardship NotesPosted in wildlife

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