Yes, it’s true. A native insect, the Douglas-fir tussock moth, is wreaking havoc in some of Washington’s forests. We often talk about the dangers of invasive species but the home-grown ones, like the tussock moth, can cause problems at times.
The Douglas-fir tussock moth is a caterpillar that, by eating the needles of conifers, can severely defoliate and damage Douglas-fir, true fir, and spruce trees in the western United States. Also, hairs from the caterpillars and their cocoons can give people an allergic reaction because their tubular hairs carry toxins inside.
The tussock moth can cause defoliation that may kill or top-kill some trees. It also can weaken additional trees that may eventually be attacked by bark beetles. And it can delay tree growth for several years. For example, a very large and severe outbreak in the Blue Mountains of Oregon and Washington from 1972 to 1974 killed two out of every five trees in the heavily defoliated areas.
Outbreak areas
Outbreaks of the Douglas-fir tussock moth appear to develop almost explosively, and then usually subside abruptly after a year or two. These outbreaks happen every seven to eleven years in eastern Washington. During a severe outbreak, caterpillars are found crawling on the ground or on trees, brush, and buildings, but once an outbreak subsides, finding caterpillars is difficult.
DNR’s Forest Health Alert of 2010 shows the outbreak status of this conifer defoliating caterpillar.
An outbreak is developing in the mountains south of Spokane, along the Idaho border and in northern Idaho. Reportedly, the defoliation is light, primarily in grand fir and it’s not expected to kill many trees directly. Another outbreak in Okanogan County started expanding in 2008. It peaked at 3,500 acres affected in 2009; however, the damage has slowed due to natural controls on the moths. Natural controls can be any combination of virus, parasites, predators, disease, or starvation that kills the moth — usually, it happens by the fourth year of the outbreak.
But, at the same time that the damage slowed in Okanogan County, there was an increase in areas affected in Spokane County, climbing to about 570 acres in Washington. The affected area is along the Idaho border on Mica Peak, Tekoa Mountain, and Gelbert Mountain. Almost 9,000 acres are defoliated in Kootenai and Benewah counties in Idaho.
If caught early in the outbreak, egg masses can be analyzed to determine the level of virus and parasitism. This information can be used to predict the duration and possibly predict severity of the outbreak.
Next steps – monitoring
DNR scientists placed pheromone traps on the Washington side of the affected area to monitor the population. The number of fresh egg masses found in 2010 indicates that this outbreak could spread in 2011. Affected landowners will be notified of the outbreak status and are asked to look for signs of tussock moth. New monitoring traps will be placed in 2011 and the extent of new defoliation will be mapped from the air. Forest stands can be made more resilient to tussock moth damage by careful thinning of Douglas-fir and true firs while retaining resistant pines and larch.
DNR’s Forest Health Program provides technical assistance on tree and forest health care for a variety of public and private landowners. The program also provides early warnings of pest epidemics or reports on new pests. This is done through annual surveys ranging from aerial and ground surveys to pheromone (attractant) trapping.
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