Recently I have begun to observe damage to ornamental blue spruce throughout the area of Colville, Washington. Upon closer inspection of these trees, I found that they are being defoliated by the Douglas-fir tussock moth. The Douglas-fir tussock moth is a native defoliator of Douglas-fir, true firs (such as grand fir) and spruce. For reasons unknown, a year or two prior to an outbreak of Douglas-fir tussock moth on forested land, we tend to see defoliation of ornamental trees such as blue spruce. Given the number and area of defoliated blue spruce I have been seeing, it is likely we will have an outbreak of the Douglas-fir tussock moth sometime within the next two years. Unfortunately, no relationship has been found between the location of the sentinel trees and the forested areas that will be defoliated in the future. In other words, we know that there will likely be an outbreak, but we do not know exactly where it will occur.
Outbreaks of Douglas-fir tussock moth are cyclical, typically occurring every 7 to 14 years. On average, outbreaks last 2 to 4 years. The last outbreak began in 2008 (Figure 1); therefore, we are due for another.
Lifecycle of the Douglas-fir tussock moth
The Douglas-fir tussock moth spends the winter months in the egg stage. Eggs are protected in gray, hairy masses that are approximately an inch in size. An egg mass can contain as many as 350 eggs. The eggs will hatch in late May or early June, depending upon temperatures.
The caterpillar (larvae) will be present from June through August. The caterpillars are quite hairy, with two long hairy tufts projecting from the head and the rear end. They also have four dense tufts of hair on their back, called tussocks, which are whitish in coloration with red tips.
The caterpillars pupate in July-August. Cocoons are grayish-brown, about one inch in size, and can be found on the foliage and trunk of trees as well as in the understory.
The pupae develop into moths and begin emerging in late July. They will continue to be active through November. The females are gray-brown, with large abdomens and are wingless. The males have gray-brown forewings and reddish-brown hind wings. They also have large, feathery antennae.
Damage: What to look for
The larvae feed on new needles in the upper crown first. These needles will turn an orangish-brown color (Figure 2). Overtime, the caterpillars will disperse to the lower crown and begin defoliating needles there. The larvae will feed on both new and old needles, sometimes completely defoliating the tree.
In addition to defoliation, you may find the moth in one of its life stages, depending upon the time of year. Another thing to look for is silk and/or frass ( poop) on the branches (Figure 3).
Defoliation by the Douglas-fir tussock moth can cause top and branch kill, which can lead to reduced vigor and growth loss. This can increase susceptibility to bark beetle attack or infection by diseases. Complete defoliation or several years in a row of defoliation can lead to mortality.
Douglas-fir tussock moth is usually controlled over time by natural enemies such as predators, parasites, viruses, cold temperatures, and/ or starvation (eating themselves out of house and home), but can also be managed through use of insecticide treatments.
Bacillus thuringiensis var. kurstaki (Bt) is an insecticide that is specific to Lepidoptera larvae. Its specificity is advantageous because it does not affect other insects, such as the natural enemies that help reduce populations. Because Bt is specific to the larval stage, it would have to be applied when the Douglas-fir tussock moth is in the caterpillar stage.
In forested settings, severe damage can be prevented through thinning. It would be best to remove host trees (Douglas-fir, grand fir, Engelmann spruce) and favor the retention of non-host trees (ponderosa pine, western larch, lodgepole pine) thereby reducing the amount of food available. Additionally, thinning breaks the crown continuity within a stand, so that when the caterpillars disperse, many will fall to the ground and dessicate or be eaten by passing birds.
If you think you may have a sentinel tree on your property, I would love to know about it! Feel free to contact me via Melissa.Fischer@dnr.wa.gov.
By Melissa Fischer, forest health specialist, Washington State Department of Natural Resources, Northeast Region, firstname.lastname@example.org