Douglas-fir Tussock Moth

What is the Douglas-fir Tussock Moth?

The caterpillars of the Douglas-fir Tussock moth chew on the needles of spruces, douglas fir, and true firs. The young caterpillars have long black/grey hair turning brightly colored as they mature. A mature larva is 1-1.5 inches long with a grayish brown body and black head. The moth spends the winter as an egg within an egg mass. In the early spring it will hatch and slowly move to feed on the new growth of the tree. This occurs shortly after bud break usually in the month of May. By mid-summer the larvae become full grown and in most cases migrate away from the infested tree. At this time they pupate in close proximity to the infested trees and emerge in late summer.

What are signs of the Douglas-fir Tussock Moth?

The first sign of attack will appear in the early spring as the young larvae feed on the current years growth. The insect will defoliate trees extensively by first concentrating on the tops of the trees. During a severe attack the tree will be left with little or no foliage and look very bare. With repeated attacks or outbreaks over multiple seasons entire trees may die or be weakened to the point of potential risk of other fatal attacks such as bark beetles.

Our Approach

Although there are many natural controls of this particular insect, chemical controls are recommended in certain cases. Natural controls include weather patterns, natural predators, as well as the nuclear polyhedrosis virus  that can keep populations of the insect low. The Douglas-fir tussock moth tends to follow a cyclical pattern with outbreaks occuring every 8-12 years and lasting 2-4 years. Chemical control is applied in the early spring before egg hatch. A follow up application is also suggested 3-4 weeks after the initial control is implemented.

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