Douglas-fir Tussock Moth

Orgyia pseudotsugata

 

Douglas-fir tussock moth damage T. Gray

Host: Douglas-fir, true firs, spruce.

Adult Douglas-fir tussock moth L. Livingston

Distribution: Throughout range of host species; location outbreaks common along the western shore of Flathead Lake.
Identification: Needle loss often concentrated at top of tree; presence of insect year round (eggs in winter, caterpillars in spring, adults flying in late summer).

Damage: Moths feed on current and previous years’ growth. During outbreaks, entire tree may be defoliated and killed.

Ecology: Greatest damage usually occurs on ridgetops and older, dense stands of susceptible hosts. Outbreaks are cyclical and generally last for two years before a natural virus drastically reduces the population.

Life Cycle: Females do not fly. Upon emerging from the pupal case, they mate and lays eggs. The eggs overwinter in a mass, encased with hairs from the mother’s body. Eggs hatch at approximately the same time as budburst. Larvae feed on new foliage and pupate in late summer. Moths emerge August through September.

Douglas-fir tussock moth with pupal case J. Dewey	Douglas-fir tussock moth larvae L. Livingston

Management

Silvicultural manipulations: Reduce overstory tree density to enhance the vigor of residual trees by alleviating competition for light, water, and nutrient resources. Vigorous trees will more effectively rebound after defoliation subsides. Specifically remove older, decadent trees that are most susceptible to Douglas-fir tussock moth damage. When thinning, retain pole-sized, intermediate trees along with non-host, early seral species, such as ponderosa pine and western larch. Removing host species from understory will encourage growth of early seral species and may disrupt vertical dispersion of caterpillars.

Douglas-fir tussock moth feeding damage D. McComb

 

Insecticides: Various chemical and bacterial treatments are currently registered for Douglas-fir tussock moth control. The bacterial insecticide, Bacillus thuringiensis kurstaki (commonly referred to as Btk) is naturally occurring and only affects moths, butterflies, and skippers. Insecticide applications must be done at critical times and require monitoring of insect development. As always, closely read and follow the label! Please contact the DNRC Forest Pest Management Program coordinator or a licensed forest pesticide applicator to discuss this option.

 

Douglas-fir Tussock Moth Links
Forest Insect and Disease Leaflet (FIDL)		Field Guide - Identification		Management Guide

Additional Common Forest Insects in Montana

Douglas-fir Beetle	Red Turpentine Beetle
Douglas-fir Tussock Moth	Spruce Beetle
Fir Engraver	Western Pine Beetle
Mountain Pine Beetle	Western Spruce Budworm
Engraver Beetles (Ips species)