Douglas-fir Tussock Moth
|Douglas-fir tussock moth damage
Host: Douglas-fir, true firs, spruce.
|Adult Douglas-fir tussock moth
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
|Douglas-fir tussock moth larvae
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
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)|