MISSOULA, MT—Douglas-fir trees throughout western Montana have been damaged by an outbreak of Douglas-fir tussock moth (DFTM). It is localized to the historic outbreak regions of Missoula, Plains, Thompson Falls, Polson, Kalispell, Columbia Falls, and the Flathead Lake at this time.

The Montana Department of Natural Resources (DNRC) and the USDA Forest Service are currently monitoring the scope and extent of damage. For the previous 1-2 years, DFTM have damaged the tops of ornamental Colorado blue spruce, which is typical prior to noticeable damage in Douglas-fir. DFTM are voracious feeders that can cause alarming damage to trees, but a virus naturally builds up in the tree population by the third year of an outbreak. DNRC estimates the insects are in their second year of activity, although surveys are underway to identify areas where the outbreak is in earlier stages and damage may continue.

Please note Western Spruce Budworm (WSBW) are also active in many of the outbreak areas. WSBW can be distinguished from DFTM by observing the larvae. WSBW are hairless and have cream colored dots along their backs, whereas DFTM have very distinguishable tufts of hair along their backs.

DNRC anticipates healthy trees will rebound from the defoliation, but landowners should consider thinning stands to reduce competition for light, water, and nutrients. Trees growing in overstocked stands will likely suffer greater damage, and young or suppressed trees often do not have enough nutrient reserves to withstand the damage. Spraying insecticides is not typically warranted unless defoliation is intolerable. Spray must be applied when DFTM are still actively feeding.

While DFTM is not dangerous to the public, the hairs on the moth can cause an allergic reaction in humans known as tussockosis. The most common symptoms are irritation to the skin and lungs. Citizens are advised to avoid handling any larvae in areas where the outbreak may be severe.

For more information, contact the Montana DNRC Forestry Division at (406) 542-4300.