Douglas-fir Tussock moth outbreak damaging trees – causing allergic reactions

Douglas-fir trees throughout western Montana are being damaged by an outbreak of Douglas-fir tussock moth (DFTM). It is localized to the historic outbreak regions including Missoula. Caterpillars and the egg masses of the Douglas-fir tussock moth can cause "tussockosis."

 The Montana Department of Natural Resources and the USDA Forest Service are currently monitoring the scope and extent of damage. For the previous one to two years, DFTM have damaged the tops of ornamental Colorado blue spruce, which is typical prior to noticeable damage in Douglas-fir forests. 

DFTM are voracious feeders that can cause alarming damage to trees, but a virus naturally builds up by the third year of an outbreak and controls the outbreak. Insects in the highly visible patches are estimated to be in their second year of activity, although surveys are underway to identify areas where the outbreak is in earlier stages and damage may continue in the coming years. 

Healthy trees may rebound from the defoliation, but young, densely stocked or drought-stressed trees may die. 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. More trees may die if defoliation continues or bark beetles move into the stand. 

Landowners should consider thinning stands to reduce competition for light, water, and nutrients. Some areas may not yet be showing evidence of DFTM but could be damaged in the next year or two. Spraying insecticides is not typically warranted unless defoliation is intolerable on individual high value trees. Spray must be applied when DFTM are still actively feeding next spring or early summer. 

While DFTM is not dangerous to the public, the hairs on the caterpillars as well as their egg masses can cause an allergic reaction in humans known as tussockosis. Itching is the most common complaint but adverse health effects can include rashes with welts or blisters, watery eyes, runny nose, cough and less commonly, shortness of breath, wheezing and chest tightness.

Avoid handling larvae and areas where the outbreak may be more severe. There have been many reports of the caterpillars from huckleberry pickers in the area.

"The situation surrounding the Douglas-fir tussock moth is still evolving," says Amy Gannon, DNRC entomologist. "The good news is Montana has a lot less Douglas-fir tussock moths than surrounding states and people are likely not having sustained contact with the larvae."