Douglas-fir trees damaged by tussock moth outbreak
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July 23, 2020
Montana Department of Natural Resources and Conservation
The tussock moth larvae feeding on Douglas-fir needles
MISSOULA -Douglas-fir trees throughout western Montana are being damaged by an outbreak of Douglas-fir tussock moth (DFTM). Damage is particularly notable throughout Missoula, Kalispell and along Flathead Lake. The Montana Department of Natural Resources (DNRC) and the USDA Forest Service are currently monitoring the scope and extent of damage.
DFTM are voracious feeders that can cause alarming damage to trees, but a virus naturally builds up by approximately the third year of an outbreak and controls the caterpillar population. Insects in the highly visible patches are estimated to be in their third year of activity, although additional areas may develop where damage continues in the coming years.
Healthy trees may rebound from the damage, whereas young densely stocked or drought-stressed trees may not have enough nutrient reserves to withstand the damage. More trees may die if severe defoliation continues or bark beetles move into the stand.
Montana Department of Natural Resources and Conservation
Large Douglas-fir have been completely defoliated by the tussock moth caterpillars.
Landowners should consider thinning stands to reduce competition for light, water, and nutrients. Spraying insecticides is not typically warranted unless defoliation or caterpillar presence is intolerable. Insecticides must be applied when DFTM are still actively feeding next spring or early summer.
Hairs on the DFTM caterpillars can cause a respiratory reaction in humans known as tussockosis. The hairs can also cause skin irritation. Avoid handling caterpillars and consider wearing long sleeves and pants in areas where the outbreak may be more severe.
Please note that western spruce budworm (WSBW) is also active in many of the same areas. WSBW can be distinguished from DFTM by observing the caterpillars. WSBW are hairless and have cream colored dots along their backs, whereas DFTM have very distinguishable tufts of hair along their backs.
Please see below for a photo of Douglas-fir affected by DFTM and a photo of the DFTM larva:
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