Drive south of Coeur d’Alene along Highway 95 and it’s hard to miss the hillsides of brown trees, particularly in the Worley-Plummer area. What, you may ask, is causing this? The culprit is Douglas-fir Tussock Moth (DFTM), a forest pest that feeds on the needles of Douglas-fir, true firs and ornamental spruce trees.
DFTM is a native forest insect found throughout the Inland Northwest, but it usually causes noticeable damage only in drier ecosystems south of Interstate 90. Another defoliator, the Western Spruce Budworm, also infests trees in this region, but its damage is generally more concentrated on Forest Service land in North Idaho. During the caterpillar life-cycle stage, both of these insects cause damage by feeding on needles, and have the potential to cause severe tree defoliation.
Typically, DFTM outbreaks occur approximately every 7-10 years (once per decade), with damage lasting for 2-4 years. A naturally-occurring virus usually causes the population levels to crash. During outbreaks, trees suffer from growth loss, top kill, and, in some instances, outright mortality. Working in conjunction with DFTM, bark beetles have the potential to take advantage of weaken trees and increase tree mortality levels.
If your forest is infected with DFTM, there are numerous approaches to managing the insect. Doing nothing is a viable option. Most trees that are attacked by the insect will incur reduced growth, but will recover. Spraying registered insecticides is another approach to consider. Proper timing of the application is critical, and generally takes place in June. If you are considering a salvage harvest, the most important point to remember is that most defoliated trees will survive and it is generally best to wait until the following spring before deciding whether to cut infected trees. If a tree has healthy buds on terminal branches, it will grow new needles. Repeated defoliation is the most damaging to trees.
In terms of long-term management on drier sites, consider shifting away from growing fir species in favor of larch and pine trees, which are usually much less susceptible to major pests, such as root disease, and provide valuable forest diversity in areas where Douglas-fir and grand fir are common.