Douglas-fir tussock moth outbreak seen in North Idaho

The summer of 2020 saw the collapse of one outbreak of Douglas-fir tussock moth in southern Idaho near Smiths Ferry. However, new outbreaks in North Idaho forests were found in August 2020. The damaged trees are easily seen from Interstate 90 and have a reddish hue to the tops and outer branches.

Damage was observed in two general areas in Idaho; in the Silver Valley along the I-90 corridor, and east of Clarkia. In Silver Valley, surveys found few egg masses and evidence of parasites and virus in the populations near Wallace and Mullan. This indicates the Silver Valley moth populations appear unhealthy and will likely collapse next year. In contrast, the Douglas-fir tussock moth populations found east of Clarkia near the Floodwood State Forest and south of Avery appear to be healthy and are building, and defoliation will probably increase next year.

About 13,700 acres of defoliation occurred in northern Idaho in 2020, but damage also occurred in western Montana. Aerial surveys mapped over 73,000 total acres of defoliation of Douglas-fir and grand fir between the two states.

Douglas-fir tussock moth is a native caterpillar throughout the West and typically has outbreaks in northern Idaho every eight to 12 years that last two to four years. The last outbreak in northern Idaho occurred from 2010-2012 and reached 68,000 acres of defoliation in 2011. Southern Idaho is on a different outbreak schedule. The recent southern Idaho outbreak affected over 200,000 acres at its peak in 2019.

The caterpillars eat green tree needles and prefer to feed on grand fir and Douglas-fir. After severe defoliation, entire trees or treetops may die, but trees usually make a full recovery from light or moderate defoliation. Outbreaks end on their own due to caterpillar starvation, natural enemies such as parasitic wasps and flies, as well as a viral disease that is specific to this species.

The Idaho Department of Lands and U.S. Forest Service conduct annual surveys to determine moth population levels in areas where outbreaks have historically occurred. Historical outbreak areas include the Coeur d’Alene Indian Reservation, McCroskey State Park and the Moscow Mountain area of Benewah and Latah Counties.

Trapping in these areas found high trap catches of adult moths, indicating building moth populations, but no defoliation has yet occurred. Fall surveys for Douglas-fir tussock moth egg masses, the overwintering life stage of the insect, are helpful to predict where defoliation may occur in 2021. No egg masses were found in the usual outbreak areas of Latah and Benewah counties this fall, but it is possible that egg masses were too high in the trees to see from the ground.

Douglas-fir tussock moths have a natural inclination to move upwards in a tree, and egg masses will be concentrated in treetops until populations are very large. Therefore, defoliation in the historic outbreak areas is still possible in 2021. High trap captures were also found at four sites in the Nez Perce National Forest near Elk City, and egg masses were found at one of these sites, so defoliation is also possible in this area.

Share