The Idaho Department of Lands says several new outbreaks of Douglas fir Tussock were reported across the north central portion of the state this summer. IDL said the new outbreaks were found in the Silver Valley along I-90 as well as the mountains east of Clarkia.
Department officials say surveys found few egg masses and evidence of parasites and virus in the populations near Wallace and Mullan, indicating the Silver Valley moth populations appear unhealthy and will likely collapse next year. Meanwhile, the Douglas fir tussock moth populations found near the Floodwood State Forest outside of Clarkia appear to be healthy and are building, and defoliation will probably increase next year.
According to IDL roughly 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.
The summer of 2020 saw the collapse of one outbreak of Douglas-fir tussock moth in southern Idaho near Smiths Ferry
Previous Douglas fir tussock moth activity: Douglas-fir tussock moth is a native caterpillar throughout the West and typically has outbreaks in northern Idaho every 8-12 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.
Douglas-fir tussock moth caterpillars eat green tree needles, which is called defoliation, 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.
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