Small Forest Landowner News Douglas-fir Tussock Moth Outbreaks Return to Eastern Washington, Starting in Unexpected Places By Glenn Kohler, DNR forest entomologist,
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[email protected]) The Douglas-fir tussock moth caterpillar is arguably a photogenic insect, but when populations boom every seven to 10 years, there can be an ugly side: Hungry tussock moth caterpillars feed on both new and old foliage of their preferred hosts (Douglas-fir, true firs, and spruce). Chewed foliage dries and turns brown, giving affected trees a scorched appearance starting in early July. If most of the foliage is removed, this can result in tree mortality or top-kill in a single season, potentially killing up to 40 percent of host trees in an infested stand. The trees most vulnerable to mortality are those with the least foliage, such as young trees and those growing in dense stands. Fortunately, tussock moth outbreaks are short-lived, due to a number of natural enemies and disease that collapse local outbreak populations after a year or two of defoliation damage. Also, mostly because the adult females are wingless and incapable of flight, the damage footprint of tussock moth outbreaks tends to be smaller and more scattered than the western spruce budworm, another notorious Eastern Washington forest pest that prefers similar hosts. Douglas-fir tussock moth (Orgyia pseudotsugata) is in the “tussock moth” subfamily Lymantriinae, which includes the non-native and much more destructive gypsy moth. Unlike gypsy moths, the tussock moth is native to western North America and its outbreaks are a natural part of dry forest ecosystems in the West.