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The observable defoliation caused by Douglas-fir tussock moths is the reddish half-chewed needles concentrated in the tops of trees and is most visible in mid-to late-August.

Portions of the Caribou-Targhee National Forest are experiencing an outbreak of Douglas-fir tussock moths just west of Driggs, near Big Hole Mountains and on nearby private lands. This hotspot is part of a larger outbreak synchronously occurring throughout much of southern Idaho.

The observable defoliation is the reddish half-chewed needles concentrated in the tops of trees and is most visible in mid-to late-August. While trees can recover from heavy defoliation, those repeatedly defoliated of 90% or more of its needles have a high probability of dying. “Many trees may look dead due to their red needles,” said Russ Oakes, Targhee Zone Silviculturist, “However, typically they are not dead and should not be cut for firewood. A good rule of thumb is if the tree has any green needles, do not cut.”

People visiting areas with heavy defoliation and lots of caterpillars or egg-masses should use caution. The moth’s caterpillar long bristly hairs may cause allergic-like reactions for some individuals, particularly in August and September when the caterpillars are peaking. A rash called tussockosis can persist for a few hours to several weeks. Recreations can worsen with exposure and can compromise airways for some sensitive individuals. For most people a topical over-the-counter treatment can treat allergic symptoms, but people with severe reactions should seek medical attention.

For more information go to the Forest Service website

As caterpillars of tussock moths form their cocoons in August less defoliation will occur this year. Later this fall they emerge as adult gray mottled moths and lay eggs. This particular moth overwinters in egg-masses laid during the fall. Sampling these egg-masses this fall will help determine the extent of defoliation expected in 2021.

Tussock moth populations began to increase in 2017. Typically, outbreaks last 3-4 years and subside when natural enemies, such as a virus specific to the caterpillars and parasitic wasps, crash the outbreak. Options for control are somewhat limited over large areas and natural controls are often the most appropriate means to control this moth. This is estimated to be year 3 of the outbreak cycle for the infestation near Driggs.