The Caribou-Targhee National Forest is experiencing an outbreak of Douglas-fir tussock moths/caterpillars west of Driggs, near Big Hole Mountains and private lands.
This hotspot is part of a larger outbreak that is synchronously occurring throughout southern Idaho. Outbreak typically last four years and this year three of the cycle for the caterpillars near Driggs. Caterpillar populations rise and fall rapidly and are not predicted to continue past 2020.
The moth’s tree-damaging caterpillar has long bristly hairs that may cause allergic-like reactions for some individuals. A rash called tussockosis can persist from a few hours to several weeks. Individuals do not have to come in contact with the caterpillar to be effected as the hairs may be airborne. Reactions can worsen with exposure and can compromise airways for some individuals.
Tussock moth population began to increase in 2016 following with outbreaks typically lasting 3-4 years and subside when natural enemies such as a virus specific to the caterpillars and non-stinging parasitic wasps crash the outbreak within 4 years. There is expected to be one more year of the outbreak and caterpillar populations should be gone within about 2 weeks.
Defoliation is now visible and looks like reddish half-chewed needles and tends to be worse in the tops of trees. While the trees may look dead because the caterpillars feed on the needles, they are not, and should not be cut for fuelwood. If trees have any green needles, please do not cut.
Options for control are somewhat limited over a large areas and natural controls are really best. Dense stands of trees growing in harsh sites are impacted most severely. Trees defoliated over 90 percent have a high probability of dying. Dry sites, ridges and dense stands tend to have more damage because trees have less water to recover from feeding damage. For more information: https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsbdev2_043384.pdf