Idaho Tussock Moth Outbreak Almost Over, But Be Wary Of Fuzzy Caterpillars

May 10, 2019

If you’ve seen naked-looking trees up and down the Highway 55 corridor, they’re not necessarily dead. 


An outbreak of Douglas-fir tussock moth is in its final year, but affected trees in the Boise and Payette National Forests aren't out of the proverbial woods just yet.



"The trees have to be very defoliated to die just from the caterpillars eating them," explains Laura Lowrey, an entomologist with the Forest Health Protection division of the U.S. Forest Service.

A tree that's lost 90% or more of its needles is at risk of mortality, she says.

Fir trees that've had their needles eaten off are in stress, though — and that makes them more vulnerable to bark beetles and other pests.

The onset of green-up means recovery may be on the horizon for naked trees, but it depends on how severely they were defoliated and how much water is available during the growing season, Lowrey says. Conditions like heat and drought are also at play. 

Before things get better, they'll likely get worse. Lowrey says she expects to see one more year of defoliation before the tussock moth outbreak has run its course.


Defoliated areas are pictured in red. Some of the hardest-hit spots on the forest are between Cascade and Smith's Ferry.
Credit U.S. Forest Service

The Douglas-fir tussock moth is native to Idaho, and outbreaks happen on roughly a 10-year cycle. An outbreak usually lasts for about four years, with the third and fourth years being particularly noticeable for defoliation.


The outbreaks are cyclical, and they tend to peter out on their own.


"Just because of the sheer numbers of the caterpillars, they end up starving by year four of the outbreak, which we're expected to be in in 2019," she says.


(Apart from exhausting their own food supply, caterpillar populations are also kept in check by an endemic virus which turns them into liquified, oozy goo — sort of like a caterpillar zombie apocalypse.)


Even though the current outbreak is wrapping up, campers and hikers should still be careful in affected parts of the forest. Brushing up against caterpillar hairs can cause an allergic reaction.


"Tussockosis is this allergic reaction to the caterpillar hairs. Some people just get hives from the hairs. But in some people it can go systemic and cause them to have a hard time breathing," Lowrey says. “It’s better to camp in areas that don’t have an outbreak than those that do, just to be on the safe side.”


Sage Hen Reservoir just outside Smith's Ferry is one popular camping spot where there are expected to be a lot of caterpillars this summer.


Fir trees around Sage Hen Reservoir outside Smith's Ferry were heavily defoliated in 2018. Entomologists anticipate further defoliation in 2019 until the tussock moth outbreak runs its course.
Credit U.S. Forest Service

“[Caterpillars] usually hatch beginning of June in the Sage Hen area,” Lowrey says.

The Forest Service is asking those harvesting firewood to be sure they're not cutting down trees that aren't dead yet. Even though tussock-affected trees may be missing most their needles, many will recover when the outbreak passes.


For more information on tussock moths in Idaho, check out the following report:

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