A Douglas fir tussock moth.

While humankind bemoans the coronavirus, Douglas fir trees in the Big Hole mountains west of Driggs will benefit from a virus outbreak to knock down an ugly infestation of tussock moths.

The Caribou-Targhee National Forest is reporting an estimated 250 acres of defoliated Douglas fir trees brought on by tussock moths. The sight can be seen from the highway and in the distance from the town of Driggs. At the same time, even larger portions of southern Idaho are being attacked by tussock moths.

“It looks pretty bad,” said Carl Jorgensen an entomologist with the National Forest Service. “These small outbreaks come every decade or two. The last outbreak in the Big Holes and parts of the Caribou (National Forest) I think was the early to mid-90s when it was really noticeable.”

Jorgensen said the tussock moths feast on the needles of the trees causing them to turn brown and shed damaged needles. He said that while the tree looks bad, they generally recover and don’t die off.

“If defoliation gets to be above 90% or greater of the crown of an individual tree, those trees have a potential of dying,” he said.

Officials want to emphasize that although the trees may look dead, many will recover and shouldn’t be cut down for firewood.

“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.”

Jorgensen said tussock moth outbreaks typically run on a four-year cycle with the caterpillars dying off during the fourth year from predators and a virus. Regional entomologist Nicole Bisang plans to survey the area in the next few weeks to determine what stage the moths are in.

“We estimate that tussock moth populations began to increase in this area in 2017,” Bisang said. “Typically, outbreaks last three to four years and subside when natural enemies, such as a virus specific to the caterpillars and parasitic wasps, crash the outbreak. This is estimated to be year three of the outbreak cycle for the infestation near Driggs.”

Jorgensen said a much larger infestation in the southwest part of the state is subsiding after a four-year outbreak.

“Generally, we let nature play its course,” he said.

Another way the population can crash is if the caterpillars eat up all the vegetation.

“If they do eat themselves out of house and home, the caterpillars will end up starving, and you’ll see very weak looking cocoons and egg masses,” Jorgensen said. “And that’s another indication that the population is crashing back down.”

He said tussock moth outbreaks are part of nature.

“Ecologically, this is one of the ways that the forests change,” he said. “It’s interesting, and it looks bad, and it may have some associated mortality, but the forest will be there after the outbreak is over.”