The rust-colored wave of death that has rolled across portions of the Sawtooth National Recreation Area has slowed. What that means is mountain pine beetles have killed most of the eligible lodgepole pine trees in the Sawtooth Valley and Stanley Basin.
"Basically, they've attacked and killed so many of our susceptible lodepole pine, meaning eight inches and bigger in diameter," said SNRA Forester Jim Rineholt. "Now they're starting to go down to as small as six-inch trees, and that's a sign that most of the host material has been killed off."
Beginning in the early part of this decade, mountain pine beetles began moving south from the Salmon River canyon and areas in the foothills of the northern White Cloud Mountains. Assisted by warmer-than-normal weather and a uniformly aged forest, the beetles rapidly swept south across the glacial moraines of the Sawtooth Mountains and through the western foothills of the White Cloud Mountains.
They swept over Galena Summit, where they also killed 800-year-old white bark pine trees. They are now at work in the northern Wood River Valley, though foresters say the more diverse forest on this side of the summit will slow the beetles' progress.
"Probably east of the Sawtooth Valley, pretty much everything's been attacked and killed," Rineholt said. "Even on the west side of the moraines.
"The thing that's changed is we're seeing more of our white bark pine being attacked and killed, which is tough to see because a lot of those trees are more than 800 years old. It's an important species for holding back snowpack and for the watersheds."
Rineholt said white bark pine trees only grow at elevations higher than 7,000 feet, "pretty much the tippy tops of all the ridges."
As mountain pine beetles have moved across the landscape, red-brown pine needles and dead trees were left in their wake.
The beetles have killed more than 8,000 trees in SNRA campgrounds alone. Numbers of dead trees in the SNRA as a whole are more appropriately measured in acres rather than trees, because so many have fallen prey. In the SNRA, about 90,000 acres of lodgepole pine trees—the entire lodgepole ecosystem—were infested to some degree.
Mountain pine beetles are "the most important native bark beetle pest of mature pines in the Western United States," according to a Forest Service information pamphlet. "Epidemics can build rapidly and kill hundreds to millions of mature trees each year."
Mountain pine beetles experience four life stages: egg, larva, pupa and adult. The beetles have one generation per year and typically take one year to complete a life cycle. Most spend winters as larvae, when they eat, and kill, host trees. Adults emerge from host trees and infest others during July and August.