Forests around the Wood River Valley have been invaded by two more species of tree-eating insects.
A stand of Douglas fir on the south side of Greenhorn Gulch has been attacked by tussock moth caterpillars, and several hundred acres around the Boulder View and Spruce Creek residential areas, north of Ketchum, have become infested by Western spruce budworm, also the larva of a moth. Despite its name, the budworm also subsists primarily on Douglas fir.
Both species of insects eat the trees' needles, eventually defoliating them.
Local Douglas fir have already been hard hit by the Douglas fir beetle, which arrived in the area about three years ago. In the Sawtooth Valley, more than a million lodgepole pines have been killed over the past eight years by an infestation of the related mountain pine beetle. Both species have larvae that feed on the phloem beneath trees' bark.
"This is really a banner period for forest insects," U.S. Forest Service regional entomologist Dayle Bennett said. "We have most of the major forest insects occurring now."
Bennett said the infestations are all part of natural cycles, though the damage done by Douglas fir beetles has been exacerbated by years of drought, which reduce the trees' ability to withstand attacks.
Bennett confirmed the cause of the recent de-greening of the forest early this month.
He said the tussock moth infestations occur somewhere in the Wood River Valley every seven to 10 years, but are short-lived, lasting only three to four years. He said the caterpillars are eventually killed by a virus, whose population builds up as the moth population does.
Bennett said most trees will recover each spring, though they can be killed by additional pressure inflicted by drought and bark beetle attacks.
"It's hard to predict what the impact will be," he said.
Bennett said Western spruce budworm infestations have defoliated trees throughout southern Idaho, Utah, western Montana and parts of Washington and Oregon.
"When the infestations reach outbreak status, it usually covers a good part of the West," he said.
Bennett said the budworm attacks cause less mortality than the tussock moths do, but can last much longer, from two to 30 years. He said the last big outbreak in the West stretched from the late 1970s to the mid 1980s.
"Typically with the budworm, it looks worse than it really is," he said. "Within four to five years, you usually don't see anything."
Bennett said the infestations usually collapse after one or two cold, wet springs, when the caterpillars become more susceptible to disease.
Biological and chemical measures are available to treat infestations by both types of moths, but due to their cost are usually only used on a very localized basis.
Ketchum District Ranger Kurt Nelson said he has no plans to treat the current infestations.
"It's a short-term problem," Nelson said. "We just have to be a little cognizant that we can't control everything."
Nelson said the Sawtooth National Forest has sprayed insecticide on some lodgepole pines affected by the mountain pine beetle at important spots such as campgrounds. However, he said, even with multiple sprayings, the enormous volume of insects is killing many trees anyway.
"We're not going to stop it," he said, "It's not as if it's one spray and we're good to go."
Bennett said the mountain pine beetle infestation is slowing as most of the susceptible trees die. He said the beetle larvae primarily attack trees that are at least 100 years old and at least eight inches in diameter.
"If the trees don't burn at that stage, mountain pine beetles build up, and they kill a large percentage of those trees," he said.
He said smaller trees don't have enough phloem to be appetizing to the beetle larvae.
Sawtooth National Recreation Area forester Jim Rineholt said records show that the last mountain pine beetle infestation occurred almost 100 years ago.
Bennett said the Douglas fir beetle infestation as well will probably not last much longer, though its life span could be increased by more drought. He said that with normal amounts of moisture, the trees generate enough pitch to drive the beetles out before they can bore in and lay their eggs.
Nelson said the Forest Service has been placing bundles of pheromones that tell beetles the trees are already occupied on Douglas firs on Bald Mountain. He said at least 400 trees have been treated each spring for the past two years.
"These large-diameter, older trees are worth keeping in the system," he said. "We're hoping we can hold on to those trees until we get some more moisture and there's a collapse of the beetles."
