Douglas-fir bark beetle invasion is altering Aspen’s point of view
State foresters, local partners scrambling to ease the spread
There is a low-key battle with bark beetles underway on the flanks of Aspen Mountain.
Colorado State Forest Service workers marched down steep slopes two weeks ago, stapling thousands of pheromone packets to trees to try to create a protective bubble on Ute Mountain to the east and Shadow Mountain to the west.
The pheromones are designed to trick Douglas fir beetles into thinking there are no suitable trees they can burrow into and leave their eggs.
“We call it the no-vacancy pheromone,” said Kamie Long, a state forester based in Grand Junction and a coordinator for the Aspen project.
The high stakes in the battle are the magnificent, tree-lined views of the lower slopes of Aspen’s southern edge and heightened wildfire danger from dead and dying trees.
Already, a significant number of dead trees are evident on both Ute and Shadow Mountain. The recent victims stand out with rust-colored needles while trees that succumbed to the beetles before are reduced to skeleton trunks and branches.
“You’re starting to see several years of mortality right on Aspen Mountain,” said Dan West, a forest entomologist with the Colorado State Forest Service.
West regularly flies over vast areas of western Colorado as part of the state agency’s annual aerial survey on forest health. He noticed a couple of years ago that Douglas fir bark beetles were really gaining ground in the Roaring Fork Valley. The primary problem is that the ongoing drought weakens the trees. The beetles sense vulnerability and attack the susceptible trees.
The Lake Christine Fire exacerbated the problem in July 2018. Douglas fir trees that weren’t torched outright were damaged and left vulnerable. The bark beetles, which are always present, feasted.
“Unfortunately for Pitkin County, it’s forever on the increase after the Lake Christine Fire,” West said.
The annual forest surveys have noted Douglas-fir bark beetle outbreaks around Ruedi Reservoir and in Fryingpan Valley. West said there is no doubt that is a result of the fire on Basalt Mountain and surrounding terrain.
The outbreak of Douglas fir bark beetles on Aspen Mountain is more likely part of the steady advance north from Gunnison County and mountain areas to the south over the past decade or so.
The visual perspective West gained two years ago was striking.
“That was the red-flag moment of, ‘Uh-oh, we need to do something now,” he said.
West and Long said the Douglas fir bark beetle is a native insect in Colorado and plays a useful role in normal circumstances. They attack the weak and diseased.
“They are Mother Nature’s way of thinning a forest without a wildfire event,” Long said.
Unlike other bark beetles, the Douglas fir variety go for larger trees, 8 inches in diameter and bigger. In normal conditions, or at least what used to be normal wet conditions in Colorado, the well-watered trees would produce sap to drown the invaders. But drought-stressed trees cannot produce enough sap for the job. Beetles create galleries under the bark and invade in numbers that overwhelm the defenses.
“And the whole tree goes kaput,” Long said. “It’s a tough world.”
The pheromone packets create a scent through pockets of forest that tell the invaders there are no places to settle. The 13 foresters from around the state spread out at regular intervals and strategically stapled the packets onto trees. The packets don’t have to go specifically onto Douglas firs to be effective. The idea is to place enough packets at regular intervals to trick the beetles into moving along. West called it creating a “pheromone plume.”
The tactic isn’t a substitute for life-saving moisture. It’s a temporary fix.
“We’re trying to give the trees a chance to rebound from the drought,” Long said.
West said it takes at least two years of normal moisture or above for the trees to bounce back from drought. While the Aspen area’s snowfall was decent this winter, the snowpack disappeared rapidly because of warm winds, dry conditions and sun-absorbing dust on snow.
“The trees are just holding on as best they can,” West said.
After seeing signs of mortality from the air, he contacted city of Aspen forester Dave Coon to discuss a plan to try to salvage the Douglas fir trees on Ute and Shadow mountains. They arranged a summit in August with multiple local stakeholder groups that agreed to help fund the state forest service’s pheromone placement. In addition to the state forest service and city of Aspen, partners include Aspen Fire District, Pitkin County Open Space and Trails, Aspen Skiing Co. and Aspen Center for Environmental Studies.
Long said the timing of placement of the pheromone packets is critical. They last for 90 days, so placement must be after enough snow melts to allow negotiating the woods but before the beetles emerge, typically in late May or early June.
When the beetles emerge, they immediately seek a partner and then the female seeks a host tree where she lays the eggs under the bark. The larvae winter over, mature and emerge in spring, starting the cycle over.
The foresters started on Ute Mountain on May 25, gaining access to the steep slopes from the lower terminal of the Bell Mountain chair and working their way down. They transferred over to Shadow Mountain the following day but were only able to place packets on portions of the east-facing slope. The west side was too steep.
Long said the goal isn’t to eradicate the bark beetles, just to trick them into thinking the Douglas fir trees are occupied. The pheromone is a repellent rather than an insecticide.
Adjacent private property owners were offered a chance to join the effort. Only the Little Ajax Homeowners Association, representing 14 townhome owners on West Hopkins Avenue, took up the offer.
Long was back on the slopes with state forest service intern Trevor Lawrence and Coon to place lures or beetle traps south of the areas where the pheromone packets were placed. The traps will be checked every two weeks to determine how active the beetles are and in what concentrations. They are baited with a pheromone just the opposite of those used in the packets. In the traps, they use the “vacancy” pheromone.
The cost of this year’s effort hasn’t been calculated yet. The state foresters mapped out terrain where they were able to place packets and areas that were too steep. The idea is to share that information with the local partners so they can undertake or hire contractors to undertake the work next year, if moisture levels are once again under snuff.
Coon said the city government would watch the results closely.
“The city has a lot at stake in the forests surrounding Aspen,” he said.
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