A nonnative tree-killing insect is invading northern Utah, attacking subalpine fir and potentially triggering yet another die-off of the region’s long-stressed conifer forests.

Introduced from Europe into the Pacific Northwest about a century ago, the balsam woolly adelgid (BWA), or Adelges piceae, was first detected in Utah in 2017 and has been spreading around the Wasatch Mountains, visibly affecting many of the popular recreation canyons outside Salt Lake City.

New research from the University of Utah, conducted in partnership with the U.S. Forest Service, has documented the current extent of the adelgid infestation and created a model for predicting its severity around the Uinta-Wasatch-Cache National Forest.

The study documented a clear relationship between the infestation’s severity and temperature, according to lead author Mickey Campbell, a research assistant professor in the Department of Geography (soon to be merged with the Environmental Studies program and renamed the School of Environment, Society, and Sustainability.)

“We took that climate-to-severity relationship along with a series of climate projections and we were able to map current and future exposure to BWA damage at a high spatial resolution,” Campbell said. “The idea [is], in 2040, 2060, 2080 and 2100, based on these different climate projections, determining how exposed these areas are to the potentially damaging effects of BWA. And indeed, we find that for an insect that prefers warmer areas, a warming climate is going to provide it with more opportunity to cause damage.”

The role of climate change

The study appears this month in the journal Forest Ecology and Management. Co-authors include U Biology Professor William Anderegg, director of the Wilkes Center for Climate Science and Policy. [The center hosts its annual Climate Summit on May 14-15, where Anderegg will give opening remarks.]

According to Anderegg, the new study suggests climate change is playing a role in Utah’s adelgid infestation.

“The main pieces of evidence are how strongly temperature is related to the spread and severity of BWA,” said Anderegg, a specialist in forest ecology. “That tells us at the very least as temperatures go up, we should be concerned about more spread and higher severity infestation.”

The study, which was funded by the Forest Service, is also intended to provide land managers with tools to predict and mitigate future damage as the BWA infestation spreads. The team created an online interactive dashboard that illustrates how the tiny insect damage is expected to unfold in the Uinta-Wasatch-Cache National Forest.

Covering the Wasatch, Uinta, Bear River and a few lesser mountain ranges in northern Utah, this national forest is among the nation’s busiest for recreation. It features five major ski areas that border several others and sees more visits than all of Utah’s national parks combined.

“Places like Brighton, Solitude, Alta and Snowbird [in the Cottonwood canyons] are likely to see quite significant increases in adelgid’s effects over the next several decades,” Campbell said. “The Wasatch is such a hub for recreation. And if you have a bunch of dead trees on the landscape, it really affects the aesthetics. It affects everyone’s enjoyment of these iconic landscapes.”

Nothing like native beetles

Aside from an appetite for conifers, this new arrival to Utah has little in common with the native bark beetles that have devastated stands of lodgepole pine, ponderosa and Engelmann spruce all over the West in recent decades. While beetles gnaw and burrow through the phloem under the trees’ bark, the much smaller, flightless adelgid sucks out the trees’ fluids and leaves behind toxic saliva.

“It inserts its stylet, which is like a feeding tube, between the cells of the bark, and eventually finds parenchyma cells and it feeds on the fluids and starches that are contained within those parenchyma cells,” said study co-author Justin Williams, an entomologist with the Forest Service’s Forest Health Protection program.

While slurping, the insect excretes a substance that weakens the tree’s defenses and causes damage that impedes the flow of nutrients between root and crown, he said. The saliva promotes abnormal cell growth that results in the tell-tale signs of “gouting,” the appearance of swollen branch nodes.

“As the infestation grows and the population of BWA grows,” Williams said, “the damage becomes more severe and those translocation effects become more severe, which eventually causes the tree to die.”

That process takes three to five years to kill the tree, although some trees could survive, while white fir withstand infestation with no apparent ill effects.

As its first order of business, the research team documented the current extent of Utah’s BWA infestation by staking out 58 plots in the Wasatch and Uinta mountains, each 30 meters in diameter, covering just under one-fifth of an acre and each containing 150 to 200 trees. Every fir tree larger than 5 centimeters in diameter was evaluated on the severity of infestations based on the presence of gouting, woolly deposits adelgid left on the trunk, crown deformities, and dead needles.

The research concluded 41% of the study area’s subalpine fir biomass is climatically exposed to some level of damage. And by 2100 under even moderate climate projections, 79% will be exposed, with 37% predicted to feature relatively high severity.

BWA’s slow march east and south

Initially, the team wanted to conduct satellite-based imaging, but that proved difficult to determine the extent of the adelgid infestation.

“There are so many other things going on in the forest that show up as forest condition changes in the satellite data, including fire, timber harvesting, and health degradation due to other insects or drought,” Campbell said. “And the balsam woolly adelgid signal is really subtle. It takes years to kill a tree. It’s not like a bark beetle that comes in and causes widespread mortality in a forest stand.”

BWA appears to attack only true fir species, leaving other conifers, including Douglas fir, untouched. It has appeared in the Uintas, but only in that range’s western fringes.

In Farmington Canyon north of Salt Lake City, where BWA was first documented in Utah, subalpine fir is experiencing near-total mortality.

“Maybe that’s because it’s been there the longest,” Campbell said, “but also maybe there’s something geographic about that area that makes it more susceptible to damage from the insect.”

Utah and Colorado lie toward the southern reaches of the subalpine fir range, which stretches north and west through the Pacific Northwest to Alaska. The adelgid has been moving south and east for decades and has long occupied Idaho’s forests. The insect’s progress is slow because it must rely on the wind or birds for dispersal.

“It’s in its dispersal stage for a fairly short time, typically late June, early July,” Williams said. “If there are two generations per year, then there’s another set of crawlers in the fall. But once it gets past that crawler stage, it becomes a sessile organism and it doesn’t move after that.”

Bark beetles, by contrast, can quickly fly from tree to tree for much of the year.

In both the adelgid and the beetle infestations in Western forests, scientists suspect the warming climate plays an important role, but aren’t exactly sure how because so many stressors are contributing to tree mortality, including drought, wildfire and disease.

“Maybe the warm temperatures are actually making the tree more susceptible. So maybe it’s not necessarily that the BWA prefers these climates,” Campbell said. “Maybe it’s just that the BWA does more damage in these climates because the trees are already stressed from warmer temperatures.”

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The study, titled “Quantifying current and potential future impacts of balsam woolly adelgid infestation on forest biomass,” appears in the current edition of Forest Ecology and Management. The research is funded by the U.S. Forest Service.

• Brian Maffly Science writer, University of Utah Communications
  801-573-2382 brian.maffly@utah.edu
• Mickey Campbell assistant research professor, Department of Geography
  ‭(845) 417-8603‬ mickey.campbell@geog.utah.edu