Beetle kill is continuing its march through the San Juan Mountains toward the high country around Silverton, according to an aerial survey that tracks outbreaks on an annual basis.

Every year, the U.S. Forest Service and Colorado State Forest Service take to the skies, flying above millions of forested acres across Colorado to see what new areas have been affected by beetle outbreaks.

Because of the COVID-19 pandemic, flights in 2020 were limited to priority areas, cutting the scope of the survey in about half, from an estimated 30 million acres in 2019 to about 16 million acres in 2020.

Still, the aerial survey was able to monitor impacted areas around Southwest Colorado.

The spruce beetle epidemic started on Wolf Creek Pass in the late 1990s. Though native to the land, the insect’s toll was exacerbated as drought, warmer winters and dense forests created perfect conditions for rapid spread.

Over the past two decades, the spruce beetle has torn through more than 884,000 acres – about 25% – of the Rio Grande and San Juan national forests, which total about 3.6 million acres, though not all of that land is spruce forest.

In more recent years, beetle kill across Southwest Colorado has finally shown signs of slowing down, but only because most trees at risk have fallen prey to the massive outbreak, aerial surveys showed.

Mark Loveall, supervisory forester for the Colorado State Forest Service in Durango, said flights in 2020 showed much of the same.

Most new spruce beetle outbreaks happened within isolated tree stands that in past years have avoided the infestation, in areas west of Lemon and Vallecito reservoirs and within the Durango-to-Silverton corridor.

High-elevation areas east of Silverton were hit especially hard, Loveall said, though the survey had not generated the exact amount of newly impacted acreage as of Friday.

“It’s working its way west,” Loveall said.

The fact the outbreak has been slowing down should be taken with a grain of salt, as it means the beetle has ripped through all its food, the Englemann spruce trees, leaving behind swaths of dead stands.

“I think it’ll still be in small pockets,” Loveall said. “But it does seem like it will wind down in the next few years. It can only move west so far.”

And even though the outbreak across the state is waning, the aerial survey determined spruce beetle is still the most damaging forest pest in Colorado, with perfect conditions for its continued infestation.

With prolonged drought in the region, trees’ ability to fight off the insect is weakened. With warmer winters, more beetles are able to survive the cold months. And dense forest stands allow beetles to travel with ease.

For the most part, it’s hard to do anything proactive to stop the outbreak, researchers have said. Most areas affected are in steep, difficult terrain. So outside some spots where tree thinning makes sense, logging is largely not feasible.

“Unfortunately, our dry conditions are optimal for insect epidemics and tree diseases in many parts of the Rocky Mountains,” Tammy Angel, acting regional forester for the Forest Service’s Rocky Mountain Region, said in a statement.

“Where possible, managing forests for age and species diversity can increase resiliency while ensuring diverse wildlife habitat, cleaner air and water, timber and grazing resources, and greener, safer landscapes for recreation,” she said.

Forest Service representatives were not available for an interview for this story.

And as if the spruce beetle outbreak wasn’t bad enough, Loveall said new insects and disease are now causing alarm for other areas of the forest.

Around Purgatory Resort, the native western spruce budworm is weakening trees, making it less likely for trees to survive harsh drought conditions.

“Combined with drought, you can have die-off,” he said. “It’s something important to watch out for.”

For the past few years, the roundheaded pine beetle has ripped through ponderosa pines near Dolores. While this year’s aerial survey showed the spread was declining, the beetles’ impact is still expanding, Loveall said.

“That one is definitely a concern because it started out in small pockets ... then they all started growing together in bigger kill areas,” he said.

Beetle spread in a given year is also impacted by how much moisture a region gets. As of this week, snowpack in Southwest Colorado is 90% of normal averages, and continued snow will be critical, especially as fire season approaches.

“Those big beetle outbreaks make for a lot of potential dry fuel in the forest, and that’s always concerning,” Loveall said.

jromeo@durangoherald.com