Stuffing vests and backpacks with smelly plastic packets about the size of a deck of playing cards, 13 Colorado State Forest Service foresters and staff scoured 10,000-foot-tall ski hills this summer to send Douglas-fir beetles a message — you’re not welcome here.

“We’re using their own communication system against them,” said CSFS Supervisory Forester Kamie Long, who organized the project to assist the City of Aspen.

The packets start a conversation that speak the language of love among these pests — pheromones. The packets slowly release chemicals that mimic natural beetle excretions, allowing foresters to fool live beetles into staying away from healthy trees. This gives stressed forests a fighting chance against these native bark beetles that have reached endemic levels in parts of the state. In turn, that helps protect our forests for the future, keeps living trees on the landscape and helps lower wildfire risk in nearby communities.

Conversations through chemicals

group of people hold equipment and stand in circle next to truck.
Colorado State Forest Service staff and volunteers plan their course of action to help keep live beetles away from healthy trees. CSFS photo.

Some of Colorado’s native bark beetles — like Douglas-fir beetles and spruce beetles — like to talk, a lot. Their communication is one of nature’s exceptional and extremely subtle acts of self-preservation.

Throughout our forests, these beetles chirp and chatter with both sounds and chemical secretions, gossiping about locations of the best trees to invade (those with at least an 8-inch diameter are preferred), chattering to attract mates, ward off rivals and let others know when a tree they’ve inhabited is getting too crowded.

Female Douglas-fir and spruce beetles launch out as the explorers of the species, finding trees to attack, then calling to others when they find a suitable buffet.

“They land on a tree or crawl up the tree bowl, basically tasting that bark to see if they have the enzymes to break down that tree species compound within its gut,” said Dan West, the CSFS state entomologist. When they find a tree that tastes good, they mine through the outer bark to get to the inner layer, the phloem, where carbohydrates and nutrition are found.

“They take compounds within the tree, oxidize them in their guts, and then out of the back end of a bark beetle comes what’s known as an aggregation pheromone,” West said. “That is the chemical signal that says, ‘Hey, this tree is a species that we can all attack, and it’s weak enough that I’ve been able to get inside of it.”

Then the migration is on. Nearby beetles key in on that chemical pheromone and earmark the tree as a gathering point. They can arrive by the hundreds, until a massive fir — already in distress from other environmental factors — is weakened and killed by quarter-inch-long pests.

However, the beetles are intelligent explorers. Once they meet and mate under the cover of tree bark, the chemical messages they send out begin to shift.

“After a male mates with a female, he starts to produce what’s known as an anti-aggregation pheromone,” West said. “That pheromone serves the benefit of spacing bark beetles out under the bark. It’s the no-vacancy signal that says this tree is pretty much full, go elsewhere.”

The female beetle travels vertically up the tree, laying eggs on each side of a line, called a gallery, Long explained. As the beetle’s eggs hatch into larvae, they move horizontally, so there needs to be space for the baby beetles to grow and have enough to eat.

“Each beetle pair needs enough space for their young to mature,” she said. Once the beetles feel like the tree is full and there’s no more room for more broods, then that “no vacancy” message is sent out in force. “If other beetles come in, then their galleries are going to cross and the babies aren’t going to survive.”

That’s where foresters step in, stapling these plastic packets — full of a phermone called MCH — within a stand of trees. The MCH, just like a beetle’s natural pheromones, carries the artificial no-vacancy message out from the tree the packet is stapled to.

“So, as beetles are flying through the forest, they’re realizing ‘Oh, wait, maybe all these trees are too full? There’s not any room for my babies,’ and so they will continue to fly and move on,” West said.

Protecting forests through chemistry

close up of a person stapling a packet onto a tree trunk.
The pheromone packs are stapled to trees approximately every 47 ft. to tell beetles there is “no vacancy” in this tree. CSFS photo.

Bark beetles are incredibly smart attackers in other ways as well. They target trees with a diameter more than 8 inches and usually leave smaller trees alone.

“That sets up the stand to continue to have that food resource in another 100 years or 150 years,” said West. “Bark beetles are really intelligent in keeping a resource for their grandchildren or great-grandchildren that will eventually be their next food stores.”

It was 1971 before scientists discovered some bark beetles emitted this “no vacancy” anti-aggregate pheromone, according to the U.S. Forest Service. Shortly after, scientists reproduced tiny pouches of the pheromone and proved it could deter beetles from landing on healthy trees.

That MCH compound has been commercially available since 1992 and is used in forests around the country. It is distributed in small bubble packs that are stapled to healthy, uninfected trees in a forest stand. The pheromone is placed about every 47 feet and leaks out of the bubbles to fill the air with the message that the trees are at full capacity — even though the beetles haven’t ever touched them.

It’s important to remember bark beetles are native to Colorado’s forests and serve an ecological role that’s beneficial, in small doses. The goal hasn’t ever been to completely eradicate these forest pests.

“All the trees affected with other diseases, with parasites, or that have been struck by lightning or chopped on by campers, you name it, those are the trees that we want to cycle through the forest, start the decomposition process and allow these forests to recycle nutrients and bark beetles are the first to wave of doing that,” West said.

But the growing number of bark beetles around the state can have a massive impact on trees.

“We are just trying to slow its spread through the forest around Aspen, in the hopes that weather patterns change and the trees can fight on their own in the future,” Long added.

The MCH packets are only one piece of the forestry toolbag to stave off bark beetles. The CSFS currently uses MCH technology in the southwest and Front Range areas of the state, from Mineral, Saugache and Gunnision counties to Douglas, Chaffee and Pitkin counties.

Similar anti-aggregation compounds have been found to work for mountain pine beetle, southern and western pine beetles as well.

“For the most part, it’s really been a tool for us to use in locations where we’ve got multiple insects,” West said. “Many locations have seen an uptick in defoliation from Western spruce budworm in Douglas-fir, and that predisposes those trees to further beetle attack. We use MCH as a tool to follow up after these caterpillars have injured the trees from eating the needles.”

The Aspen project

two people are on a very steep mountainside covered with trees
The project site in Aspen, Colo., was at an elevation of 10,000 feet and the slopes were up to 85%. Sometimes the staff and volunters had to crawl on hands and knees to staple the 4,100 MCH packets onto trees. CSFS photo.

Douglas-fir beetle activity has been increasing in the Roaring Fork Valley since the Lake Christine Fire in 2018, largely because of multiple years without enough water. Environmental conditions like large, catastrophic wildfires and drought are now compounding to create the perfect catalyst for beetle outbreaks.

“What we’re seeing after large wildland fires, we’re seeing these beautiful populations of beetles build up on the periphery of these fire areas,” West explained. “They’re using those trees that might have been slightly injured by the fire but still have viable carbohydrates underneath the bark. They’re using those as a food source.

“Why didn’t we see it in the past? It’s because we allowed small fires to burn through forest and that gave us these mosaic patterns. That allowed species diversity in our forest and allowed to age diversity in our forest that we basically have eliminated with fire exclusion,” he said.

For the past year, a partnership of agencies and CSFS experts watched bark beetle destruction  spread over the face of Aspen Mountain, an iconic destination for outdoor-loving tourists and adventure seekers from around the world. It’s a place where the scenery has a direct impact on the economy.

“Aspen needs visitors to come to the city, they need recreation. That’s an important part of their income,” Long said. “So, while we want to protect trees, we also have to be strategic with our placement of our MCH, as well as the time and money it takes to do so.”

The partners agreed that the slopes above Aspen were good places to deter Douglas-fir beetle attacks, with the goal of steering the beetles to a part of the forest that doesn’t have Douglas-fir trees growing. This year kicked off the three-year project to use MCH to herd the beetles to other areas, save the living Douglas-firs on the slopes and lower the risk of wildfire for the city.

MCH was the most nimble tool for the job because of the terrain — the project site was at 10,000 feet on slopes that sometimes slant to 85 percent. In June, West, Long and the CSFS staff scaled Ute and Shadow mountains, sometimes on hands and knees, stapling 4,100 MCH packets throughout the trees.

“To put that into perspective, across the state of Colorado in numerous counties where we’ve seen a big increase in Doug-fir beetle activity, we typically have been hanging around 30,000 packet statewide each year,” West said.

Once on the slopes, they found another stressor within the Doug-fir forest. A parasite called dwarf mistletoe is creating additional stress on the trees.

“It takes all its food and water from the tree. So in drought conditions, the tree is already stressed from lack of water, and now they’re supporting a parasitic plant. So it’s like a double whammy,” Long said. With the MCH packets in place, the Doug-firs can focus their energy and resources on dealing with the mistletoe until drought lessens, Long said.

Part of the three-year project includes monitoring beetle movement in the places the MCH packets are hung. That includes putting out lures and funnel traps to get a count of the beetles that might be flying around the area. These lures use the opposite communication, they have the vacancy pheromone in them that the beetles release when they first attack a tree. The City of Aspen will monitor the traps, collect beetles and send them to the CSFS to count.

Foresters are optimistic that the packet project will help keep Aspen’s iconic vistas in prime condition.

“These treatments of MCH are really tipping the scales to our favor in areas where we typically wouldn’t be able to access the forest with forest management equipment. We’re now able to get into these places on foot to get us out of the window of drought conditions where beetles can build their populations quickly,” West said. “We use these tools in our toolbox to hopefully let those trees build their defenses back up once the precipitation turns back on and we can let Mother Nature take its course from there until the next event comes along.”