Bark beetles reducing healthy forests into kindling; scientists say that’s good news

Amanda Pampuro

UNCOMPAHGRE NATIONAL FOREST, Colo. (CN) — Fire can be a healthy part of forest ecosystems like this one, replenishing the soil with nutrients and clearing space for new growth.

It can be part of a natural cycle, like when lightning sparked several blazes in the Uncompahgre National Forest in early July. But climate change also creates the conditions for bigger and more out-of-control wildfires, turning natural disasters into bigger catastrophes. By press time — almost two weeks after Democratic Governor Jared Polis declared a state of emergency — the Turner Gulch and Sowbelly fires had consumed nearly 20,000 acres of forest in Western Colorado and were still burning.

Like wildfire, the unassuming mountain pine beetle belongs in the Rocky Mountains. Also like wildfire, these insects can bring down large numbers of trees, ruining picturesque forest views.

But fire and mountain pine beetles aren’t just like one another; they also interact with each other, with these beetles making wildfires worse and vice versa. As Colorado’s climate dries and warms, ecologists and nature-lovers alike are watching as both forces leave bigger scars on the landscape.

Once one knows what to look for, it’s hard to unsee the damage that bark beetles leave behind.

“When you get a large area of infection, you can absolutely tell,” Paul Talley, executive director of the Colorado Trail Foundation, said in an interview. “There'll be millions of dead trees.”

Broadly known as bark beetles, these bugs thrive in warmer and drier weather, stressing out pine trees while increasing the odds and devastation of forest fires.

Though each beetle is barely the size of a pencil eraser, they’re one of the Rocky Mountain’s most devastating natural disturbances. A swarm can quickly erase vibrant green forest, dotting the landscape with countless rust-colored corpses.

All related through the genus Dendroctonus — meaning “tree killer” — mountain pine bark beetles feed on high-elevation pine trees.

In an evolutionary arms race, these trees and bugs have been adapting alongside each other since the last Ice Age. Each species of bark beetle feeds on a particular type of tree, which in turn has developed specific chemical defenses. Knowing that the spruce beetle, D. rufipennis, feeds on spruce trees, it’s easy to surmise what the Douglas fir (D. pseudotsugae), lodgepole pine (D. murrayanae) and ponderosa pine (D. ponderosae) beetles feed on.

Climate change is disturbing this natural balance, supercharging bark beetles while stymieing trees’ ability to fight back.

Since the mid-1990s, so-called blooms of bark beetles have affected nearly 80% of Colorado’s 4.2 million acres of pine forest, reducing decades-old trees into firewood. In the process, they’ve literally laid the groundwork for some of the state’s most devastating forest fires, from the 2016 Beaver Creek Fire in Walden to the 2020 East Troublesome Fire in Grand County.

As the Turner Gulch Fire spread through western Colorado, Dan West, an entomologist with the Colorado State Forest Service compared the fire’s path with a map of historic beetle activity.

“That’s definitely an area where there has been piñon ips,” West said over the phone. “There’s been a change in fuel content due to bark beetles.”

Bark beetles are not simply a Colorado problem. Mexico City, northern Germany, and the Pacific Northwest have all battled similar climate-driven bursts of beetles in recent years with pesticides, strategic replanting and research.

Still, West says climate change isn’t the only force creating conditions ripe for bark beetles. Thanks to Smokey the Bear, Americans for decades took a zero-tolerance approach to wildfires. That’s led to denser forests and bigger buffets for these pests.

“We have been really good at putting fires out,” West said, “which means that a lot of our trees are getting bigger and bigger and bigger.”

Despite rendering postcard views into wildfire fodder, West does not call these beetles a pest.

Like fire, they’re just a part of nature here, filling a vital biological niche in their native habitat. In the long term, experts say they even make forests healthier.

“Bark beetles serve as the ecological sanitizers of the forest,” said West, who helps manage Colorado’s 24 million acres of state forestland.

One paper, published in the journal Nature in 2020, points to the surprising ways bark beetles are reshaping the landscape, for better or worse.

In spaces where thick pine canopy once shaded the forest floor, the arrival of sunlight supports new plant growth, making room for greater numbers and varieties of flowers. That in turn helps boost bee and pollinator populations. In that way, “there are benefits to forest biodiversity and forest ecosystem services that bark beetle outbreaks can actually provide,” said Seth Davis, an associate professor of forest entomology at Colorado State University and an author of the Nature paper.

All of that is little solace to Coloradans watching with dismay as the state’s biggest trees come down. After all, “people have a strong emotional connection to forests and trees,” Davis said.

“When a lot of trees die, they think this is bad and the forest must be sick,” he said. Arguing otherwise is a hard sell for Coloradans who love trees and don’t care for bugs — which is to say, most people.

Pine trees use their needles to convert energy from the sun into sugar. They send that fuel to their roots through the phloem layer, a tree’s version of the circulatory system.

That’s what these beetles target. Burrowing through the bark, they feed off the tree’s food supplies while also destroying the pipeline that distributes it. In the process, the trees starve to death from the inside out.

But evergreen trees aren’t defenseless; after all, they’ve been evolving alongside these bugs since the Pleistocene. Over the past 2 million years, they’ve developed their own ways of fighting back.

"As the insect attacks a plant, the plant isn't just going to sit there,” said Barbara Bentz, a research scientist with the Rocky Mountain Research Station who has studied these native insects for 35 years. Rather, “they evolve defenses against the insect.”

Those defenses include terpenes and hormones to ward off attack, as well as resin to physically block pests. Recall the last time you tried to wash tree sap off your hands, and you'll get a sense of how the thick, sticky substance can slow down a swarm of bugs.

Less rain and higher temperatures throw this arms race out of balance. For one thing, less water means less resin for trees, weakening their defenses.

Bark beetles also become more active in hotter weather, as high temperatures prompt beetles to wake from hibernation and seek out trees in which to lay eggs.

“They're opportunistic,” Bentz explained. “If there's a number of stressed trees and the weather is right, they can have a big population explosion.”

Given the immense damage these beetles cause, it may seem counterintuitive that many ecologists advocate for a hands-off approach in the Rocky Mountains.

A healthy forest is more than just trees — it’s also “the processes that drive what's going on,” Bentz said. That includes bark beetles, which have “been here millions of years.” Like fire — another natural process in the forests of the Western United States — they “drive what tree species are there.”

Compared to the rabbit pace of bark-beetle reproduction, trees are fighting back slowly — so slowly that it’s hard for people to directly witness. And yet researchers say they’re fighting back all the same, as the most resilient trees survive beetle feasts and pass on their genes.

It can take decades for a new pine tree to produce its first viable seeds. Ecosystems change slowly, across timescales that dwarf human lifespans. While it may seem like the beetles are winning the fight against trees, the war is millions of years old and nowhere near ending. If anything, bark beetles are just winning the latest battle.

This holistic view of forestry helps explain why state and federal officials don’t always interfere when bark beetle outbreaks occur in wild, unpopulated areas across Colorado.

The trouble comes when beetles encroach on populated areas and campgrounds, as hollowed-out trees pose risks of falling and feeding forest fires.

Cities like Aspen have implemented a robust beetle deterrent program, hanging pheromone packets to bolster tree defenses against their burrowing foe. Cutting down vulnerable trees or trimming back excess growth can also help provide a protective barrier.

When it comes to replacing trees killed by beetle outbreaks, state officials recommend that Coloradans replant the same types of trees. That may seem paradoxical: If beetles have just destroyed acres of Douglas fir, why replant that same vulnerable type of tree?

The answer gets back to the holistic view of ecosystems — and the faith among naturalists that in the long run, native trees will find a way to fight back against native beetles.

Introducing new trees to an area “could really throw an ecosystem out of whack,” said Kristin Garrison, director of forest planning and implementation at the Colorado State Forest Service. That’s on top of other hazards that new tree species could bring. For example, while bristlecone pines are more resistant to beetles than Douglas firs, they also have thinner bark – making them more vulnerable to fire. Worse, any species without natural predators might grow unchecked, choking out other wildlife.

It may feel unsatisfying to leave these tree-butchering beetles to their own devices — but such is life in the Anthropocene. As the effects of climate change become more obvious, ecosystems must find ways to adapt, even if those changes are unwelcome or even scary to humans.

Outside of big cities, groups like the Colorado Trail Foundation are making sure that recreation areas also stay safe. Since 1987, the nonprofit has organized volunteers to maintain the 567 miles of trail between Denver and Durango in the southwestern part of the state.

Their work includes fixing bridges, trimming growth and increasingly, clearing downed trees — a task that is becoming more important amid wildfires and beetle outbreaks.

“We tell people to make sure, when they're hiking, to be aware of trees,” said Talley, the foundation’s executive director. “You never know what windstorm or rainstorm will make a tree fall. And once the tree dies, it will fall.”

Though bark beetles are as native to Colorado forests as the blue spruce and the lark bunting, their booming numbers are also a reflection of a warming and drying climate. While bark beetle outbreaks are indeed normal here, Talley hesitates to use that term without first inserting the word “new.”

“Sometimes people see something like a bark beetle, and they say, ‘That's what nature does. It regenerates. It has these cycles,’” Talley said. “I agree with that — and yet, these are definitely issues of a changing climate. It’s the new normal.”