Extreme drought. Soaring temperatures. Decades of fire suppression. It’s a perfect recipe for the kinds of wildfires now tearing through the West.
But there’s another ingredient that could make fires even more severe, and it’s just the size of a grain of rice: bark beetles.
These tiny insects prey on a large number of tree species, which can make some forests, especially in the American West, more susceptible to severe wildfires. In fact, there’s some indication that beetle-killed trees have helped fuel the Bootleg Fire currently raging in Oregon.
Now climate change stands to add fuel to the problem. Rising temperatures are causing the populations of bark beetles to balloon, while also worsening droughts that make trees more susceptible to beetle attacks. Beetle infestations may in turn be contributing to climate change, by transforming forests from carbon sinks to carbon emitters.
“In the past, outbreaks were more limited in extent or intensity,” said Rebecca Wayman, a forest ecologist at the University of California Davis. “Now that we’ve seen such severe outbreaks with huge proportions of trees being killed, the effect on fire is certainly high on people’s minds.”
Beetles are one of many environmental variables that shape wildfires, forest experts like Wayman said. But their spread is a reminder that these variables — from drought and fire suppression to tiny pests — interact and can amplify one another. Climate change changes the balance, and is helping turn many forests into tinderboxes.
Meet the bizarre bark beetle
Bark beetles aren’t your typical charismatic species, but what they lack in looks they make up for in talent.
Of the roughly 600 species in the US, a dozen or so are known to kill swaths of forests by boring into the wood, disrupting the flow of nutrients into the tree. First, they have to overwhelm the tree’s defense system with a chemical trick. Once a beetle arrives at a tree, it emits a pheromone that functions as a beacon, drawing in tens to hundreds of other beetles for a coordinated attack. Those beetles are also known to carry fungi with them, which some species use to help feed their offspring. Together, the beetles and fungi kill the tree.
Remarkably, once the tree is full of beetles, the insects emit another pheromone that tells other beetles not to join them, according to Chris Fettig, a research entomologist at the US Forest Service who studies bark beetles. “That switches the behavior and functions as a ‘no vacancy’ sign’,” he said.
Bark beetles are native to the US and a natural part of forest ecosystems, but as their numbers have grown, they’ve utterly decimated US forests. In fact, bark beetles have killed more trees in the past three decades than all wildfires in the western US combined, according to a recent Forest Service report that Fettig co-authored. One of the worst years in history was 2009, when bark beetles infested nearly 9 million acres of western forests.
More recent data suggests that bark beetle infestations have killed fewer acres of forest in the past five years, on average, compared to the late 2000s, partly because there are simply fewer host trees for the beetles to infest, Fettig said. But he and other researchers warn that beetle-spurred tree die-offs are worsening in other ways.
“These events are becoming larger and more severe and garnering more attention,” Fettig said. “Many of us are concerned that it’s an indication of what the future is going to look like in several different [eco]systems.”
Climate change and fire suppression help bark beetles kill trees
Climate change will have an enormous effect on animals of all kinds, and these beetles are no exception. In fact, rising temperatures are largely to blame for the recent beetle boom, researchers say.
For one, they mean more of the insects are able to survive winter, Fettig said. Plus, research suggests that hotter summer weather can speed up the time it takes for some species to complete their life cycles, leading to population growth.
Climate change is also making droughts more frequent and severe in some places, so there’s less water available to trees. To avoid drying out, trees close microscopic structures called stomata that absorb carbon dioxide. Without access to the carbon in CO2 — which trees use for their defense system — they have a harder time surviving a beetle attack. “Climatic water stress can have profound effects on tree susceptibility to bark beetle attack,” the authors of the study wrote.
Making matters worse, decades of fire suppression and a lack of forest management have allowed forests to grow dense, so more trees are competing for less water. “We can’t just call this climate change,” Wayman said. “We have to look at the history of how we’ve been managing — or not managing — forests over the last couple of hundred years. That’s also been contributing to this water stress.”
Dead trees can fuel forest fires
When trees are infested with bark beetles, they start to lose water. Research has found that the twigs and needles of lodgepole pine, for example, can lose 80 to 90 percent of their water content within a year of an attack. As you can imagine, dry trees tend to be more flammable and burn hotter than wet ones. Infestations can also reduce the amount of forest canopy cover, allowing fire-spreading winds to more easily whip through the forest.
Some research already supports the idea that bark beetle outbreaks may fuel intense wildfires. Earlier this year, Wayman published a paper analyzing the footprint of two fires in California’s Sierra Nevada mountains, including the Rough Fire of 2015 and the Cedar Fire of 2016. They collectively burned more than 70,000 hectares. As she and her co-authors found, a recent bark beetle infestation in the area increased the severity of the two wildfires, as measured by how much vegetation they destroyed within their footprint. (Weather was still a more important factor, she said.)
“There’s an influence of these dead trees on the landscape,” Wayman said. “They did increase the likelihood that live trees in that location would die.”
Bark beetles have also been implicated in some of the most destructive fires of the past decade, including the 2020 Creek Fire — the largest wildfire in California state history — and in the more recent Bootleg Fire in Oregon. As much as 90 percent of the Creek Fire’s fuel was timber killed by bark beetles, USA Today’s Joshua Yeager and Mark Olalde reported in 2020.
Yet it’s too simple to say beetle outbreaks always fuel wildfires, and a lot of research has failed to find a connection.
In some cases, the fire is burning at such a high intensity already, perhaps fueled by extreme weather, that it doesn’t really matter whether the trees are dead before the blazes tear through, Wayman said. “Does [tree mortality] even matter when you have 96-degree temperatures and 50-mile-per-hour winds?”
The role of beetles in fires also depends heavily on how long it’s been since the last outbreak, which affects how much fuel is available, Wayman said. While her study looked at the impact of a recent beetle attack, other studies that assessed fires that burned many years after an infestation have suggested that past outbreaks aren’t as worrying. One explanation is that decay reduces fuel from dead trees, Fettig said.
While the overall role of beetles remains uncertain, it’s clear that they’ve killed a lot of forests and left a lot of fuel behind. “The scale of present tree mortality is so large that greater potential for ‘mass fire’ exists in the coming decades,” Fettig and several other researchers wrote in 2018.
Prescribed burns will help forests survive beetle attacks
So what can we do? Until large-scale climate solutions reduce the likelihood of extreme temperatures and severe droughts, firefighters can try a different approach: burning or thinning forests.
This might sound counterproductive: Why burn forests to save them? But proactive forest management — which can involve prescribed and controlled burns and removing certain tree species from a forest — is the most effective way to reduce bark beetle outbreaks, according to Sharon Hood, a research ecologist at the US Forest Service.
One way we know it works is that Indigenous people used controlled burns to manage forests in the US for thousands of years, shaping the very ecosystems that we’re now racing to save. Understanding “the historical disturbance regime, and looking into the past for things that influenced our forests, can really help guide current and future management,” Hood said.
Prescribed burns, for example, not only lower the risk of wildfires but also reduce the density of trees, and thus the competition for water among them. Hydrated trees have an easier time fighting beetles. Controlled burns can also stimulate trees’ defenses, Hood said.
“With climate change, we know there are a few things that we as individuals can do, but it’s a global problem,” Wayman said. “Whereas on the forest management side, we can act on that. We can’t throw up our hands and say, ‘This is out of our control.’ We can take action.”