ALBUQUERQUE, N.M. — Writer Bill deBuys once described the sound of an army of bark beetles munching on New Mexico’s northern mountain piñon trees as “a faint mechanical drone, as of a thousand tiny chisels rhythmically chipping away.”
I asked entomologist Barbara Bentz last week about the sound. Bentz, based at the U.S. Forest Service’s Rocky Mountain Research Station in Logan, Utah, makes her living studying how beetles kill trees, and she’s busy these days.
From the mountains of New Mexico all the way north to Canada, beetle populations are in overdrive, drilling into the bark of trees to lay their eggs. Given the right conditions, a wave of offspring can overwhelm a forest. And with rising temperatures, conditions have been right a lot lately.
Consider biologist Jeff Mitton’s field plot on Niwot Ridge in the Rockies above Boulder, Colo. In the first decade of the 21st century, temperatures on Niwot Ridge were 3 degrees Fahrenheit higher than in the 1970s on average, and the mountain pine beetles have been on a binge.
Mountain pine beetles typically fly in August, but a few years back Mitton and a colleague saw them flying in June, two months early. After two years in the field documenting the beetles’ life cycles to figure out what was going on, Mitton and colleague Scott Ferrenberg have published a provocative explanation: Instead of one life cycle in a year, they think, rising temperatures are allowing the pine beetles time and ripe conditions enough for two.
“The more beetles in the air, the more trees that get hit,” Mitton told me. “The more trees that get hit, the more trees die.”
I chased down Mitton and Bentz because beetle-caused tree death here in New Mexico is a big deal.
In 2002 and 2003, a combination of drought and higher temperatures hammered the piñon forests of northern New Mexico and the Four Corners region. In his new book “A Great Aridness,” deBuys, a New Mexico writer, documents the carnage in sad, elegant detail.
It was beetles that did in the trees, their “thousand tiny chisels rhythmically chipping away,” but it was high temperatures and drought that set the stage, stressing the forest to the breaking point before the beetles moved in for the kill.
The British evolutionary biologist J.B.S. Haldane famously quipped that his studies suggested God “had an inordinate fondness for beetles.” By which he meant that the subset of insects known as beetles is an extraordinarily diverse group of organisms.
Most beetles, even the subset that burrows into trees’ bark to lay eggs, are not capable of wiping out forests in a population explosion, Bentz explained. The ones that can cause such damage respond to a warming climate in different ways. But across the board, there is widespread evidence that rising temperatures are changing the dynamic, giving the beetles the upper hand, she said.
Scientists say the mountain pine beetle, the species Mitton studied outside Boulder, is not a major problem in the mountains of New Mexico, at least not yet. But we are not immune.
Northern Arizona University forest ecologist Neil Cobb and his colleagues documented the connection between warm weather and a population explosion of a different bark beetle, known by its Latin name Ips confusus. During the heat of the 2002-03 drought, Cobb and his colleagues did not collect bark beetle life cycle data, but he thinks there is a strong possibility the extremely warm weather during those two years enabled an extra bark beetle life cycle. They called it a “global-change-type drought.”
Scientists also have documented spruce beetles, which hits forests mostly to the north, going through their life cycle in a single year rather than two as a result of rising temperatures.
Science moves cautiously, and Mitton’s paper is the first to claim evidence of multiple mountain pine beetle generations in a single year. Our lack of knowledge about the details of bark beetle life cycles in a warming climate is widespread, according to Bentz. “Most of the species, there’s just not very much information,” she said.
But if he is right, the results suggest significant new threats to western North America’s trees. “You’re changing forests across the landscape,” Mitton said.
That is why Bentz will be out in the woods again this year. “We’re out there when the trees are being attacked,” she said.
She’ll be listening for deBuys’ “thousand tiny chisels.”