The Plight of the Bark Beetle

Written by Quentin Septer

Note: This is a modified excerpt from my book, Where Land Becomes Sky: Life and Death Along the Colorado Trail, available now.

Beneath the conifers that surround me, piles of rust-colored pine needles have fallen from fragile branches. Heaps of fine, saw-dust like powder lay at the base of the trees, accumulating in the wrinkled fissures of their bark. Clumps of resin ooze from pine boughs, flowing from swaths of stripped bark and the boring holes of insects. Red needles cling weakly to the brittle twigs of still standing evergreens. Other trees have toppled over altogether, laying dead and dry on deathbeds of pine needles.

Over recent years, bark beetles have plagued forests across North America, infesting millions of acres from Mexico up into the northern reaches of Canada. Since the year 2000, the beetles have killed more than 100,000 square miles of evergreens across the American West. The beetles have killed 1.8 million acres of spruce and 3.4 million acres of pines in Colorado alone. Across the state of Colorado and the nation at large, vast expanses of “ghost forests”—in the words of one forester from the U.S. Forest Service’s Intermountain Forest and Range Experiment Station—stand dead on the landscape. The thickets of the Pike National Forest through which I now cycle represent just one of many regions ravaged by the bark beetle, but these trees prove emblematic of a phenomenon continental in scale.

More than 600 species of bark beetle inhabit North America. Several of these species have infested Colorado’s forests over recent decades. The mountain pine beetle (Dedroctonus ponderosae), the spruce beetle (Dendroctonus rufipennis), the Douglas-fir beetle (Dendroctonus pseudotsugae) and other species of the Dendroctonus genus are among the most notorious of the pests. The insects, no larger than a grain of rice, bore into the bark of infested conifers, laying their eggs in early autumn. During the fall, winter, and early spring months, the growing larvae live and feast on an infested tree’s phloem—plant tissue that distributes sugars throughout a tree like an arboreous circulatory system. With enough bark beetles feeding on a given tree’s phloem, nutrients fail to flow through an infested tree’s bark. The trees begin to starve, so to speak.

But it’s not always the pine beetles themselves that kill infested trees. The beetles carry with them a blue-stain fungus. “Blue-stain fungi depend on bark beetles for their dispersal,” a 2019 study published in Nature states, “whereas fungi increase the efficacy of beetle attacks since the fungi help to neutralize tree defenses.” It’s an insidious kind of alliance. Once introduced into a tree’s interior by a boring bark beetle, these blue-stain fungi go on to infect the tree’s xylem—vascular tissue through which water flows from a tree’s roots to its leaves. The fungus can completely penetrate a tree’s sapwood within a year, and as the fungus proliferates, it blocks water flow throughout an infested evergreen.

This combination of burgeoning insects feeding on a tree’s phloem and blue-stain fungi obstructing water flow proves lethal to an infested conifer. As nutrients fail to flow through the evergreen, and as water fails to reach the tree’s extremities, its pine needles turn red and rust-colored, scarcely clinging to their resident branches. With time, the needles fall to the forest floor; a sign that the tree is dying.

Come mid-summer, larvae mature into adult beetles. They emerge from the bark of their home trees and fly on to infest still other evergreens. Mountain pine beetles were long thought to reproduce only once per year in Colorado. But Jeffery B. Mitton, Professor of Ecology and Evolutionary Biology at the University of Colorado Boulder, recently discovered populations of mountain pine beetles yielding two generations per year in the state. “The season is much longer. The beetles are now starting in May instead of July,” Mitton says, and “at least some of the beetles are pulling off two generations per year. We think this is one of the reasons that this epidemic is unprecedented in scale and intensity.” Other pine beetle species are even more prolific. Up to nine generations of the southern pine beetle, for instance, can be born within a single year in the warmest part of the beetle’s range down in Florida. Be it two generations of mountain pine beetles here in Colorado or nine generations of the southern pine beetle in Florida, the result is the same: an outright assault on a region’s evergreens.

Healthy conifers can defend against modest bark beetle invasions. Chemical compounds called terpenoids saturate the sap of the trees, killing off boring bark beetles and thwarting their attacks. But when bark beetles invade a tree by the hundreds and thousands, the tree’s defenses are overrun. By way of chemical signals called pheromones, bark beetles recruit fellow troops to join the attack. Swarms of beetles amass in the bark of a single tree. And once the tree is overwhelmed, fully occupied by boring beetles, the legions fly on to infest still other trees. In this manner of boom and bust swarming from evergreen to evergreen, the insects can spread like wildfire across a landscape. 

The woods of North America have coexisted with bark beetles for millenia. The pests are a normal, natural part of the ecosystem. But bark beetle epidemics over recent decades have swelled to unnatural proportions. Historically, frigid winter temperatures kept bark beetle populations at bay. Temperatures of thirty degrees Fahrenheit below zero, when sustained over a period of several days, are cold enough to kill bark beetle larvae. But as winter temperatures have risen over recent decades, greater numbers of the beetles now survive the milder chill of Colorado’s winter months. These increased populations, coupled with trees stressed by drought, forest fires, and increasingly sweltering summer temperatures (along with unnaturally dense forests) creates a perfect storm. Under such conditions, trees grow less capable of producing sap saturated with defense chemicals at concentrations capable of killing invading bark beetles. The bark beetles, meanwhile, multiply and fly on to attack vast expanses of timber en masse, devastating millions of acres of forest across Colorado and the North American continent at large.

All told, one in 14 of Colorado’s evergreens have been killed by the region’s most recent wave of bark beetle infestations. That’s roughly 834 million trees. “The mountain pine beetle has been attacking forests for millions of years,” says Professor Jeffery Mitton. “This cycle is different. This one is unprecedented both in its intensity at a site and also in its geographic extent. This thing goes from New Mexico, a thousand miles north to the Yukon territories; from the Front Range of Colorado to the Pacific Ocean. This thing is already ten times bigger than the second biggest epidemic in history.” 

And all of these dead and dying trees can fuel the flames of wildfires, too. The woody debris left in the wake of a pine beetle infestation provides ample kindling for wildfires to grow out of control. It comes as no surprise then, that some of the largest wildfires ever seen in the state of Colorado, the East Troublesome Fire and the Cameron Peak Fire among them, burnt through stands of beetle-killed timber. “A combination of weather, terrain, and beetle-killed lodgepole pine contributed to the significant fire growth,” officials at the Forest Service reported as the East Troublesome Fire burned—the second largest wildfire in Colorado state history to date.

Colorado’s current bark beetle outbreak continues to kill trees in droves across the state. There is a silver lining, however. Some of Colorado’s forests once hard hit by the pests are beginning to recover. A study published in the journal of Ecology in February of 2020 found that high-elevation forests in southern Colorado have begun to regenerate from recent bark beetle infestations, though the composition of those forests—which tree species are growing where—has morphed in the beetles’ aftermath. In beetle-stricken woods, spruce have largely given way to aspen and fir. Meanwhile, other studies predict worsening bark beetle infestations in the decades to come, as atmospheric and surface temperatures continue to rise and droughts grow more common and severe with a changing climate. 

“Within about five years, almost every large lodgepole in the state will be killed,” Jeffery Mitton said of Colorado’s ongoing bark beetle infestations. That was back in 2017. Only time will tell how forests across Colorado and North America at large will cope with and recover from the plight of the bark beetle.