A Bug in the System
Climate change is fueling the biggest outbreak ever of tree-killing bark beetles. The insects are decimating conifer forests from Alaska to Arizona - and raising concerns that they could reach the Upper Midwest.
There’s no place to pull off on this stretch of the serpentine road leading up Wyoming’s Signal Mountain, so Phil Townsend just stops the car in the middle of his lane. He hops out and darts across the road to get a closer look at a towering lodgepole pine. It’s a substantial tree, fatter than a telephone pole and 80 feet tall, with full, lofty branches full of needles—but Townsend has already written it off.
“This tree is dead,” says the CALS forest ecologist. “It just doesn’t know it yet.”
Townsend points to the dozens of tubes of yellow pitch sticking out from the tree’s trunk. Each one, he explains, marks the point of attack of a mountain pine beetle.
As the beetle bored in, the tree exuded resin in an attempt to trap it, but the effort failed. The pitch tubes are gritty with fresh sawdust, but otherwise empty. The beetles tunneled through the sticky wax and are now chewing into the phloem, the tree’s nutrient pipeline, creating galleries where they’ll deposit their eggs.
“This is a fresh attack,” he explains. “They will eat the phloem and girdle the tree. This tree is not going to succeed.” Townsend climbs back in the car and heads to the summit to get a clear view of the sea of conifers that blanket the hillsides in this part of Grand Teton National Park. A typical sightseer at the mountaintop lookout would describe the forest as green and healthy, but Townsend points out various spots that show a subtle yellow sheen. “Next year those trees will all be red. The year after that, they’ll be gray.”
This color pattern has become all too familiar to those who live, work or play in the vast forests of the mountainous West. From Arizona to Alaska, matchhead-sized beetles are turning conifers from green to rusty red to driftwood gray. Trees in the Jackson Valley, which runs along the base of the Tetons south of Yellowstone National Park, are among the latest casualties.
Back in Madison, CALS forest entomologist Ken Raffa offers a grand assessment. And the picture is grim. “We’re talking about dozens of millions of acres across the West where it’s almost 100 percent mortality,” he says. “We’re talking about transformations of entire ecosystems.”
Changes in climate, Raffa says, are enabling this swathe of destruction. Warmer summers and milder winters have boosted beetle populations at a time when drought stress, fire suppression and other management practices have left forests ripe for attack. As a result, what used to be intermittent, isolated flare-ups of native beetles have exploded into the largest known insect outbreak in North American history.
“As conditions have gotten warmer, the outbreaks have gotten more frequent and more large-scale. The outbreaks are normal, but the size of the current outbreak is unprecedented,” Raffa says. “The enormity is such that it has transformed lodgepole pine in British Columbia from a carbon sink to a carbon source. So it’s taking something that’s normally sequestering carbon and turned it into something that’s releasing carbon,” he says. “That has implications for global carbon cycles and global warming.”
Raffa has a good perspective on what’s normal and what’s not about the mountain pine beetle and its close relatives. He has been studying the insects since his grad student days in the 1970s, and he has continued that line of research at Wisconsin. Those studies inform his work related to some of the Badger State’s problem beetles—in particular the fir engraver, a serious pest in red pine plantations.
Most of his work focuses on the thresholds—the tipping points at which an endemic, low-level population surges to the point where it can successfully infest a stand of healthy trees, or an entire forest, or—as is now the case—a major forest ecosystem. While the infestations he’s studying can be epic in size, his approach is to think small.
“The critical features that drive whether or not these large-scale outbreaks can occur are happening at the scale of the individual beetles confronting biochemicals at the point where they enter the tree,” he explains. “Those fine-scale processes ultimately determine if outbreaks can take place.”
Bark beetles have been killing conifers in western North America for at least 38 million years. Fossilized wood from the region shows evidence of Dendroctonus (the name means “tree killer”) beetle galleries. But the attacks usually have been few, far between and short-lived. Beetle populations would burgeon when trees were under stress, kill off the oldest, biggest trees, and then die back when that food source was gone.
Outbreaks were less common because trees that evolved with the beetles are very good at defending themselves. As a lodgepole pine oozes sticky resin to block the beetle’s path, it is also killing its own tissue at the attack site and flooding the area with heavy doses of insecticidal chemicals. This leaves the beetle trapped in a toxic environment with nothing to eat. Raffa calls it a “scorched earth strategy.”
“Trees are vicious,” he says. “That’s why an outbreak occurs in only one area maybe every 30 years. Most of the time, things are weighted very much against the beetle.”
The only way the beetles can defeat a healthy tree is through a swift mass attack. The first ones to the tree sound the charge by releasing chemical attractants that draw hundreds or thousands more, enough to deplete the resin and weaken the tree before it can bring its defenses up to speed. The bigger the beetle population, the better the odds of success.
This is where warmer weather gives the beetle a leg up. Higher temperatures boost beetle numbers by speeding up the life cycle and reducing overwinter mortality. The minus-40-degree cold snaps needed to freeze beetle larvae have become rare occurrences. Warmer temperatures have also allowed beetles to thrive in places that used to be inhospitably cold.
“What we’re seeing now is the mountain pine beetle getting into higher latitudes and higher elevations,” says Raffa. As it makes itself at home farther up in the mountains, the beetle has devastated one of the Rockies’ longest-living conifers, the white bark pine. It’s what’s known as a naïve species, explains Monica Turner, a UW-Madison landscape ecologist who studies the Yellowstone National Park ecosystem. Because it evolved at higher elevations where cold temperatures used to suppress beetle populations, the tree didn’t develop the same level of ability to defend itself.
“Now it’s being hammered by the mountain pine beetle. I think we could lose this forest type, or see it very much reduced,” Turner says. And that could bring a cascade of impacts, she adds. Grizzly bears, for example, fatten up on white bark seeds before hibernation. Losing this food source could force them to move down slope where they’re more likely to encounter people. The beetle’s move into more northern latitudes opens up what some have called the “doomsday scenario,” in which the beetle takes a right turn in Canada and make its way across the boreal forest to the Midwest. The worries began when mountain pine beetles crossed the Rocky Mountains from British Columbia into northern Alberta.
“This is frightening, because lodgepole pine is a sister species to jack pine, and the two hybridize in Alberta,” Raffa says. Mountain pine beetles have been killing hybrid lodgepole/jack pine forest in Alberta, and in 2010, for the first time, have successfully reproduced in jack pine, a dominant species in the forest that stretches across Canada. “For the first time ever, the mountain pine beetle is physically connected with Wisconsin’s forest,” he says. What are the chances of the beetle making it to Wisconsin? “The short answer is that nobody knows,” says Raffa. But he hopes to get some answers through collaborative research with UW-Madison microbiologist Cameron Currie.
Cameron Currie studies partnerships between insects and microorganisms, and he’s learned a lot about how insects employ microbes in the struggle against predators, competitors and prey. His earliest studies focused on species of ants that cultivate fungus to feed their young. The ants, Currie discovered, carry antibiotic-producing bacteria that protect their fungus crop from predators. He sees parallels between this system and that of the tree-killing beetles.
Mountain pine beetles also employ fungi, both to help break down tree tissue and to provide a food source for beetle larvae and adults. Currie suspects that bacteria also play a role in the beetle system. If so, he says, they may help the beetle adapt to a new host tree species.
“Microbes have a much faster turnover rate than beetles, so they evolve and adapt much faster,” he points out. “If it is the microbes that mediate the defenses of the hosts, it may be only a matter of time before the microbes adapt in a way that they are able to overcome the defenses of jack pine.”
Knowledge and methodology developed in Currie’s ant work is helping address that question. For example, Aaron Adams, a postdoctoral fellow in Ken Raffa’s lab, is conducting a series of experiments to see how well the mountain pine beetle’s microbial firepower matches up with the jack pine’s biochemical defenses.
“I am looking at how the differences in the chemical defense from one tree species to another affects the ability of the beetle’s microbial associates—both fungi and bacteria—to survive and grow,” he says.
Adams is culturing fungi and bac-teria from the beetle galleries in thepresence of defensive chemicals from both lodgepole and jack pine. “I’m growing fungi from the beetle in the presence of lodgepole pine monoterpenes, which are a major component in the tree’s chemical defense, and in the presence of jack pine monoterpenes. I want to see if jack pine chemistry has the same ability to inhibit growth of the fungi,” Adams says. He’s taking a similar tack with bacteria collected from the beetles, seeing if they have the ability to convert these chemicals to a less toxic form.
He’s also taking a genomic approach, examining DNA sequences extracted from bacteria associated with the beetle to learn what traits they carry and what role they might be playing in the tree attack.
As they probe this microbial system, the researchers hope to also get information about its strengths and weaknesses, which could help forest managers better predict whether a beetle attack will suc-ceed or fail. They may also find some ways to harness these microbes’ abilities for bioremediation or energy production.
Canadian pulp mills are already using bacteria to break down monoterpenes in wastewater. Adams thinks the beetle galleries may contain strains that do this more efficiently. Currie, who is affiliated with the Great Lakes Bioenergy Research Center, hopes the beetle research will yield organisms that can help solve the puzzle of how to efficiently convert cellulosic plant material to ethanol. “We are looking at the possibility that within the bark beetle system there are microbes that help us break down plant cell walls,” he says.
The idea that anything good could come from the mountain pine beetle is a hard sell in the West. Most people there see the beetle as nothing but trouble, and one of the biggest worries is fire. In an area that’s already coping with years of drought, it seems like a no-brainer that millions of acres of trees killed by beetles are a conflagration waiting to happen. But it’s not that simple, says Monica Turner, who has been conducting research about fires in the Yellowstone area for many years.
“Throughout the West, a lot of people assume that when beetles come through and kill many of the trees in the forest, it will increase the risk of subsequent fires. But if you look in the literature, there is not a whole lot of evidence to support it,” Turner says. “It is a lot of anecdotes and people making assumptions.”
In fact, beetle outbreaks may actually decrease the chance of the most dangerous types of wildfire, according to research by Martin Simard, one of Turner’s former grad students. Simard compared the amount of fuel available in undisturbed stands with that in stands that had undergone recent beetle attacks and others that had been attacked 25 years ago. He found that the beetle-attacked forests had significantly less fuel in the canopy.
“This is important,” says Turner, “because the active crown fires in the Rocky Mountains, in the lodgepole pine, are carried primarily through the crown, not in the big logs on the ground. The green needles have resins in them that are very flammable and the tree crowns are close together, so the fires can go right through them.
“When you take all of those kinds of changes in the fuels and you run them in a model that simulates fire behavior, you find that the likelihood of a severe crown fire always goes down after the beetles, and it stays down for at least 25 to 35 years,” Turner says. “There is an increased risk in what we call passive crown fires—what firefighters call torching—where a single tree will light up. But you never get an increase in the risk of severe crown fire. Those are the most dangerous, the ones that threaten people’s houses and things of that nature.”
Another big worry following a major tree kill, either from beetles or fire, is the potential for losing nutrients—especially nitrates—from the system.
“It can be a problem if nitrates get into streams and ground water,” Turner says. “It happens in a lot of the eastern forests. But what we are finding in Yellowstone is that the forests seem to have pretty effective mechanisms for conserving their nitrogen. We are not finding evidence that they ‘leak’ when they are disturbed.”
Research by Jake Griffin, another of Turner’s students, suggests that the trees that survive the beetle attack, along with other vegetation, are quick to take up the nitrogen. But that might change if forest managers opt to bring in loggers to remove the beetle-killed trees.
“Will the amount of nitrate lost from the system increase if you come in with heavy equipment and cut out the remaining trees, along with [performing] the other operations that disturb the soils?” Turner asks. Griffin’s research will help answer this question.
The impacts of the mountain pine beetle do not stop at the edge of the forest. The insects are changing the face of the landscape across the West and beyond—and that’s the scale at which Phil Townsend wants to look at them. For this he needs a tool that lets him step back to get the widest view possible. He’s studying the beetle infestations from space.
“Our work is not tree-based or stand-based. We’re looking over a much larger area. We’re studying how an insect outbreak affects forest characteristics—forest composition, water quality, carbon sequestration,” Townsend explains. “We use satellite remote sensing to develop models of mountain pine beetle infestations.”
One advantage of satellite data is that it has been collected since the 1970s, long before the current beetle outbreak. This lets Townsend’s team track the beetles back through time.
“We can go back to 40 years’ worth of data to look at patterns of how the damage spreads,” he explains. “For instance, we’ve learned that mountain pine beetle infestation usually starts at mid-slope. It quickly fills in the valleys and then moves upslope. It’s like a front marching up the mountainside. This reflects the mountain pine beetles’ mass attack strategy. They storm the trees.”
Before Townsend can make sense of what he sees from space, he has to know what’s happening on the ground. So he starts in the woods, marking off a plot, counting the trees and noting their stage of attack. Later he plugs that data into a computer model that compares the area’s damage to its infrared signature. By analyzing data from dozens of plots across Greater Yellowstone, Townsend has assembled a library of signatures for different types and levels of disturbance.
“We’re not just trying to map whether a stand is dead or has been attacked,” he says. “We’re trying to map the actual percentage of the forest that is damaged, because ultimately we convert this into carbon, nitrogen—all sorts of different things. We’re trying to quantify the effects of this outbreak on the dynamics of the whole system.”
Collecting data in the woods poses some interesting logistics. Beetle outbreaks often occur in remote places, accessible only by foot or horseback. Getting there can be fun, but it takes a lot of time, so Townsend has worked up a shortcut. Using cameras and a spotting scope, he can measure damage on a remote hillside from the side of a road.
But while hiking into the woods takes time and effort, it yields more than raw data. Getting nose to bark with a lodgepole pine can offer a perspective that you can’t get from a satellite. The day after his trip up Signal Mountain, walking through a different forest a few miles to the east, Townsend stops and points to a wad of pitch on yet another big lodgepole pine.
“This tree was successful,” he says. “You can see where the beetle bored in and the pitch came out, and it actually knocked the beetle out. Here’s the beetle stuck in the pitch. This tree defended itself. It may be one of the few trees that actually does succeed.”
“Sometimes the tree does win,” he adds with a smile. “That’s neat to see.”
SIDEBAR — Audio Assault
Confusing beetles with their own language might be an effective way to control them.
Richard Hofstetter MS’96 is taking what you might call a double-talking Dr. Doolittle approach to controlling bark beetles.
For the past five years or so, Hofstetter, a professor of entomology at Northern Arizona University, has been listening in on bark beetle conversation – acoustic signals the insects use to communicate. And as he has learned the beetles’ language, he has found ways to use it against them.
The research began with an effort to simply bombard the beetles with a variety of loud and discordant sounds – everything from heavy metal to Rush Limbaugh played backwards. but Hofstetter’s team didn’t have a lot of success until they focused on the beetles’ own sounds, and began to decode which signals were associated with various behaviors.
“Playing sound alone is not going to work,” he explains. “You have to play biologically relevant sounds, such as their own signals or the sound of a competitor or predator By knowing their communications signals, we can interfere and divert behaviors or cause adverse behavior.”
Some of this involves messing with the insects’ love lives. A well-timed broadcast of a male beetle’s mating call can really spoil a mating pair’s intimate moments.
“We have seen the female leave her partner and tunnel directly to the speaker, where she sits and waits for ‘something’ to happen,” he says. “The male tunneled the opposite way.” They got even more dramatic results by playing a male beetle’s aggression call: the male mated with the female, then killed her.
Now that Hofstetter has a pretty good idea of how to use the technology to disrupt the beetles inside the tree, he’s hoping he can use it to discourage them from attacking the tree in the first place. He’s also looking for applications beyond the bark beetles, He is collaborating with a Michigan State University scientist to see if the acoustic approach could be used to mange the emerald ash borer, an invasive species that is threatening the survival of ash trees in many states, including Wisconsin.
SIDEBAR — The Forest Before and After
Landsat images taken before and after a severe crown fire in Greater Yellowstone. The false-color images show the undisturbed forest (deep green), the burned areas (purple), and other features such as lakes (black), scrubland (pinkish), and rocky/snowy mountain tops (white and light blue).
The effects of bark beetle outbreaks are less striking than those of fire because beetles only kill some of the big tees and spare the understory vegetation and the soil. In this pair of Landsat images taken before (1999) and after (2007) a beetle outbreak, the forest in the lower half of the image (south of the river) has been severely attacked (purple). Each image is approximately 20 km wide.This article was posted in Environment, Features, Spring 2011 and tagged audio assault, bark beetles, Emerald ash borer, Entomology, Forest ecology, Mountain Pine Beetle, Richard Hofstetter, sidebar, The Forest Before and After.
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