Beetle Destruction - Photo by Don Becker, courtesy U.S. Geological Survey.

Beetles of Mass Destruction

How the mountain pine beetle could take out Minnesota's forests


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By Sara Specht

Black Hills

Mountain pine beetle larvae are exposed beneath pine bark to test for cold tolerance. At right, a pine forest in the Black Hills turns red from beetle kill. Photo by Derek Rosenberger

You hear the term “outbreak” and you picture a disease, a tiny virus flaring up and spreading uncontrolled through a population. The word “epidemic” conjures images of scientists struggling to contain the bug as it expands and jumps to new locations and hosts.

It turns out those expressions apply just as well when the bugs are winged and the victims are trees. 

Now try this word on for size: hyper-epidemic.

That is what scientists are calling the current outbreak of mountain pine beetles cutting through massive swaths of pine forests across the western United States and Canada. Raging since the early 1990s and at ten times the magnitude of any recorded outbreak—and growing—this is a forest insect epidemic on a scale no one has ever seen before. 

Now forest entomologist Brian Aukema, an associate professor in the Department of Entomology who cut his teeth on the mountain pine beetle outbreak in British Columbia, is leading research efforts to slow the insect’s march toward Minnesota and to understand what the impact will be if it does eventually arrive.

“If you look at the areas of pine killed out there so far, it’s greater than the size of the state of Minnesota,” Aukema says. “Imagine driving from Albert Lea to Lake of the Woods, and from Fargo all the way out to Hudson, and seeing nothing but dead pine all the way. What would happen here if it got into our pure stands of white, red or Jack pine, we have no idea, which is what we’re trying to figure out with this project.”

Bugs gone wild

Unlike most of the looming pest invasions that make the news, the mountain pine beetle isn’t invasive. It’s native to the United States and has existed for millennia in a boom and bust cycle with the local pines west of the Rocky Mountains. It exists in small numbers for decades at a time until there are enough mature pines nearby—it exclusively attacks trees over about 60 years old—when the population explodes into an outbreak until it runs out of trees or the climate turns cold enough to stop it.

pitch tubes

The insects kill by breeding in and tunneling through a trees water-conducting tissues just under the bark. On the tree exterior, this results in popcorn-shaped masses of resin, called "pitch tubes", where the beetles have entered.

During an outbreak, the tiny insect, about the size of a grain of rice, burrows into and tunnels through just under the bark of a tree. Once a year, around late July, all of the adults fly to find new hosts, and winds can blow the swarm hundreds of miles. A female leads the way and releases pheromones to attract huge numbers of beetles to attack a single tree. Feeding and colonization can kill a tree in weeks, but it can take a year for the telltale red coloring to show up signaling a beetle-killed tree. Within a few years, the tree will be gray, leaving a trail of ghost forests in the wake of the swarm.

Twelve years ago, the mountain pine beetle swarm crossed the continental divide of the Rockies into northern Canada for the first time. The beetle, a generalist that will feed on almost any breed of pine it runs across, found a new species, the jack pine, dug in and began moving east, attacking nearly every new species it encountered. That ribbon of jack pine stretches to Minnesota and beyond, where it meets other new menu items, like red and white pine.

There are two main environmental differences from previous outbreaks that have encouraged this one to continue expanding. The first is a result of decades of forest fire prevention across the country, resulting in a plethora of older pines across the landscape. The other is the global trend of climate change that has left much of North America with slowly warming winters.

“The mountain pine beetle is doing what it’s always done—it kills all the pine at once, and then 60 years later it has another forest to go through,” says Aukema. “But after Smokey the Bear—”only you can prevent forest fires”—the lodgepole and ponderosa pine forests grew up to be mature and overmature, and they were ripe for the beetle. That, combined with gradually warming winter temperatures in western North America, is completely exacerbating the current outbreak.”

Derailing the eastern advance

The eastern front of the pine beetle occupation in the U.S. today is the Black Hills region of South Dakota. Together with the Minnesota Department of Agriculture (MDA), the U.S. Forest Service and the Minnesota Department of Natural Resources, Aukema’s team is funded with a three-year Legislative-Citizen Committee on Minnesota Resources grant aimed at both preventing the introduction of mountain pine beetle into the state and understanding how the insect could affect its nearly 200 million mature pine trees.

Following two pine log inspections in 2012 that revealed mountain pine beetles transported into the state, once in building materials and once in firewood—luckily already dead in both instances—the team recognized another potential infestation pathway into Minnesota. After conducting surveys of businesses that might transport in pine wood, in January 2015 the MDA instituted a quarantine on freshly cut pine logs with bark on them shipped from infested states. The move has been met with statewide support.

“It’s funny, because with anything else we do, there is always someone who’s unhappy with it,” says Mark Abrahamson, a supervisor in the plant protection division of the MDA. “But the mountain pine beetle is something that everyone hates. No one has had any complaints and I haven’t heard any reports of businesses objecting.”

Brian Aukema and Derek Rosenberger

Brian Aukema (left) and Derek Rosenberger examine debarked samples of Minnesota pines that have been exposed to mountain pine beetle in South Dakota.

While everyone hopes the quarantine will slow or halt the beetle in South Dakota, the northern arm of the outbreak continues to follow the Canadian boreal forest in this direction. To prepare for that invasion, Aukema is working to understand how the insect will fare in Minnesota’s forests. In order to kill enough beetles to stop an outbreak, the temperature needs to drop to -40 degrees for a few days, or experience a dramatic cold snap in the early fall or late spring when the beetle isn’t physiologically ready for it. According to analysis of historical climatic data by Derek Rosenberger, a graduate student in Aukema’s lab, northern Minnesota has been experiencing slowly warming winters for more than 50 years, enough to be very susceptible to mountain pine beetle attack should the beetles arrive.

To test how the beetle will respond to the state’s pine species, Aukema's team has, for the past two years, transported freshly cut and wax-sealed trees from the Cloquet Forestry Center out to the Black Hills where there is an active beetle population. After introducing females into the Minnesota pines, the researchers track whether the insects attack, colonize and breed there, and if so, do the young survive and thrive.

“All of our early data is showing that yes, mountain pine beetle appears to do quite well in our eastern pines. Very well,” Aukema says. “But there are a lot of questions still to be asked in this system. It’s hard to read a story about a quarantine and understand ‘what’s the actual risk to my pines?’ We want to quantify that risk and translate it to people who can use it.”

This summer will mark the second full year of data collection from the field trials, while Aukema’s team has also been performing further testing on the insect’s cold tolerance. He hopes to have a clear picture of how the beetle will survive in Minnesota’s climate by late fall. In particular, he wants to translate their findings into answers that can inform land management policy and practices that will be relevant for the state and the region.

In the meantime, he is already formulating new avenues of research into the insect’s introduction into the state’s unique forest ecology, preparing to face the beetles' potential assault on Minnesota’s pines.

Current Mountain Pine Beetle Outbreak Range

Map of Pine Beetle Outbreak

The mountain pine beetle is native to western North America, where it would survive at an endemic level for years until ideal conditions prompted an outbreak. Those outbreaks could last a decade before the epidemic subsided due to either a lack of new hosts or a substantial cold snap.

The current mountain pine beetle epidemic that began in the early 1990s is now the largest forest insect outbreak in modern recorded history.

When they fly in the summer, the beetle can get caught in wind currents and can spread up to 500 miles in a year. In 2003 the beetle breached the geoclimatic barrier of the Rocky Mountains that has historically halted its spread north into Canada. From there the insect continued to move east through the Canadian boreal forest.

So far the mountain pine beetle has devastated more than 125 million acres for mature pine forests in North America, including 45 million acres in Canada. Currently, the eastern edge of the U.S. outbreak is the Black Hills region of South Dakota. The Canadian outbreak is following a path of jack pines that stretches through Minnesota's lakes region, which the beetle, left unchecked, could potentially follow through to the East Coast and down into southern pines.