Eastern larch beetles, tiny burrowing bugs native to Minnesota, are exploding in number across the state's northern forest and have killed or damaged about a third of the state's tamarack trees — one of the first clear signs of a rapidly changing climate.

With earlier springs and warmer winters, there's nothing to stop the bug's relentless march across Minnesota, leaving state foresters helpless in the face of an onslaught that seems destined to eradicate the graceful deciduous pine admired for its striking deep gold color in the fall.

"It's a fantastic example of climate change in action," said Brian Aukema, a University of Minnesota professor who studies larch beetles and other forest insects. "That insect is telling us that tamarack no longer belongs here."

The tamarack assault is the first, but not likely the last, infestation driven by climatic changes that eventually could help convert Minnesota's boreal forest to hardwood, scrub and eventually savanna, forest scientists say.

A second insect threat looming is the mountain pine bark beetle, which has managed to jump the Rockies from its native region west of the mountains thanks to warmer winters and is eating its way toward Minnesota via the jack pines in Canada. And the emerald ash borer, an invasive insect from Asia, is expected to accelerate its destruction of the ubiquitous shade trees in Minnesota and elsewhere across the North as average winter temperatures climb in the coming decades.

"There are a lot of things that could come here if our winters warm up," said Lee Frelich, a professor of forest ecology at the U who studies the impact of climate change.

But the larch beetle shows how a homegrown pest can quickly evolve into a major threat with a relatively minor shift in climate.

Foresters in Minnesota and elsewhere in the pine forests around the Great Lakes have long seen it as a benign pest that remained in balance with the natural systems around it. Wasps were an effective predator, and frigid winters kept its numbers in check. Sometimes the bug would take out stands of mature larch trees, but the outbreak would quickly fizzle.

But starting in 2001, foresters observing the vast tamarack bogs of northern Koochiching and Lake of the Woods counties in Minnesota noticed outbreaks that got started — and just kept going. Now, more than a quarter-million acres scattered across the state — an area as large as Anoka County — are filled with dead and dying tamarack trees.

"We've never recorded an outbreak like this before," said Brian Schwingle, a forest health specialist with the Minnesota Department of Natural Resources. "It's unprecedented."

'I was shocked'

In 2010 Aukema arrived at the U after years studying pine bark beetles in British Columbia as they destroyed a section of forest the size of Minnesota. He was asked by DNR foresters to take a look at larch beetles.

"I was shocked by the mortality in Minnesota," he said.

One of his graduate students, Fraser McKee, began spending a lot of time sloshing through tamarack bogs in northern Minnesota tracking the life cycle of the tiny beetles.

Scientists thought they understood the entomology well. The adult beetles spend winter under the bark of a tamarack tree or beneath the snow at its base. In April they emerge and fly to new host trees, tunneling through their bark. The females send a scent to attract males, and together they build galleries of tunnels and niches for eggs, girdling the tree and destroying its circulatory system in the process. Then they do it again, and some do it a third time in a single summer.

But the broods of offspring don't fly. They stay where they are and hunker down for the next winter. Those that survive the subzero temperatures emerge as reproducing adults the following spring.

In short, they produce one generation per year. Or they used to.

A few decades ago, that part of Minnesota saw winter temperatures plummet to 35 below eight or nine times a year. But since 2000, that's only happened twice, said Kenny Blumenfeld, a senior climatologist with the Minnesota State Climate Office. Minnesota now has more frequent warm spells in winter and earlier springs, and average summer temperatures are increasing about one-tenth of a degree per decade, he said. Average winter temperatures are rising 10 times faster than that, he said, and even faster in the state's northern reaches.

"It's the kind of change that's hard for natural systems to keep up with," Blumenfeld said.

'They can fly!'

During the unusually hot, dry summer of 2012, McKee tracked beetles on about 150 trees in Lake of the Woods County and began to notice something unusual. Trees were under attack at a time when the beetles were not supposed to be flying. Not only that, but the beetles were the reddish-brown color of young adults and carried a remarkable amount of fat for an insect that supposedly had survived a northern Minnesota winter.

He brought some back to his lab and that fall started growing a second generation by replicating summer temperatures and light. One day he opened the glass container and the insects flew up toward the lab's fluorescent lights.

It was, Aukema said, a eureka moment.

It proved, he said, that "they reproduce without going through winter — and they can fly," he said.

McKee theorizes that some of the bugs born early in the summer now get a temperature cue that flips a genetic switch and makes them into reproducing adults. In addition, more of them are surviving the warmer winters.

Aukema is now figuring out just exactly what those temperature points are, but the mystery of Minnesota's larch beetles has largely been solved.

"They grow at an exponential rate," said Jessica Hartshorn, a forest health specialist with the DNR.

A stronger commercial market for tamarack wood would help the tree survive, foresters said. Stands at risk of an infestation could be logged, stopping the beetle's spread while encouraging new growth. Tamarack was once a precious commodity; the mature trees had perfectly straight trunks that, in the mid- to late 1800s, were ideal for the growing country's railroad ties and telephone poles.

Now their progeny are mature, covering 7 percent of the state's 15.7 million acres of timberland. But less than 50,000 cords are sold each year in Minnesota, compared to 1.5 million cords of aspen.

Harder to harvest

"We are dealing with more acres than the market can absorb," said Kristen Bergstrand, DNR's forest marketing coordinator. The primary reason, she said, is that tamaracks have to be harvested in winter because they grow in soggy bogs and peatlands that are impossible for loggers to navigate in the summer. And even winter access is getting more difficult as winters warm and ground beneath the snow is less likely to be frozen solid, she said.

Similar larch beetle infestations have attacked Canada and the southern edge of the boreal forest around the Great Lakes.

Foresters said they don't know what will happen in the tamarack bogs once the trees disappear. The water table could rise because the trees are no longer there to soak it up, and shrubs will likely move in to replace them.

"For me that's the unanswered abyss," said Hartshorn. "But it seems like the bellwether for what we're headed into."

Josephine Marcotty • 612-673-7394