Pheromones from tiny beetles could help save Minnesota's tamarack trees

The first sign of an attack is usually a smell, an aroma that can be quite pleasant, like a freshly cut Christmas tree. It emanates from some of the deepest swamps and most hidden recesses of Minnesota's forests. But along with that aroma, there is something else in the air, something sweet to a destructive type of bark beetle. It's a trail of pheromones signaling that it is time to feast.

Scientists at the University of Minnesota have identified the chemicals and compounds that eastern larch beetles produce to communicate with one other. The hope is that those compounds can be manipulated to disrupt that communication and slow an outbreak of the swarming insect that has killed tens of millions of tamarack trees in Minnesota.

"We know a lot more now about how these beetles attack trees and signal to each other and find mates than we ever did before," said Brian Aukema, a forestry entomologist at the University of Minnesota whose lab has been leading research into the outbreak. "We're refining and refining our understanding, and if we can understand exactly how this communication signal is working, then we can use it in tree protection."

Until the last few years, little was known about the eastern larch beetle, and it had never been enough of a problem to merit deep study. The native beetle is found everywhere tamaracks are found, and it had lived in relative harmony with the Minnesota pine trees for some 14,000 years, since the glaciers retreated from the last ice age.

In the early 2000s, an outbreak of the beetles began that has not ended. Trees that had never been bothered by the insects before suddenly could not fight them off.

The reason for the outbreak, Aukema's lab found, is that the winters had become shorter and less severe. The beetles here at the very southern range of the boreal forest now have just enough warm days between deep freezes to reproduce multiple times. Healthy tamaracks can typically fend off one generation of young larvae. But they're defenseless against two or more.

Scientists have been racing to understand the once-benign beetle to see if there is anything that can be done to keep tamaracks in Minnesota as the climate continues to warm. Disrupting their communication may be one such path.

It's the female beetle that decides which tree to attack, Aukema said.

As soon as she starts to chew through the bark, a race for survival begins between the tree and the beetle, he said.

The insect emits a pheromone — seudenol — that attracts any other larch beetle that detects it.

"That's how you end up with thousands of these beetles on a live tree," Aukema said. "But the tree is not a passive organism; it's going to start exuding some resin."

That appealing smell of fresh-cut pine is resin, a sap-like defense mechanism that trees release when they are wounded to plug holes in their bark. Resin is highly toxic to larch beetles, and it also attracts their predators, such as checkered beetles.

A checkered beetle can land on a tree, cut off an eastern larch beetle's legs, split it in half and eat its insides in about 30 seconds, Aukema said.

But if enough larch beetles are attacking the tree at once, they'll overwhelm it and exhaust its defenses.

Seudenol, which is also emitted by other types of bark beetles out west, can be a powerful lure for the larch beetle and potentially used as a trap, Aukema said.

The females emit another pheromone to attract mates called frontalin. Aukema and his team found that females do not stop producing frontalin after they mate.

"That surprised us because if you've mated, you're not trying to attract additional mates and the last thing you want is more beetles to come colonize the same tree because there's only so much food under the bark," he said.

But then he saw how beetles react to higher concentrations of frontalin.

A little of the pheromone attracts beetles. A lot of it repels them.

"It's like catching a whiff of a neighbor's grill," he said. "You smell a small amount of smoke on the grill and you want to go closer. You get too close and you're standing straight over that smoke, and you want to move away."

Large amounts of frontalin signal to other beetles that a tree is full and they should move on. It's possible that high concentrations of the compound could be produced, placed near a tamarack tree and used as a beetle repellent.

Larch beetles are the size of grains of rice. Aukema and his team learned about the compounds they respond to by traveling to a special U.S. Department of Agriculture lab in Louisiana that can attach micro-electrons to a beetle's head and measure brain impulses as compounds the size of particle of sawdust are dragged over their antennae.

"It's so painstaking, it's like watching neurosurgery," Aukema said.

The project is paid for with state lottery proceeds that go to Minnesota's environmental and natural resources trust fund.

Aukema doesn't know if any of the chemicals would be able to save large swaths of the state's tamaracks. Pheromone repellents and lures have been used on other tree species in Idaho and other western states with mixed success, and are often best at saving individual trees or small areas.

Time may be running out.

Before European settlement, tamaracks were the most common tree in Minnesota, spreading across some 6 million acres. Their pencil-straight trunks made them ideal for railroad ties and telephone poles and they were quickly logged out of the uplands that cover most of the state, and never grew back. They survived in the lowland swamps and peatlands, where most other tree species cannot survive and logging is much more difficult and can happen only when the ground is frozen.

Before the outbreak, there were about 1.4 million acres of tamaracks in the state. The beetles have partially or completely devoured about 1.1 million of those acres.