Three years after state and federal officials first released thousands of tiny stingless wasps on an island in the Mississippi River to combat the destructive emerald ash borer, researchers are using a new technique to measure the effectiveness of the wasps.
Rob Venette, a research biologist for the U.S. Forest Service in St. Paul, has set up a system of cages draped with very fine mesh around two dozen infested ash trees at the park. The mesh keeps the wasps from escaping.
Venette plans to return to the site this fall to see if the wasps have attacked the emerald ash borer. In the spring, he'll check to see how they survived the winter. If it works, it will be the first time researchers recover the wasps.
"This would give us more encouragement to pursue even more releases even more aggressively," he said.
Great River Bluffs State Park is rugged land, with panoramic views of forested bluffs that straddle the Mississippi River. It's also ground zero in the state's battle against the emerald ash borer, known to researchers as EAB.
"We're in kind of a bowl and it's this big open sunny bowl where the trees have been stressed by fire and now drought as well, so they're very attractive for EAB," said Monika Chandler, a project coordinator for the Minnesota Department of Agriculture, as she stood at the bottom of a bluff in the park. "So this is an area that's kind of a sink for EAB."
State and federal officials have released more stingless wasps in the area than anywhere else in Minnesota. The wasps are a natural predator to the emerald ash borer. This year alone, 25,000 have been introduced in the 100-acre area. But even that won't be enough to save the trees there.
Since 2010, approximately 130,000 stingless wasps have been introduced statewide.
"We're likely to lose these particular trees unfortunately," Chandler said. "But our hope is that if the wasps become established, they can help to protect other trees and the next generation of ash as it comes along."
Still, the spread of emerald ash borer is inevitable, she said.
Last month, researchers found the beetle in Superior, Wis. State and federal officials say it's a matter of time before it makes its way into the Duluth area. That's why researchers are counting on the wasps to manage the emerald ash borer population, which is already in Houston, Winona, Ramsey and Hennepin counties.
At another site in the park, Chandler's colleague Jon Osthus used a pocket knife to peel back the bark of an infested ash tree.
"There's a lot more galleries," said Osthus, an emerald ash borer biocontrol coordinator for the Minnesota Department of Agriculture. "And there's the larvae."
Using the tip of the blade, Osthus pulled out the grub-like beetle. Cream-colored, it was about three quarters of an inch long.
Even though the trees have been treated with wasps, Chandler said they're more attractive to the emerald ash borer because it's been so dry this year.
"EAB disrupts the movement of water and nutrient in the tree," she said. "When you compound that with drought stress, the trees are hit so much harder."
Minnesota is home to about 1 billion ash trees -- more than any other state in the country. Researchers believe the wasps may be slowing the spread of the beetle in those trees, but they're not entirely sure.
The problem with using the tiny wasps, also known as parasitoids, is that once they're released, it's hard to know exactly where they go.
"It really is a question of finding a needle in a haystack," Venette said. "We have indirect evidence that it's working. Sites where parasitoids have been released, the populations have not built up as quickly as we would have expected and there has to be a reason for that."
There are several reasons to be optimistic. Researchers believe the emerald ash borer infestations were found early in Minnesota. Also, not all the ash trees in southeastern Minnesota are infested.
The results of Venette's cage study will help researchers determine whether to continue to release wasps as a strategy in the battle against the tiny invasive beetle.