Tiny stingless wasps released in Hartley Park to slow spread of emerald ash borer
Minnesota Department of Agriculture officials released three species of stingless wasps in Duluth's Hartley Park Thursday morning, in hopes that the tiny parasitic insect will slow the spread of an invasive menace to local ash trees — the emerald ash borer.
Jonathan Osthus, a biologist and coordinator of the state's biocontrol efforts, explained that two of the wasp species introduced in Duluth attack the ash borer in its larval state, and another destroys the pest's eggs.
The wasps are about the size of gnats, and they have no interest in humans, according to Osthus.
"You'd never notice them," he said.
What people may notice, however, are the signs of efforts to establish a feeding population of the parasitic wasps in Hartley. Those signs, strapped to the trunks of ash trees, include pill bottles stuffed with coffee filter paper that's laden with wasp eggs and chunks of wood filled with wasp larvae.
Osthus and his crew also released some adult wasps during their Thursday visit to Hartley.
This summer, Hartley Park became the the eighth site in the state where the wasps are actively being introduced, and Osthus said it's the northernmost release to date.
He expressed confidence the wasps will be able to survive and reproduce in the area nonetheless, noting that Lake Superior helps to mitigate winter temperatures in Duluth.
The wasps will be released this year and next, then the Department of Agriculture will survey the area and attempt to trap them to determine how successfully they are naturally reproducing.
Osthus' expectations remain modest. He acknowledged that the outlook for most mature ash trees in forested areas of Duluth remains dim. But he's optimistic the wasp could give some younger ash trees a shot at survival.
"These biocontrols are the only management option we have at the forest-landscape level," Osthus said.
Dale Sellner, parks and grounds maintenance supervisor for the city of Duluth, said some high-value ash trees planted in boulevards will receive injections of insecticide to spare them from the emerald ash borer, but those types of controls are impractical in a forest setting such as Hartley, where ash trees are the predominant species.
The wasps being released in Hartley were reared at a U.S. Department of Agriculture facility in Brighton, Mich. Osthus said the insects are raised in a carefully controlled setting and estimated the cost to be about $4 to $6 per wasp. But the federal government picks up the tab and provides the wasps to the Minnesota Department of Agriculture free of charge to assist in its efforts to slow the state's growing emerald ash borer infestation.
About 5,000 of the parasitic wasps are expected to be released in Hartley Park this year. Osthus said the wasps have adapted specifically to feed on the emerald ash borer as a host and are not expected to pose a threat to any other species.
The wasps can fly up to 4½ miles at a stretch, and Osthus said their introduction in Hartley will likely benefit other parts of the city, as well.
"It moves with the emerald ash borer," he said of the hungry wasp.
The invasive and voracious ash borer has no natural native predators in North America and has decimated large populations of ash trees elsewhere in the U.S. Minnesota appears especially vulnerable, as it is home to an estimated 1 billion ash trees — more than any other state in the nation.
Sellner said this year, a number of local ash trees appear to be reaching a "tipping point."
"We're seeing the physical death of trees occurring, which means the emerald ash borer has probably been here for four to five years now, so the sense of urgency is higher than ever," he said.
Emerald ash borers were first found in Superior in 2013. An infestation on Duluth's Park Point was discovered in late 2015, and in the fall of 2016 the borers were uncovered near Hartley Park.
Osthus noted that substantial populations of the pest are now feasting actively in Hartley. He pointed to tell-tale splits in the bark of a black ash then used a knife to peel back a piece of bark to reveal a gallery of burrows.
In its larval state, the emerald ash borer burrows through the inner bark of ash trees and disrupts the movement of nutrients through the phloem, effectively girdling and starving the tree.