Emerald ash borer threat: Tiny wasps released in Minnesota to save trees

CHISAGO COUNTY, Minn. (FOX 9) - Minnesota is releasing thousands of tiny wasps into its woods, and they could be the ash tree's best shot at survival.

Battling the emerald ash borer

What we know:

The emerald ash borer has been devastating Minnesota's forests since it was first discovered in the state in 2009. Researchers believe the pest mostly spreads through firewood carried by humans, though it can also fly. The damage it leaves behind is hard to miss — stripped bark, leafless limbs and dying trees.

At Ki-Chi-Saga Park in Chisago County, the destruction is visible just steps into the woods. The park spans 97 acres and is made up of roughly 90% ash trees, putting nearly the entire canopy at risk.

"You can see these marks here, here. Coming all the way up the tree," said Jackson McDowell, Chisago County Parks supervisor.

Insecticide treatments can slow the emerald ash borer, but they're only a temporary fix and come at a significant cost — making them an impractical long-term solution for a park of this size.

A tiny but powerful ally 

Dig deeper:

Minnesota's Department of Agriculture has released about 8,000 wasps into Ki-Chi-Saga Park over the last two years as a biological response to the emerald ash borer. Most of the wasps are in their pupal and larval stages, housed in small pull boxes or bark barrels attached to the trees.

"These wasps, especially the egg parasitoid, is very, very small. You probably would never even see it," said Angie Ambourn, a Minnesota Department of Agriculture entomologist.

The adult wasps are only about the size of a gnat and can only be seen clearly under a microscope. Critically, they do not sting humans. The wasps are native to Asia — the same region as the emerald ash borer — and can only survive by acting as parasites on the pest's eggs and larvae. The department has released them at dozens of locations across some of the 60 counties where emerald ash borer-infested trees have been confirmed.

"There are also some studies that have already been done that are showing good establishment and that these, you know, that our parasitoids are persisting even after we've done the releases for years after we done those releases," said Ambourn.

FOX 9's Corin Hoggard first visited the park last July after the first batch of wasps was introduced. It's difficult to tell with the naked eye whether the wasps survived the winter, but McDowell says the signs are encouraging.

"We still have a canopy and we still have a lot of green leaves, so we're hopeful," said McDowell.

What we don't know:

Beyond buying time, scientists hope the wasps may give ash trees the opportunity to evolve their own natural defenses against the emerald ash borer.

Whether that long-term outcome is achievable remains to be seen.