Wasps released in Forest Grove to help combat spread of emerald ash borer

PORTLAND, Ore. (KOIN) – When the emerald ash borer was first found in Forest Grove in June 2022, the Oregon Department of Forestry and the Oregon Department of Agriculture took the discovery very seriously and immediately began strategizing on how to slow its spread. 

The state has had time to prepare, since the invasive insect has already wreaked havoc on ash trees in 34 other states. It wasn’t a matter of if the emerald ash borer would make its way to Oregon, it was a matter of when. 

The discovery in Forest Grove was the first time the destructive insect had been found in the west coast states of Oregon, Washington and California. ODA believes the emerald ash borer may have been in the area for 3 to 5 years before it was discovered, since it can take several years before an ash tree shows symptoms of an infestation. 

Now, one year after that discovery, ODF and ODA are taking a multi-pronged approach to slow the insect’s spread. 

There’s no easy way to eradicate the bug once it arrives in an area, but if scientists and forestry experts can slow its reproduction and migration, they can better protect some trees and remove trees that could serve as hosts. 

As of June, ODF had confirmed emerald ash borers were present in 50 trees in Washington County. More than 9,000 trees have been surveyed. 

So far, ODA said the only location where the emerald ash borer has been found in Oregon is around the city of Forest Grove. 

On June 7, ODA for the first time released natural enemies of the emerald ash borer in Oregon. These natural enemies are all wasps that can parasitize emerald ash borer larvae. 

The wasps are called Tetrastichus planipennisi, Spathius galinae, and Oobius agrili and none of them have the ability to sting. 

Like the emerald ash borer, these wasps drill through tree bark to lay their eggs. However, instead of simply laying their eggs under the bark, the wasps lay them directly on emerald ash borer larvae. When the wasp larvae hatch, they eat the host larvae, therefore preventing the emerald ash borers from maturing and reproducing. 

The U.S. Department of Agriculture produces the wasps with the intention of using them against the emerald ash borer. Last spring, the USDA intentionally infested bolts of ash wood with emerald ash borers and then allowed the wasps to penetrate the bark and lay their eggs into the emerald ash borer larvae. 

When the wasps were about to emerge from their larval stage, the bolts were shipped overnight to Oregon. The next day, ODA staff hung 14 of them at two locations near Forest Grove and released 13 adult Spathius galinae to start targeting the emerald ash borers near the city. 

ODA expects to receive more of the wasps in the coming weeks. 

In the fall, ODA will collect wood samples from the wasp sites and look for emerald ash borer larvae under the bark to see if the wasps were able to parasitize them with their eggs. 

In addition to the wasps, ODA and other local agencies are also trying to slow the spread of emerald ash borers with more than 100 “trap trees.” 

These trap trees were selected around Forest Grove, about 2 miles from where the emerald ash borers were first seen. To turn the ash trees into traps, ODA girdles them, meaning it saws away bark at the bottom of the tree, so that the trees send out distress signals and attract female emerald ash borers looking for a place to lay their eggs. 

The ash trees near a girdled tree will be treated with an insecticide that ODF said should prevent the emerald ash borers from infecting them. Instead, the intent is to concentrate the egg-laying to the trap tree. 

Next fall, ODF said the trees will be cut down to see if there are any emerald ash borer larvae inside the bark. This will help officials know which way the invasive insects are spreading. 

While ODA is working closer to the city, ODF is creating trap trees 15 to 30 miles from Forest Grove to help determine if emerald ash borers are spreading into outlying areas. ODF will monitor these areas with drones to track the emerald ash borer impact over time. 

Ash trees within a mile of where the emerald ash borers were first detected have been cut down over the last year. ODA expects they’ll have removed 58 trees by June 30. 

A quarantine remains in place in Washington County that prevents people from moving ash and white fringe tree material out of Washington County. This includes logs, stumps, green lumber, nursery stock, scion wood, root stock, chips and mulch, roots and branches, and firewood of any hardwood species. 

Emerald ash borers have killed up to 99% of the ash trees in some North American locations. At least five ash species native to the Central U.S. have become critically endangered as the emerald ash borer spreads across the country. 

The emerald ash borer is about a half-inch long and an eighth of an inch wide. It’s known for its metallic, shiny green color.