Emerald ash borer spreads deeper into Minnesota

The invasive emerald ash borer is continuing its path through Minnesota, putting the state’s wealth of ash trees at near-inevitable risk. However, by respecting material quarantines and managing their own trees, whether infected or unaffected, people can at least hit the brakes on infestations, giving their neighbors time to prepare.

For the Minnesota Department of Agriculture, new EAB finds are not very surprising at this point.

“(EAB) is going to keep popping up in new locations, unfortunately,” said Jonathan Osthus, MDA EAB technical assistance coordinator. “People need to prepare themselves. If you’re in an area not known to be infested … once you’re within 10-15 miles of a known infestation, you really need to think about whether you’re going to treat your trees or remove them if they’re in a location in your yard where they could cause damage to people or property.”

EABs, native to northeastern Asia, are normally a bright, metallic green, about a half inch long and an eighth of an inch wide as adults.

With EAB’s recent detection in Todd County, there are now 60 counties in Minnesota with known EAB infestations, broadly covering the state’s south, east and central regions. The average spread rate of the insect without human-assisted movement is about a couple of miles per year, so if the quarantine is maintained, it will at least reduce how fast EAB moves through the region.

“The insect is cryptic in nature,” Osthus said. “By the time you find it in a new location, it’s already been there for a number of years; it spreads beyond what is visible, so it’s one of those things where you’re always behind once you find it. … When it (EAB) gets into a community or area, it takes 12-plus years to kill the majority of the mature ash.”

Materials regulated under the quarantine include logs, tree waste, wood chips and mulch from ash trees, as well as all non-coniferous firewood. EAB can remain present in a pile of ash wood for over a year after its cutting.

There are three types of ash trees native to Minnesota: black ash, green ash and white ash. Black ash is found throughout two-thirds of the state, especially in low, wet areas in north central Minnesota; green ash is the most common, growing throughout the state; and there are pockets of white ash in southeast Minnesota’s upland areas. All three species are highly susceptible to EAB, with green and black ash being the most at risk.

“They don’t really have any natural resistance,” Osthus said. “It’s a 98%-99% mortality rate for mature ash trees once emerald ash borer gets in the area.”

It usually takes about five years for a tree to die from an EAB infestation.

The main EAB symptom MDA looks for when conducting early detection efforts is woodpecker damage to ash trees, mainly located in the middle to upper tree canopy, where the birds are feeding on EAB larvae below the bark. The resulting bark loss will leave bare, blonde patches on the tree with shallow woodpecker marks.

Infestations start from the upper levels of the tree and work their way down. By the time symptoms appear on a tree trunk — such as EAB’s characteristic serpentine tunnels below the bark — the tree is far along in its destruction.

“You’ll see the thinning canopy and dying crown, which should make it pretty obvious,” Osthus said. “That woodpecker damage shows up before there’s any real, obvious canopy decline.”

For people who want to protect their ash trees, there are insecticide options available; trees can be saved if they are healthy and the insecticide applications are done properly.

“You need to have 70% of a healthy canopy or more for it (the tree) to be a good candidate to treat,” Osthus said. “Even lightly infested trees can be treated, as long as it has a healthy enough canopy.”

If a tree is mature, with a 15-inch trunk diameter about 4.5 feet above the ground, MDA recommends bringing in a professional to assess the tree and apply the needed chemicals, which is usually done by trunk injection. Soil drenches available at garden stores are viable options for smaller trees. These treatments need to be reapplied annually or every other year, depending on the chemical used.

When it comes to replanting after removing ash trees, MDA advises property owners to lean toward species diversity. People can check with their University of Minnesota Extension to get planting recommendations for their part of the state.

“Planting with diversity leaves your yard and forest more resilient against a future pest outbreak or diseases that may come through,” Osthus said. “Emerald ash borer is the most recent devastating one, but there will likely be another one in the future. … A lot of people want to replant with a nice maple tree because they have that beautiful fall color; maybe think about planting something else. Maples are overrepresented in most communities.”

Osthus also encourages people to reach out to their area Department of Natural Resources or forester to develop a forest management plan; these resources can sometimes offer guidelines and cost-share options for certain activities.

In the meantime, slowing the spread of EAB by respecting the quarantine and letting infected ash trees die as slowly as possible will help Minnesota communities financially as well as environmentally, spreading out the mitigation-related costs over a longer period.

“It’s kind of inevitable that it’s going to cause a lot of damage in the area,” Osthus said. “You just want that to happen at a slower rate so the impact doesn’t happen all at once and it’s not as costly. … Even if your tree looks healthy, it’s going to be impacted at some point in the near future, so figure out what you have for ash trees in your yard and … decide whether treatment is even an option that’s worthwhile.”