Stepping into a forest this time of year – amid all of the dormant trees – might seem like walking into a graveyard. That’s especially true in some forests where, between the dense leaf litter and fallen branches of maples, oaks and chestnuts, lie hundreds of trees whose demise came prematurely.

All of them were ash trees.

Once flourishing groves of native ash trees – mighty hardwoods that were as popular as street trees as they were for making quality baseball bats – have been dying rapidly for two decades because of a small, stubborn pest: the emerald ash borer.  

Recently, new ash trees have been spotted growing in forests, parks and backyards. This has given hope to ash-tree fans that perhaps the hardwood could be making a comeback. Experts say the plague isn’t over, but researchers have found that some ash trees are resistant to the ash borer, and the scientists are looking for ways to grow forests of resistant ash trees. And they are asking property owners who see healthy ash trees to report their locations to help with the project.

The insect is a bright, green “jewel beetle” that originated in northeast Asia. It came to the U.S. in about 2002, arriving in the Great Lakes port of Detroit, Michigan, in wooden shipping crates. It decimated Detroit’s street trees and the ash trees in its parks and surrounding forests before spreading south into Ohio and across North America. 

The number of ash trees has steadily declined since then. According to data from the U.S. Forest Service’s Forest Inventory and Analysis program, Ohio’s forests had an estimated 260.6 million ash trees in 2004. By 2022, the forest service estimated there were 172.9 million ash trees, a reduction of about 33%.

Peter Lowe, senior coordinator of horticulture education and community impact at Dawes Arboretum in Jacksontown, south of Newark in Licking County, said the ash borers are still lurking in the forests.

“The threat is not over; what’s happening is the ash trees need to be of a certain size for the emerald ash borer to want to eat them,” Lowe said. “The majority of trees now aren’t at the appropriate stages to be killed off and that is why the ash borer has not been around as much.”

The squiggly path of the emerald ash borer can be seen under the bark of this dead ash tree in the Denison University Biological Reserve. Credit: Shaye Phillips

Ash borers don’t start attacking trees until they’re about four inches in diameter, he said. When they chew through the bark, they burrow into the phloem layer, where they cut off the veins that transport nutrients to the tree. As the beetles burrow deeper, they also disrupt the xylem layer, where water for the tree is stored. Without nutrients and water cycling through the tree, it will wither and die. 

Luckily, the ash borers don’t target the roots. The living root structure can send up new shoots, giving the opportunity for juvenile versions of the tree, which are called ash bushes.  

“Once they grow, if the tree’s damage is under enough control –  less than 50-60% – then pesticides can be used to take care of the ash borers,” Lowe said. 

Lowe said that a common mistake some people have made is to end pesticide treatment before the infestation is gone. 

“Those trees will appear fine, but they are not,” Lowe said, adding that continued pesticide treatment is the only way to keep the bugs at bay. 

Kathleen Knight, a research ecologist with the U.S. Forest Service in Delaware, Ohio, has been working on other ways to help coax the ash trees back to life. In 2006, she set up monitoring plots in forests that had a great population of ash trees before they were killed off.  Knight has measured the trees in those plots every year since then to see how many ash trees died over time. Knight and the forest service are still monitoring those plots this year.

Knight said that over the past few years, the U.S. Forest Service has noticed a second resurgence of the ash borer. During the first invasion, a majority of the fully grown ash trees died. Now the second generation of the younger surviving trees have become a target for the tiny emerald beasts. 

“The second outbreak is bringing a lot of mortality,” Knight said. “People might notice this driving along the roads and seeing a ton of dead trees.” 

The U.S. Forest Service is working on a cross-breeding project that could get the ash population back on track. Knight and her team noticed some larger trees that survived the second wave, sparking hope that they could have some rare resistance to the ash borer. 

“There are five species of ash in Ohio (black, blue, green, pumpkin, and white), and blue ash has a lot better survival than the other species,” Knight said. 

Fresh buds on this white ash sapling give hope to Joshua Cherubini, manager of the 350-acre Denison University Biological Reserve north of Granville. Credit: Shaye Phillips

Although it appears similar to other varieties of ash, the blue ash does not seem to attract the emerald ash borer as much as the others. Knight described this as a significant breakthrough, because the blue ash could be crossbred with other trees to increase resistance. 

Jennifer Koch, Knight’s colleague and geneticist at the forest service, is searching for thriving trees for breeding programs. Knight and Koch encourage Ohioans who have found healthy ash trees to report them on an app called TreeSnap. The surviving ash trees are used to spawn more saplings, with the intent to test their resistance and replant them back in the wild. 

“We’re still trying to plant these seed orchards, and we need to test a lot more trees for that,” Knight said. More trees means more diversity, which is the key to saving the species. 

“We’re making really good progress, and all the results are really promising,” Knight said.

Experts are trying to remain positive but keep the issue in perspective. 

“It seems likely that ash will persist in Ohio’s forests into the future,” said Thomas Macy, Forest Health Program Manager at the Ohio Division of Forestry. “However, ash borers are here to stay, so the ash trees will not likely be able to become the dominant canopy trees like they once were.”

Kathy Smith, the Program Director of Ohio Forestry and a private woodland owner in the Ohio Woodland Stewards Program, shares similar concerns. 

“I think it’s great that we’re trying to develop resistance in trees; it’s just not going to happen quickly,” Smith said. “We’ll just have to be patient and see if we can find something to fill the gap that ash trees did in Ohio’s forests.” 

Ash trees play a vital role in Ohio’s forest ecosystems. Jim McCormac, a naturalist, author, photographer and columnist for The Columbus Dispatch, lives in the Columbus suburb of Worthington. Having worked for the Ohio Department of Natural Resources for 31 years, McCormac knows quite a bit about the importance of ash trees. 

This “drill hole” is a telltale sign of activity by the emerald ash borer. Credit: Shaye Phillips

“Green ashes are a major component of forested wetlands and their loss could open up wooded wetlands and pools to excessive sunlight. Too much sun allows for the wetlands to dry up, threatening the aquatic life in those habitats,” McCormac said. 

Ash trees also play a crucial role in the lumber and manufacturing industries. Ash lumber has been used for furniture, bookcases, bowling pins and baseball bats. Ash wood was the leading choice for Major League Baseball bats for a century. The loss of these products has also led to a growing gap in commodity markets. 

The loss of the once prominent trees has been a blow to Ohio’s forests, but the trees weren’t always a concern. Smith, of the Division of Forestry, remembers that when the emerald ash borer first touched down in the state two decades ago, experts were not worried about them. 

“Michigan was dealing with [the invasion] and I asked if we should start talking about it and putting it front and center, and a couple of forest health folks said to me that it wasn’t going to be a problem,” Smith said. 

Once the ash borer took over, though, it was too late to stop the invasion. The pests ended up opening the door for more invasive species to be discovered and identified.

“What we learned from that is that we have to be prepared to talk about [invasive species] – not in a panicked, scary way – but in a way that tells woodland owners or urban tree owners to be vigilant and tell us when they see one,” Smith said. “The reality is this is just going to keep happening and we need to get better prepared and be proactive to catch up.” 

The best way to identify an affected ash is to look at the bark. “Emerald ash borers drill d-shaped holes that can be seen among the top layer of bark,”  said Joshua Cherubini, manager of the 350-acre Denison University Biological Reserve, as he points out a bunch of pin-shaped holes in one of the Bio Reserve’s decayed ash trees. 

Read more: Denison’s new Bioreserve manager tackling invasive plants across 350 acres

Josh Cherubini became manager of Denison University’s 350-acre Biological Reserve in late 2023. Credit: Brin Glass

Underneath the first layer of bark, zigzag markings running along the cambium layer are a sure sign of an invasion. Another, more obvious sign is that when an ash tree dies, the top section of the tree will break off from the rest. “If you see a tree where the top is broken off, there’s a good bet that’s an ash tree,” Cherubini said. 

Cherubini walks away from another fallen ash tree, only to find a young, white ash tree growing next to the trail. Little buds have formed at the tips of the thin branches, and it appears to be relatively healthy. 

“My hope is that we still get a lot of sprouts,” Cherubini said, as he gently held up one of the branches. “I hope that there will be enough to restart the population, even though the emerald ash borer is here to stay.” 

Shaye Phillips writes for TheReportingProject.org, the nonprofit news organization of Denison University’s Journalism program, which is supported by generous donations from readers. Sign up for The Reporting Project newsletter here.