Small pest, big killer

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Although small, at just an inch and a half long, the metallic green-colored emerald ash borer is deadly. The invasive, wood-boring beetle is one of the country’s most destructive tree pests, responsible for the deaths of millions of ash trees since its discovery 15 years ago. In February it was confirmed in Clayton County. (Photos courtesy of

EAB larvae do the most damage to ash trees. Once they hatch in the tree’s crevices, they tunnel into and feed on the tissue between the bark and the sapwood. This disrupts the tree’s ability to transport water and nutrients, essentially starving the tree.

Ash trees, like the white ash shown here, are known for their opposite limbs and opposite compound leaves, which include five to nine leaflets per leaf.

Now that EAB has been confirmed in Clayton County, communities and property owners have a difficult decision to make: do nothing, remove and replace their ash trees, or protect healthy trees from the growing threat.

Responding to the EAB threat in Clayton County

By Audrey Posten, North Iowa Times Editor

The exotic beetle is metallic green in color and half an inch long—small enough to fit on a penny. But don’t let its diminutive appearance fool you. The emerald ash borer (EAB) is one of the country’s most destructive tree pests, responsible for the deaths of millions of ash trees since its discovery 15 years ago.

Earlier this year, EAB was confirmed in Clayton County, leaving communities and property owners with a difficult decision to make: do nothing, remove and replace their ash trees, or protect healthy trees from the growing threat. 

What is EAB?

“Emerald ash borer is a bad bug,” shared Mike Kintner, from the Iowa Department of Agriculture and Land Stewardship, who spoke at a meeting about EAB at the Clayton County Extension Office in Elkader April 12. “It not only attacks, but kills, the ash tree.”

“It’s here,” he warned, “and it’s going to spread.”

Kintner said EAB was first found in the United States in 2002, in Detroit, Mich., although it could have been in the country as many as 10 years before that. A native to Asia, the invasive, wood-boring beetle likely arrived in solid wood packaging material, he explained. Today, EAB has reached 30 U.S. states and two Canadian provinces.

EAB was first confirmed in Iowa in 2010, in Allamakee County, and has since spread to 45 counties. Clayton County had staved off infestation until February, when Kintner discovered it in a tree on the north side of Marquette, near the river. The state, he said, boasts 55 million ash trees. Clayton County has one of the highest concentrations, with between 2.5 million and 5 million ash trees.

“That’s kind of a problem,” Kintner said. “There’s a lot of ash out there.”

Northeast Iowa’s ash trees are predominantly white ash, followed by black ash, noted Iowa DNR District Forester Dave Asche, who also spoke at the meeting. Ash trees, he explained, are known for their opposite limbs, setting them apart from other species. They also have compound leaves, with seven to nine leaflets per leaf. They are opposites, as well, he said. In addition, ash trees are known for throwing off a lot of seed.

“Ash is a great tree. They’re quick growers, provide good shade and take drought well,” Asche noted. “People jumped on the bandwagon. That’s why we’ve got a problem.”

Although adult emerald ash borers feed on the leaves of ash trees, Kintner said it’s the larvae that do the most damage. Once they hatch in the tree’s crevices, they tunnel into and feed on the tissue between the bark and the sapwood.

“This disrupts the tree’s ability to transport water and nutrients,” he detailed. “It essentially starves the tree.”

EAB hits the top of the tree first, then progresses downward, so Kintner said signs of infestation will first be revealed through canopy dieback. The tree will look spindly and rough. Other symptoms include increased woodpecker activity, bark splits and suckering, where sprouts come out at the bottom of the tree. When adult beetles emerge through the bark, they will create small, D-shaped exit holes. Under the bark, one will see the meandering, S-shaped galleries created by the larvaes’ tunneling.

Kintner said EAB has a one-year life cycle in Iowa. Adult beetles begin emerging in May or early June, and activity continues throughout the summer. Due to their small size and similarity to other insects, the beetles can be hard to detect, Kintner added.

“A few don’t do damage,” he said. “It’s when they multiply that the tree suffers.”

EAB likes to attack the same tree and area over and over again, Kintner said. Once the tree dies, they will move to another.

The pest will fly, traveling up to 2 miles per year. However, Kintner said the biggest factor in its spread is human-assisted movement, with the distribution of EAB-infested products like nursery stock, logs, non-heat treated lumber and bark chips. Firewood is the predominant offender, so people are reminded to “burn where you buy.”

“EAB can  grow at an alarming rate,” Kintner commented. “This bug is on the move, and there’s no stopping it.”

Options available

There are options available for communities and property owners. One is to do nothing, noted another meeting speaker, Donald Lewis, a professor of entomology at Iowa State University.

“For woodland, that’s the practical approach,” he said.

Asche agreed, advising against the mass harvesting and selling of ash trees.

“For yards, ‘nothing’ may not be the best option,” Lewis said, adding, however, that “no one can make you do anything. You get to choose what to do with your trees.”

For those who want to take action, Lewis said they can either remove and replace their ash trees or utilize preventative insecticide treatments on healthy trees.

“They are all valid options,” he remarked.

When considering removal versus treatment, Lewis said one must take several factors into account. Has the tree fully matured (40 years in an urban setting or 75 in a rural setting)? Does the tree have structural problems, or are there distinct site problems, such as erosion or compaction? Could it be a liability to your home, yard or pedestrians? If so, noted Lewis, the tree should probably be removed.

“You need to set sentiment aside,” he said. “Is the tree healthy and vigorous, or is it in decline? If it’s not healthy, then it’s not a good candidate for treatment.”

Once a tree is infected with EAB, Kintner said the “bad” parts can not be pruned off, as it’s already progressed past that point. 

Both Lewis and Asche suggested that property owners with several ash trees past their prime start “thinning out the herd” now, to help manage treatment.

“Decide which ones to treat,” Lewis said. “If you wait for them all to die at once, it could bankrupt you.”

“Try to stay ahead of it financially,” Asche recommended. “If I had a bunch of yard trees, I would try taking out a few to spread the cost out.”

When selecting replacement trees, all three speakers stressed diversification. By planting one species, the trees could be mass impacted by pests and disease, just as the ash have been by EAB.

Asche advised staying away from maples, and instead going with oaks or Kentucky coffeetrees.

For property owners opting for treatment, Lewis said it’s particularly justified if trees are located in a high-risk area, within 15 miles of where EAB has been confirmed. 

“Insecticides can protect and rescue trees,” he said. “We know they work, and they are not incredibly dangerous or expensive.”

He offered several possibilities. If it’s a small tree, less than 20 inches in diameter, you can do it yourself, using a soil drench or granular products. Larger trees will require a certified applicator who will administer a trunk or soil injection or a trunk spray.

Lewis said insecticide is typically applied bi-annually, for the remainder of the tree’s life. It will kill both adults and larvae. April to mid-May is the best time to treat.

“It moves through the wood tissue and into the leaves,” he explained. “You need moist soil to move the treatment, so it’s better to do it when the leaves are growing in the spring. Without the moist soil and leaves, it would not work.”

Trunk injections, which Lewis said are the most popular form of treatment, can cost between $12 and $24 per inch.

“A two-foot tree could cost $240 to $400 every other year to treat,” he shared.

EAB in the future

Lewis said researchers are continuing to look for ways to treat ash trees less often. Kintner said work is also underway to make trees more resistant.

“EAB doesn’t attack in Asia like they do here,” he said. “Here, ash has a certain smell that attracts them. The trees there are more tolerant or resistant.”

Other insects, like a parasitic wasp that targets eggs and larvae, also help keep them in check, Kintner added.

“Last year, we started to bring them in,” he said. “We want to bring their natural enemies here and create more resistant trees. We’ve tried to create a similar environment here.”

As to the future of the ash tree, Asche said he doesn’t foresee them dying off. 

“We won’t lose them, but they will remain smaller in diameter,” he projected. “They will release the seeds,” then the growth cycle will repeat.

To learn more about emerald ash borer, visit or

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