It’s been at least 15 years since the emerald ash borer arrived and began to destroy ash trees in the Chicago area. Some homeowners who have been treating ash trees with insecticides to fend off the borer larvae may wonder: Can I stop now?
Sorry. “The borer is still around in the Chicago area,” said Julie Janoski, Plant Clinic manager at The Morton Arboretum in Lisle. “If you have an ash tree you’ve been treating and you want to keep it, you should absolutely keep up with the treatments.”
Green ash, white ash, and black ash were once among the most commonly planted trees in the Chicago area. As native tree species, they were also abundant in Midwestern woodlands. Then came the emerald ash borer. The 2020 Chicago Region Tree Census conducted by the Arboretum found the total population of ash trees in the seven-county region fell from 12.7 million in 2010 to 6.8 million in 2020, a decline of 46%.
Of the nearly 7 million ash trees left in the region, about half are in poor or declining condition. “Still, that’s a food supply,” Janoski said. “It means the borers will be with us for awhile.”
The emerald ash borer is an insect from Asia that was accidentally imported to the United States. It feeds easily on American ash trees that have no natural resistance.
As an adult, the insect is a striking metallic green beetle, but it is at the larval stage that it destroys ash trees. The wormlike larvae chew tunnels through the wood just beneath the bark, severing the vessels that carry water between the roots and the leaves. “Basically, the tree dies by drying out,” she said.
The larval-stage borer turns into an adult beetle that can fly to another tree to lay eggs. In the years since the emerald ash borer was discovered in southeast Michigan and Ontario in 2002, it has spread to 35 states as far west as Colorado, and five Canadian provinces. It was found in Illinois in 2006.
In addition to the mature ash trees that haven’t died yet, the tree census discovered hundreds of thousands of young ash trees in the Chicago region. Many of these saplings have likely sprouted from the stumps of ash trees that died and fell or were cut down. While a stump remains, the tree’s root system may be alive and able to re-sprout.
The borers only attack trees old enough to have trunks about 3 inches in diameter. “Most of the ash sprouts won’t have a chance to grow big enough, but some will,” Janoski said. “As long as stumps keep re-sprouting, they’ll provide food and habitat to support a borer population.”
If you have an ash on your property, the only way to protect it is with an insecticide that enters the tree’s wood and poisons the borers when they feed. Contact the Plant Clinic (mortonarb.org/plant-clinic) to learn about insecticide options.
Treatment is only effective against the borers as long as it is regularly reapplied. It’s most likely to succeed if it is started before a tree is severely damaged.
In an ash tree affected by emerald ash borer, the canopy of leaves will thin out or die back because nutrients and water can’t reach the top branches. Suckers, or new young branches, may sprout from the base of the tree or from lower branches. You may notice the holes of woodpeckers that drill through the bark to dine on the borers.
“We recommend that before you start treating an ash, you have a licensed professional arborist assess the tree to help you decide if it’s worth the ongoing investment,” Janoski said.
If an ash tree dies, have it removed promptly, because the standing tree will quickly become dangerous. “The dried-out wood is brittle and breaks easily,” she said.
Have the stump removed, too. “The fewer ash stumps there are to re-sprout, the less food there will be for emerald ash borers,” Janoski said. Once the stump is gone, “take the opportunity to replace the tree with one of the many beautiful tree species that aren’t susceptible to these borers.”
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or [email protected]). Beth Botts is a staff writer at the Arboretum.