In between beach days and backyard barbecues, the end of summer is a good time to examine your trees for signs of hazardous invasive insects.
The USDA has named August as Tree Check Month to encourage homeowners to look for and report new infestations of the Asian longhorned beetle, an invasive wood-boring insect that attacks maples, elms, horse chestnuts, birches, willows and other hardwood trees. “Ordinary people who spot invasive bugs are important allies of the scientists and arborists who work to control them,” said Tricia Bethke, forest pest outreach coordinator at The Morton Arboretum in Lisle. “Spotting the insects and their damage early means saving more trees.”
Here are some threats to have in mind as you look over your trees. If you see troubling signs, take pictures and contact the Arboretum’s Plant Clinic (mortonarb.org/plant-clinic).
Asian longhorned beetle (ALB): Although this Asian wood-boring insect was eradicated in Chicago in 1998, it’s still a worry in Illinois. “ALB could come back,” Bethke said. “It’s out there in other states, and our communities have plenty of the kinds of trees it prefers.” The larvae bore through the wood during the cooler months, often leading to serious damage and eventually killing the trees. Adult beetles — about 1 ¼ inch long, with black, white-spotted bodies and long, waving antennae — are most active in summer. Indications that the insect is present can be seen year-round. Look for deep, perfectly round holes in the bark, about 3/8 to ½ inch wide, where adult beetles have tunneled out of the wood. Shallow holes the size of a quarter, often with weeping sap, may be places where the beetles laid eggs. The tunnels made by the larvae are hidden inside the tree, but after pruning, you may discover them in the cut surface of the wood. For more information, see mortonarb.org/ALB.
Elm zigzag sawfly (EZS): “This is a new invader from Asia that could be a major threat to all our elms,” Bethke said. Trees that may be affected include the diminishing number of native American elms that have survived the Dutch elm disease (DED); native slippery elms; and Asian species and newer elm hybrids that have been selected to resist DED. The larvae chew long zigzag trails through elm leaves. They can consume all the leaves from a tree, although scientists are still studying how severely this threatens the trees’ lives. The adult beetles, less than ¼ inch long, are dark brown with yellow legs. EZS is well-established in Europe, and it was found in Cook County in 2024. “In one year, the beetle can fly 30 to 50 miles and produce six generations, so this insect has the potential to spread rapidly,” Bethke said. For more information, see tinyurl.com/yc8py5ar.
Spotted lanternfly: Potentially a major agricultural pest, spotted lanternfly prefers tree of heaven (Ailanthus) but will also suck sap from more than 70 other species of trees and plants. It may weaken young trees. Often mistaken for a moth, the roughly 1-inch-long adult insect has pale gray outer wings with black spots; when it opens its wings, it shows a set that is spotted with red. Adults will die in fall, but the egg masses they leave behind on tree bark and other surfaces can overwinter and hatch in spring. For more information, see mortonarb.org/lanternfly.
Spongy moth: The caterpillars of this insect (formerly known as gypsy moth) are voracious eaters and can strip all the leaves from hundreds of tree species, including oak, maple, apple, crabapple, hickory, birch, pine, spruce and willow. They have mostly hatched into adults by this time of the year. In late summer, fall and winter, you are more likely to see the egg masses on the bark of trees or on other surfaces, such as lawn furniture, siding, sculptures and cars. They are tan or light brown and 1 to 2 inches across, with a spongy, fuzzy texture. Scrape them off and drop them in a bucket of soapy water to kill them. For more information, see mortonarb.org/spongy-moth.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.
