True white grub adults have emerged in central Illinois. These one-inch-long reddish brown to brown June beetles feed at night on the leaves of crabapple, ash, oak, and other trees. Leaf edges being eaten with no observable insects present is a common symptom. Scouting at about 10:30 p.m. will catch these insects in the act. Various chemical insecticides such as carbaryl (Sevin), chlorpyrifos (Dursban), diazinon, and synthetic pyrethroids are effective controls.
Euonymus webworms (also known as euonymus caterpillars) are just appearing on European euonymus (Euonymus europaeus) in northern Illinois. Look for larvae in small webs. The webs increase in size as the larvae feed on the leaves and can become large. These webworms are very pale yellow with black spots and can grow to almost one inch long. They are controlled with Bacillus thuringiensis kurstaki, sold as Dipel, Thuricide, and other trade names, as well as other chemical insecticides.
Cankerworm damage is becoming more common on leaves of many trees throughout the state. Bacillus thuringiensis kurstaki, sold as Dipel, Thuricide, and other trade names, is effective, as are other chemical insecticides.
Honeylocust plant bugs are feeding on expanding honeylocust leaves throughout the state. The bugs are very small right now; the easiest way to find them is to shake a branch over a white piece of paper. If you see a tiny green insect crawling on the paper, take a closer look at it with your hand lens. Plant-bug feeding causes severe leaf distortion, chlorosis, and yellow-brown leaf spots. A heavy infestation may cause premature leaf drop.
(Other contributors to this article: Donna Danielson and Karel Jacobs, The Morton Arboretum)