Japanese beetle adults typically emerge during the third week of June in southern Illinois, the end of June in central Illinois, and the first week of July in northern Illinois. They are common in a two-county band along the eastern border and in a large area around Collinsville. Every major city in the state has them, but they may be scarce or unknown in rural areas in the western three-quarters of the state.
Adults are rotund, hard-shelled beetles, metallic green, with coppery to brown wing covers. They have tufts of white hair along the bottom edge of the wing covers. They vary from about 3/8 to 1/2 inch long.
The adult beetles eat small holes through leaves, damaging ornamentals. They may eat through the upper epidermis and mesophyll, leaving the lower epidermis intact; the lower leaf surface appears white at first but soon dries and turns brown. Feeding typically starts from the top of a leaf and proceeds downward. Sun-loving, these beetles sit atop a leaf and feed during the day, with damage being most severe on the upper foliage of trees and shrubs. They prefer crabapple, linden, willow, rose, grape, and birch but feed on many other species of broad-leaved trees and shrubs.
Adults are present in large numbers through July and into mid-August. A few can sometimes be found into October, but their numbers are usually not high enough to cause damage after mid-August. During July, frequent matings are observed, including a phenomenon known as “balling.” Males exit the soil and may swarm emerging females. So many males can be trying to mate with a female that they form a clump about the size of a golf ball.
Mated females tunnel into the soil to lay eggs, which hatch into white grubs that feed on turfgrass roots in late July to early August. The C-shaped white grub has six legs and a brown head. It feeds throughout the rest of the summer, causing turf to die back. In the fall, it tunnels deeper into the soil for the winter. It comes back up into the root zone to feed in the spring, pupates in the soil, and emerges as an adult.
Adults tend to feed where others have fed; so control when they first emerge is more effective than later efforts. Even handpicking can be successful if started when the beetles first appear. Beetles that are disturbed either fly or fold their legs and drop from the plant. Holding a jar of soapy water or rubbing alcohol under attacked leaves and poking at the beetles cause most of them to drop into the killing jar.
Sprays of carbaryl (Sevin) or cyfluthrin (Tempo) are effective in killing attacking beetles for 2 to 3 weeks. Because season-long control involves two or three treatments, spray only landscape plants where the damage would be obvious. For example, treat shrubs near the front entrance of a building but avoid spraying trees behind it. This strategy reduces the insecticide used and the client’s cost. (Be sure to consult with the client before allowing some landscape plants to be damaged.) Controlling these beetles has little effect on the health of the plant or the number of beetles next year.
Japanese beetle traps are available with lures that attract beetles from long distances. The lures bring beetles into the trap’s area, but many do not go into the trap. Research has shown these traps increase damage to the landscape with high beetle populations. If you are in an area with relatively few beetles, placing traps at least 50 feet away from treasured plants may provide some protection.
Because the adult beetles fly several miles each summer, treating nondamaging larval populations in the turf does not reduce the damage. Treat turf only if the number of grubs is high enough to cause damage or attracts raccoons, skunks, and birds.