Japanese beetle adults are emerging in southern Illinois; they will emerge by the end of the month in central Illinois and in early July in northern Illinois. These 1/2-inch, stout, metallic green beetles with coppery wing covers feed on a wide range of plants.
Damage is usually heaviest on linden, birch, willow, crabapple, rose, and grape. If smartweed is in the area, watch it, as Japanese beetle usually feeds on it first. Adults eat through the upper leaf surface and usually through the entire leaf, resulting in brown or lacy, skeletonized leaves. They feed at the top of the plants on the upper side of the leaves in full sun. Thus, early feeding can be overlooked on trees unless you look at the top. Binoculars work well for this.
Adults like to feed where they have fed before. Hand-removal by homeowners, particularly in the first weeks after emergence, can greatly reduce early and later feeding. A jar containing rubbing alcohol or detergent and water can be held under a feeding beetle. Poking at the beetle causes it to fold its legs and drop straight down into the jar, where it will be killed. Harvesting the beetles daily or every other day can greatly reduce damage.
Sprays of carbaryl (Sevin), cyfluthrin (Tempo), or other pyrethroids provide about 2 weeks of control per spray. Because the beetles are present in damaging numbers for about 6 weeks, repeated sprays are needed. With the beetles feeding on so many types of plants, we recommend treating only plants in noticeable locations--trees or shrubs next to building entrances and in landscape entryways; ignore the others. Beetle feeding does not appreciably harm plant health; it is primarily aesthetic damage. By reducing the number of plants treated, you avoid high insecticide use, as well as cost to the client.