Japanese beetle damage and control

By Jennifer Fishburn, University of Illinois Extension

[June 11, 2012]  Japanese beetle adults may be munching on roses, lindens, raspberries, birch trees, crabapple and apple trees in your neighborhood. The beetles are voracious foliage and fruit feeders on nearly 300 species of plants. Feeding on plants generally lasts for about six weeks.

The Japanese beetle is approximately a half-inch long and is metallic green with coppery wing covers. The beetles chew the leaf tissue between the veins, leaving a skeletonized leaf. Adults most actively feed from 9 a.m. to 3 p.m. on warm, sunny days. Normally they start feeding on the upper portions of plants and work downward. Japanese beetles prefer plants exposed to direct sunlight.

Since they are an introduced species, Japanese beetles don’t have any natural predators in the United States.

Damage to trees and shrubs is considered to be primarily aesthetic. According to Phil Nixon, Extension entomologist, "even heavily attacked trees and shrubs rarely exhibit severe dieback, because the beetles attack after the bulk of food production has already occurred in the leaves." "Photosynthetic production primarily occurs early in the season when the leaves are still soft and pliable. Japanese beetle defoliation occurs later in the growing season," he explained. "This allows one to selectively treat those trees and shrubs in very obvious landscape locations and to ignore the damage on others."

There are several control options for Japanese beetles.

When the adult beetles change hosts, they prefer foliage previously damaged by other Japanese beetles, so early hand-removal of the beetles is effective. In the late afternoon and evening, disturbed beetles fold their legs and drop to the ground. By holding a container of rubbing alcohol (isopropyl alcohol) or soapy water under beetles and poking at them, one can easily collect a pint or so in less than an hour. If this is done every day or two for the first couple of weeks after the beetles emerge, subsequent damage through the summer is reduced. Although labor-intensive, this is a viable option.

Netting is used to provide complete protection. Rosarians protect prize individual buds and blooms or even entire plants with netting. Backyard blueberry growers use netting as well. Shade cloth with a high light transmittance, spun-bound polyester row covers, netting sold in fabric stores, window screening and other meshes all work well.

Insecticides provide effective control of adult Japanese beetles. Heavily attacked plants can be sprayed with carbaryl (Sevin), cyfluthrin and pyrethroid. Always read and follow label directions for safe use of pesticides. Sevin is toxic to bees and other beneficial insects and should be sprayed in the evening. Protect natural enemies such as birds and predator insects by keeping the use of conventional pesticides to a minimum. Spray only plants where damage is very noticeable or food crops that are under attack. Plants in less obvious parts of the landscape and large trees can go untreated.

Pheromone traps are available that contain an externally produced hormonelike chemical attractive to male Japanese beetles and a floral lure attractive to female beetles. The pheromone traps are useful for detecting beetle emergence but are not recommended as a control. Research shows that beetles are attracted from a considerable distance to areas near the traps, but then switch their seeking behavior to food plants, resulting in heavier plant damage near traps. Even though the traps catch large numbers of beetles, their use is not recommended.

[By JENNIFER FISHBURN, horticulture educator, University of Illinois Extension, Logan-Menard-Sangamon Unit]