Japanese beetle control

By John Fulton

[July 11, 2013]  A few beetles have shown up in the area. They were about two weeks later than last year. Japanese beetle adults have a shiny, metallic green head and a body that is one-half to three-fourths-inch long with copper-colored wing covers. A key characteristic is prominent white tufts of hair along their sides.

These beetles have an overwhelming appetite for your favorite rose. Adults feed in herds on many deciduous trees, shrubs and vines such as linden, Japanese maple, sycamore, birch, elm and grape. They generally do not feed on dogwood, forsythia, holly and lilac. Japanese beetle adults feed on flowers and fruits and skeletonize leaves by eating the leaf tissue between the veins. Feeding is normally in the upper portions of trees. Beetles prefer plants in direct sun, so heavily wooded areas are rarely attacked.

Adults can be with us until mid-August, or a few weeks later this year due to the later start. The life cycle is similar to a June bug, only it runs a few weeks later. After mating, females lay eggs in turf, and the eggs hatch into grubs in August. Grubs feed on plant roots until cold weather drives them deeper into the soil. Adults emerge in summer of the following year.

One reason for potentially low beetle numbers is the extremely dry soil conditions we had last year. Of course, all bets are off if you watered your lawn and the neighbors didn't.

The bacterial control, milky spore sold as Doom or Grub Attack, is frequently recommended to control Japanese beetle grubs. In our area milky spore is generally not recommended, since it controls only Japanese beetle grubs. Also these grubs must already be infesting the turf for milky spore to work effectively. Pesticides commonly used for lawn grub control will also control Japanese beetle grubs.

Controlling Japanese beetle grubs does not significantly reduce the number of adult beetles the following year. The beetles are good fliers and easily travel a couple of miles in a single flight. Evidence suggests that adult beetles are attracted to previously damaged leaves. Therefore reducing feeding damage now can result in less feeding damage in the future.

Generally, pesticide sprays of cabaryl, sold as Sevin, can reduce damage for up to two weeks, but four to seven days is more likely. Sevin is toxic to bees. Synthetic pyrethroids can also be effective, but they tend to break down quickly with extreme heat. These would include permethrin and bifenthrin. The Japanese beetle repellent made from Neem has not been shown to be effective.

Picking beetles off by hand every couple of days may be just as effective as spraying. When disturbed, the beetles fold their legs and drop to the ground.

Japanese beetle traps are not recommended since they can actually increase damage by attracting more than they kill.

A number of birds such as grackles, cardinals and meadowlarks feed on adult beetles. Two native predator insects and a couple of introduced parasites may help to keep Japanese beetle populations in check. Protect natural enemies by keeping the use of conventional pesticides to a minimum.

Although damage looks devastating, Japanese beetle feeding rarely kills plants. Therefore, confine control of beetles to small fruits, garden produce, shrubs and small trees near main building entrances and other important landscape locations where damage is obvious. Protecting a newly transplanted linden tree is a good idea. Covering plants with floating row covers can protect ripening fruit and prized roses.

[By JOHN FULTON, University of Illinois Extension]