What to do? The voracious Japanese beetle is back

By John Fulton         

[JUNE 27, 2006]  After three years of heavy numbers in the Mount Pulaski, Chestnut and Latham areas, the rest of Logan County is becoming acquainted with the Japanese beetle. Japanese beetle adults have a one-half to three-fourths-inch long body with copper-colored wing covers and a shiny metallic green head. A key characteristic is prominent white tufts of hair along their sides.

They also 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. The life cycle is similar to a June bug. After mating, females lay eggs in turf. The eggs hatch into grubs in August. Grubs feed on plant roots until cold weather drives than deeper into the soil. Adults emerge in summer of the following year.

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 and not our predominant lawn grub, the annual white grub. Also Japanese beetle 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 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 off beetles 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. Covering plants with floating row covers can protect prized roses and ripening fruit. 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 shrubs and small trees near main building entrances and other important landscape locations where damage is obvious. Protecting a prize rose bush or a newly transplanted linden tree is a good idea. The U of I Arboretum decided not to treat anything two years ago and didn't lose any plants.

[John Fulton, unit leader, University of Illinois Extension, Logan County Unit]