Extension Educator, Horticulture
They're here–Japanese beetles! If you don't have these gluttonous beetles in your area, consider yourself lucky.
Japanese beetle adults have a 1/2 to 3/4 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 all the leaf tissue and leaving the veins. Adults are most active from 9am–3pm on warm, clear summer days. Feeding is normally in the upper portions of trees. Beetles prefer plants in direct sun, so heavily wooded areas are rarely attacked.
Adults are present until mid August. After mating females lay eggs in turf which 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 commonly recommended to control Japanese beetle grubs. In our area milky spore is generally not recommended since it only controls Japanese beetle grubs and not our predominate lawn grub, 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 such as diazinon will also control Japanese beetle grubs.
However, according to Phil Nixon in the U of I Extension Home, Yard and Garden Pest Newsletter, controlling Japanese beetle grubs does not significantly reduce the number of adult beetles the following year. The beetles are good fliers and easily fly 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. Sevin is toxic to bees and other beneficials. Synthetic pyrethroids can also be effective. The Japanese beetle repellent made from Neem has not been shown to be effective.
Picking them 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. Hold a can containing rubbing alcohol or water with detergent below the infested leaves. The beetles will drop into the container and be killed. Covering with floating row covers can protect prized roses and ripening fruit.
Japanese beetle traps are not recommended. Traps 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.
If you want to see Japanese beetles, go out to our Idea Garden on south Lincoln in Urbana. Because of the public nature of the garden, the tremendous number of beneficial insects, and since the damage is not life threatening for the plants, we have chosen not to spray insecticides for the Japanese beetles. We have chosen to pick them off and to not have a perfect looking garden. In addition we are offering them as souvenirs.