Extension Educator, Horticulture
Recent invasions of Japanese beetles have caused a flurry of phone calls into our office. 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.
Also characteristic of Japanese beetles is their 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. Their favorite plants are in the rose family including rose and crabapple. 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 early to mid-August after mating females lay eggs in turf which hatch into grubs in August. Grubs feed on plant and turfgrass roots until cold weather drives them deeper into the soil. Adults emerge in late May or early June of the following year.
Control can be difficult. New migrations of beetles occur daily. Generally pesticide sprays of cabaryl sold as Sevin can reduce damage for two to several days, but several applications are required to maintain control. Some home gardeners find picking them off by hand every couple of days can 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. Prized roses and ripening fruit can be protected by covering with floating row covers.
Japanese beetle traps are not recommended. University of Kentucky studies have shown that if only one or two traps are used in a garden as few as 54 percent of the beetles are captured and there is a net increase in the beetles in the area around the traps. This is true even though in the study a single trap often captured as many as 20,000 adult beetles in a single day. Traps can actually increase damage. Traps have been shown to be only effective in community-wide efforts.
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 controls 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 in from other areas.
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.
The good news is the Japanese beetle attacks on ornamental plants rarely seriously harm the health of the attacked plants although damage looks devastating. Confine control of beetles on shrubs and small trees near main building entrances and other important landscape locations where damage is obvious.