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The Homeowners Column
Controlling Japanese Beetles
Extension Educator, Horticulture
They're here ... Japanese beetles. Since early control is important, a few tips are worth repeating.
Japanese beetle adults have a 1/2 to 3/4 inch long body with copper colored wing covers and a shiny metallic green head. Kind of attractive in a bugly sort of way. A key characteristic is prominent white tufts of hair along their sides.
They also have the munchies 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 them 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. Common lawn grub controls such as halofenozide (GrubEx) and beneficial nematodes will 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. Do the math, and beetles may travel 10 to 15 miles from where they lived as larvae. Typically, one-third of the adult Japanese beetles fly to a new host each day.
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 (Sevin) can reduce damage for up to two weeks. However, Sevin is toxic to bees and other beneficials. Synthetic pyrethroids can also be effective. Informally, the repellent Neem has not been shown to be effective.
Picking them off by hand every morning 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 soapy water below the infested leaves. The beetles will drop into the container and be killed.
Japanese beetle traps are not recommended. It has been shown repeatedly that the use of these traps where a large beetle population exists results in increased plant damage compared to not using the traps. I think people use traps just for the mere satisfaction of seeing beetles trapped in the bags.
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.
At our Idea Garden on south Lincoln in Urbana we are trying several methods of control including floating row covers over the fruits, Pyola sprays (combination of canola oil and pyrethrum) and hand collecting.