The Homeowners Column

The Homeowners Column

Time to Scout for Japanese Beetles

Photo of Sandra Mason

Sandra Mason
Extension Educator, Horticulture
slmason@illinois.edu

I now talk of my garden in relation to a new season — Japanese beetle season. Let the trumpets sound. It's time to scout for Japanese beetles. 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.

Japanese beetle adults are one quarter to one half inch long 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, linden, grape, raspberry and some 350 different plants. They generally do not feed on dogwood, forsythia, holly, lilac, evergreens and hosta. Plants in the smartweed family such as Persicaria are good indicators for Japanese beetles since they usually find those first.

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 9 a.m. – 3 p.m. on warm, clear summer days. Feeding is normally in the upper portions of plants. 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. However, it only controls Japanese beetle grubs and not our predominate lawn grub, annual white grub. Common lawn grub controls such as halofenozide (GrubEx), imidocloprid (Merit) and beneficial nematodes will control several species of beetle grubs.

Controlling Japanese beetle grubs in your yard does not significantly reduce the number of adult beetles the following year, according to Phil Nixon, U of I Extension entomologist. The beetles are good fliers and easily fly a couple miles in a single flight. They 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.

Generally pesticide sprays of cabaryl (Sevin) to control the adults 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. Move the plant and the beetles will drop into the container and be killed. This is best done in the morning when they fly slower.

Japanese beetle traps are not recommended where a large beetle population exists. It has been shown repeatedly that the use of these traps results in increased plant damage compared to not using the traps.

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.

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.

Although damage looks devastating, Japanese beetle feeding rarely kills woody plants. Therefore, confine control of beetles to plants in important landscape locations or plants of particular value.

After the Japanese beetle season, the plants releaf and reflower and life goes on.

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