Be on the lookout for little, shiny green beetles feeding on your plants. Japanese beetles are just beginning to get a foothold in the Lawrence area, although the insects are common in the eastern half of the United States.
Japanese beetles are closely related to May beetles/June bugs. The major identifying characteristics are their metallic green bodies and coppery wing covers. Japanese beetles also have white tufts of hair on the ends of their abdomens if you can get close enough to see them. The adult beetles are three-eighths to one-half inch long.
Green June beetles are often confused for Japanese beetles but are larger and shinier. These big bumbling cousins prefer overripe fruit and sweet corn and are rarely found on other plants. Japanese beetles, on the other hand, feed on just about any crop or landscape plant.
Another way to identify Japanese beetles is by their feeding habits. Damage usually starts at the top of the plant as the beetles begin feeding on the upper leaf surfaces. Sometimes the beetles eat holes through the leaves, but usually completely defoliate the plant as they work their way down the stem.
If you find Japanese beetles feeding on your plants, the easiest control is to hand-pick the insects. Fill a jar at least a few inches deep with rubbing alcohol or soapy water. Pick the beetles from the plants and drop them into the jar, or gently knock them off of the plant while holding the jar below to catch them.
For heavy infestations, an insecticide could be used. However, the two insecticides that are most effective on Japanese beetles are also highly toxic to the natural predators of other common insect and mite pests. That means that spraying your Japanese beetles could lead to a spider mite problem or other insect problem. The two insecticides (cyfluthrin and carbaryl) may also have to be re-applied since they are only effective for about two weeks and Japanese beetle adults typically feed for about six weeks.
Azadirachtin is a botanical insecticide that is labeled as a repellent for Japanese beetles but also has the potential to kill natural predatory insects. Part of what makes this product safer than conventional insecticides is a shorter residual period, but that may mean more applications for effective control. Researchers at Kansas State University indicate that azadirachtin is most effective when Japanese beetle populations are high.
Traps are sometimes recommended for control of Japanese beetles, but newer research indicates that the traps may actually attract more insects to feed in an area instead of protecting the plants. Traps work better as a monitoring device to help commercial growers know when the beetles begin feeding each summer.
The situation gets worse: Japanese beetle larvae (grubs) feed under the ground on turfgrass roots. Female beetles lay eggs in the soil in late July and August. As the eggs hatch in late summer and fall, the grubs begin feeding on turf roots and continue feeding throughout the winter.
Adult beetles fly up to three miles, so controlling adults may not affect grub populations.
Several biological controls, biological insecticides, and conventional insecticides are labeled for Japanese beetle grub control in turf. Research regarding biological controls such as parasitic nematodes, milky spore, and other parasites shows conflicting results. Also, control is unnecessary until grubs number 10 or more per square foot. You can check for this by digging a one foot square of grass from your lawn and counting the number of grubs in the soil.
Green June beetles are best controlled by hand-picking or using bird net over desirable plants. If you choose to use an insecticide, read the label to determine how long you must wait to harvest fruit after the application. Since Green June beetles tend to fly slowly, some homeowners also report controlling them by swatting them with tennis rackets.
If you suspect Japanese beetles on your fruits, vegetables, landscape plants, or other crops, they can be brought into the Douglas County Extension office for positive identification.
— Jennifer Smith is the Horticulture Extension Agent for K-State Research and Extension in Douglas County and can be reached at 843-7058.