Japanese beetles are one of the exotic insects that have made their way into the United States. They damage plants due to the lack of a natural predator.
Adult Japanese beetles are between 1/4 and 1/2 inch long. They're easily identified because of their metallic green body with coppery wings. On either side of the wings are five tufts of white hairs.
"Like many perennial flowers, Japanese beetles are three to four weeks late this year," says David Robson, University of Illinois Extension horticulture educator.
Adult Japanese beetles are voracious feeders, especially on sunny days, and seemingly indiscriminate. Like most insects, they have their favorite food but will feed on anything present.
"Roses, grape vines and linden trees seem to be the favorites in Illinois," says Robson. "The Japanese beetles do not discriminate—all types of roses and lindens are fair game, even the supposedly insect- and disease-proof shrub roses. You can find the insects on viburnums, climbing hydrangeas, Shasta daisies, daylilies, porcelain vine and some fruit trees."
Generally, the beetle feeds on the tender upper leaf tissue between the veins, creating a skeletonized look to the leaf. In many cases, what's left turns brown, giving the plants a scorched appearance.
Vegetables tend to be one of the last things the beetles will eat. Most summer Illinois fruits, such as apples, peaches and pears, have a skin that is harder for beetles to digest.
"Adults can be controlled by knocking into containers of hot, soapy water," says Robson. "These efforts will probably need to be repeated daily for several weeks."
It is not recommended that gardeners use petroleum products such as gasoline to knock the beetles into. While gasoline or diesel fuel may work, disposal of the dead beetles and liquid becomes a problem.
"Japanese beetle traps have shown effectiveness, but reports indicate that they actually attract more beetles into the yard that may not be caught by the traps," says Robson. "The current recommendation is to not use traps at all, or to place them at least a city block or more from your yard."
There are several insecticides that can be used as knockdown sprays. The insecticide carbaryl, often sold as Sevin, provides some of the best control. More than likely, you will have to repeat applications every five to seven days. Other products include acephate (Orthene) and cyfluthrin (Tempo). Imidacloprid (Merit) is often sold as Japanese beetle control, but it is a systemic, which means it must be applied early in the season, usually early May, to allow the plant to absorb it.
Be sure to read and follow directions, locating "Japanese Beetle" control on the label.
Grubs can be controlled with imidacloprid (Merit) or Milky Spore disease, a bacterial toxin that will only have an effect on the Japanese beetle grub.
"Above all, maintain the health of the plant," advises Robson. "Plants may grow less and have a more difficult time recovering from stress conditions such as drought, early frosts, diseases and other insect attacks."
Plants will often recover and appear fine next year, living on stored food reserves. But, repeated defoliation in early summer will weaken many trees, shrubs and vines.
For more information, contact your local U of I Extension office for the Japanese Beetles fact sheet. Or download it at http://web.extension.uiuc.edu/regions/sifamily. Click on "Around the House" and then "Tip of the Month."
Source: David J. Robson, Extension Educator, Horticulture, drobson
