University of Illinois Extension

Japanese Beetles

“When choosing plants for your yard and garden, it can be helpful to avoid favorites of the Japanese beetle,” says Susan Grupp, a University of Illinois Extension horticulture educator. “If you don’t, you might be challenged with the problem of Japanese beetles every year.”

Japanese beetles are brightly colored insects that appear in gardens and landscapes in early summer. They have metallic green bodies, copper-colored wing covers, and tufts of white hairs. There are five tufts found along each edge of their abdomen and one more pair of tufts found at the tip of the abdomen.

“These white tufts help separate them from other similar beetles,” explained Grupp. “The adult beetle is a little less than one-half inch long and the males are a little smaller than the females.”

Eighty-one years ago, the beetles made their first appearance in the United States with their discovery in New Jersey. Since then, they have spread westward and become a serious and destructive pest.

Photo of Japanese beetle“As a grub, they feed on plant roots--primarily grasses,” she said. “As an adult, they feed on leaves and flowers of over 250 plant species.”

Japanese beetles over-winter as partially grown grubs in the soil, below the frost line and move back upward to feed on roots in the spring. They pupate near the soil surface and beginning in late June in Illinois, emerge as adult beetles.

“Then, for the next 30-45 days, beetles will feed on plants, though their favorite host seems to be rose,” she noted. “Other plants they love are flowering crabapple, flowering cherry, Japanese maple, linden, elm, viburnum, Virginia creeper, grapes, and many others.”

During this time of feeding, mating takes place. A single female can lay 40-60 eggs. Eggs will hatch in about 14 days and young grubs begin feeding on roots.

Grupp said that Japanese beetles are often found in groups. “Research has shown they release a special pheromone that attracts more males and females to feed and find mates,” she said. “Also, plants that have been fed upon may release volatile odors that can attract even more beetles.”

The beetles are most active during the warmest part of the day. Adults live for about 30-45 days, though some beetles may be found as late as September.

“Beetles feed on leaf tissue with the veins left behind, creating a skeletonized look,” she said. “They can devour entire rose petals, though, and even entire leaves if soft and thin.”

A great amount of research has been conducted by the USDA and state experiment stations to develop biological control options for Japanese beetles.

“A disease-causing bacterium called milky spore disease is produced and sold,” Grupp said. “This control is directed to the grub stage and may take several years to build up in the soil to be effective.

“It is important to note that directing control at the grub stage does not necessarily equate to reduced numbers of adults in your landscape. These beetles can fly in from neighboring yards, even from several miles away. Also, milky spore disease does not control the annual white grub that is found with Japanese beetle grubs in lawns.”

Other naturally occurring organisms can help reduce grub populations. These include parasitic wasps, toads, and birds, along with fungus diseases and nematodes. Some animals, such as moles and skunks, eat a lot of grubs. The nematode heterorhabditis bacteriophora can be effective against Japanese beetle grubs and is available at garden centers and through mail order catalogs.

“Japanese beetle traps are not recommended,” Grupp said. “Research at the University of Kentucky has shown these traps attract many more beetles to your yard--more than if you had not used them in the first place. As the beetles are approaching the trap, they stop to feed on plants and some never even make it to the trap.”

So what is the home gardener to do? “For prized landscape plants, begin scouting for beetles in late June,” said Grupp. “Be especially diligent on warm, sunny days with little wind. When you find them, just knock them off the plant into a pail of soapy water or alcohol. Interestingly, when you touch them, they just give up and fall.

“If you can keep feeding damage to a minimum when they first arrive, it is likely you will have less damage overall.”

For trees and large shrubs, this approach may not be practical. A decision has to be made as to whether or not an insecticide treatment is warranted.

“There are insecticides labeled for Japanese beetles,” she said. “Since recommendations can change from year to year, consult your local University of Illinois Extension office for the names. Always read the label and follow directions. Take special note of any techniques listed that explain how to protect beneficial insects, especially honeybees, from spray applications.”