Typically, adult beetles feed in groups on the upper leaf surface, leaving only a lace-like skeleton of veins. They can also seriously damage flowers and ripening fruit, and if large populations are present, they can defoliate a large tree. Feeding damage caused by beetles usually results in leaves turning brown, dying, and eventually falling off. Despite the amount of damage done, trees and shrubs generally are not killed. Because the loss of the leaves comes late in the season, long-term damage is minimal. By the time the damage begins, the leaves have produced most of the plant’s food for the growing season.

Japanese beetle grubs feed below ground on turf grass roots. Large areas of lawn can be destroyed in a relatively short period of time by grubs or digging animals (skunks, birds, and raccoons) that feed on grubs. First evidence of injury by grubs is a localized patch of pale, discolored, and dying turf grass, symptoms similar to drought stress. As grubs expand their feeding range, the small damaged areas enlarge and turf can easily be lifted and rolled back like carpet to reveal the grubs. The grubs will be found where the brown, damaged area meets the green normal part of the lawn, rather than in the center of the brown area. If 10 to 12 grubs exist within one square foot, treatment is warranted.

The Japanese beetle has a one-year life cycle, spending most of its life in the soil as a grub. In spring, the overwintering grubs move upward in the soil. From mid-May to June, the young larvae pupate. After pupating, adults begin to emerge from late-June to August. The female Japanese beetles release a pheromone that attracts males, causing them to congregate in groups. Mating and egg laying begins soon after emergence. Japanese beetle adults feed during the day on a wide variety of low-growing plants and later fly to tree leaves. Adults typically live for 30 to 45 days. Once mated, females lay eggs in the lawn just under the soil surface. Eggs soon hatch and young larvae begin to feed on roots of nearby grass and other plants until cold weather forces them to move deeper into the soil for the winter. During the winter months, they remain several inches deep in the soil.

Cultural management: 

For adult Japanese beetles, handpicking the beetles off isolated plants or knocking them into jars of soapy water will reduce populations. Commercial Japanese beetle traps and pheromone lures are available, however, research has shown that the use of traps may attract more beetles into the area. Eggs and young grubs cannot survive in relatively dry soils,so avoid irrigating lawns during beetle activity to help reduce grub populations.

Biological management:

Beneficial nematodes can be applied to turf starting in late July, where they infest and kill grubs. The soil needs to remain moist to keep the nematodes alive.  Nematodes may die if applied during a time of high temperatures. Products containing Heterorhabditis bacteriophora nematodes are commercially available, but are not always available in stores. They are available through mail order or internet sources.

The bacterial product, milky spore disease, (Paenibacillus popilliae, formerly Bacillus popillae), when applied to the soil can kill grubs.The bacterium reproduces in the grub. The dead grubs then decompose and release the bacterial spores. These spores will remain in the soil to infect future grubs. In southern states, milky spore disease takes two to three years before spore counts build up enough to become effective. In northern states, the bacterium does not always survive the winter and is not considered an effective tool. This product only kills grubs of Japanese beetles. Grubs of other species are not affected.

Another bacterial product, Bacillus thuringiensis galleriae, can be applied to the soil to kill beetle grubs, including those of Japanese beetles. The grubs ingest the product and the bacterium damages the grubs internally.

Chemical management:

Feeding injury from adult beetles rarely causes death of a plant. Avoid unnecessary spraying by protecting only highly valued or aesthetically visible plants from feeding injury. When using an insecticide, treat when the beetles are present. Insecticides are not always very effective since the beetle can feed on so many different species of plants.

Neem products containing the ingredient azadirachtin are effective as repellents, potentially reducing defoliation. Neem products are different from neem oil. Look for the active ingredient azadirachtin on the label.

There are a number of different grub control products and they vary in their active ingredient and their time of application. Be sure to read the label of the product so that it is applied at the right time. Use pesticides safely and wisely; read and follow label directions.

The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.

For current pesticide recommendations, contact The Morton Arboretum Plant Clinic (630-719-2424 or plantclinic@mortonarb.org).