The Japanese beetle is native to Japan and was accidentally introduced into the United States in 1916. Since that time it has slowly spread from southern Maine to Georgia, and westward from Missouri to Minnesota.
Japanese beetle adults are half-inch-long, shiny, metallic green, oval insects. They have coppery-brown wing covers with five tufts of white hairs along the sides of their bodies. The larvae of Japanese beetles are white grubs with three pairs of jointed legs and a yellow-brown head. They assume the typical C-shaped position in the soil as other grubs.
The Japanese beetle has a one-year life cycle but spends most of its life in the soil as a grub. In spring, the over-wintering, partially grown 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 and other grassy areas 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. As the soil temperatures warm in the spring, the grubs return to the surface to complete their life cycle.
Both the adult and larval stages of the Japanese beetle can be quite destructive. The adult beetles feed on over 300 species of ornamental plants with roses, lindens, crabapples, and grapes being preferred hosts. Typically, adult beetles feed in large groups on the upper leaf surface, leaving only a lace-like skeleton of veins. They can also devour 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. Trees and shrubs rarely attacked by Japanese beetles include red and silver maples, boxwood, euonymus, juniper, arborvitae, magnolia, ash, hydrangea, spruce, and yew.
The Japanese beetle grubs feed below ground on cool-season 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. If 10 to 12 grubs exist within one square foot, treatment is warranted.
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 will not protect plants from damage, and may in fact attract more beetles into the area. Tests have revealed that Neem products containing the ingredient azadirachtin are effective as repellents, potentially reducing defoliation. Neem products are different than neem oil. Look for the active ingredient azadirachtin on the label.
For grubs, because eggs and young grubs cannot survive in relatively dry soils, do not irrigate during beetle activity to help reduce grub populations. The bacterial milky spore disease, Bacillus popillae, kills grubs in the soil. The dead grubs then produce bacterial spores that remain in the soil to infect future grubs. In southern states, milky spore disease takes 2-3 years before spore counts build to become effective. In northern states, the bacterium does not always survive the winter and is not considered an effective tool.
Beneficial nematodes can be watered into 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 recommended by the University of Illinois. Beneficial nematodes are not always available in stores; they are available through mail order/internet sources.
For adult Japanese Beetles: 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 a pesticide treat when the beetles are present.
For grubs: There are a number of different grub control products and they vary in their time of application. Be sure to read the label of the product so that it is applied at the right time. Contact the Plant Clinic (630-719-2424 or email@example.com) for current recommendations. 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.