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Japanese beetles

In Minnesota, Japanese beetles (Popillia japonica) are primarily found in the Minneapolis-St. Paul Metropolitan area, and in some areas in southeast Minnesota. Japanese beetles have an exceptionally large host range, feeding on the leaves of over 300 species of plants, including apples, grapes, blueberries, raspberries, roses and plums.

Identification

Japanese beetles are 3/8 long and oval. Their head and prothorax (the area behind the head) are metallic green, while the wing covers are a shiny bronze. Look for five white patches of hair along each side of their body and two white patches on the tip of their abdomen to verify that they are Japanese beetles.

Damage

japanese-beetle

Japanese beetle

Jeff Hahn, University of Minnesota

Adult Japanese beetles feed on the leaves of many different plants including birches, basswood, apples and raspberries. They feed on the leaves between the veins, so when they are finished, they leave a skeleton of brown fibers where the leaves used to be, a type of damage referred to as skeletonizing. Damage to raspberries can occur where the beetles eat flowers along with the fruit or if the beetles kill so many leaves that the plant is weakened and cannot support fruit.

Important biology

Japanese beetles spend the winter as grubs in the soil of turf grass areas. In spring, they move near the soil surface, where they finish feeding and pupate. They emerge as adults in late June or early July and can fly a long distance to suitable feeding sites. When an adult starts feeding on a shrub, it emits a pheromone that attracts other beetles, so that beetles will aggregate on that shrub. This behavior results in large numbers of beetles feeding on one plant while neighboring plants are left alone. The female releases pheromones to attract males. After mating, adult females lay up to 60 eggs in turf grass. The eggs hatch in about one to two weeks, and the grubs feed on the roots of grasses and other plants. As the soil starts to cool in the fall, the grubs dig deeper in the soil, where they overwinter. Adults primarily feed in July and August, although you can find some activity into the fall. Japanese beetles have one generation per year.

Management

japanese-beetle

Japanese beetle feeding damage

Jeff Hahn, University of Minnesota

For fruit growers, Japanese beetles are best controlled as adults. Physical removal is a viable option for small raspberry patches. Remove the beetles by hand and put them in soapy water. Hand picking is most effective as the beetles first arrive, before they release their aggregation pheromones to attract others. The best time to handpick beetles is in the evening and early morning, when they are less active. Don't use Japanese beetle traps. Research has shown that traps attract more Japanese beetles than they catch, and will typically cause more damage to plants in a garden.

Insecticides can help manage adults. Neem extracts like Azadractin have been shown to provide short term protection, especially if only small to moderate numbers of Japanese beetles are present. There are several contact, residual insecticides including permethrin, bifenthrin, malathion or carbaryl. When using insecticides, always read and follow the label. Be sure the specific product you intend to use is labeled for raspberries


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