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.
Japanese beetles are 3/8 long and oval. Their head and prothorax (the area behind the head) are a green metallic color, 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.
Adult Japanese beetles feed on the leaves of many different plants including birch, basswood, apple as well as blueberry. They feed on the leaves between the veins, so when they are finished, there is a skeleton of brown fibers where the leaves used to be. Crop losses can occur where the beetles eat fruit along with the leaves or if the beetles damage so many leaves that the plant is weakened. Blueberry plants that have been severely chewed by Japanese beetles will be susceptible to winter injury.
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 huge 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.
For fruit growers, Japanese beetles are best controlled as adults. Physical removal is a viable option for small blueberry 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.
Some growers have been able to minimize damage from Japanese beetles by placing a fabric barrier such as cheesecloth over the plants. Be sure to use a fabric with a mesh less than 1/4 inch. This netting keeps both birds and Japanese beetles off the blueberry plants.
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. Gardeners can also use conventional insecticides including permethrin, bifenthrin, malathion or carbaryl. When using insecticides, always read and follow the directions on the label.