Across the eastern United States, midsummer increasingly finds gardeners woefully seeing favorite crops and flowers being decimated by hungry Japanese beetles. First introduced into the United States accidentally in 1916 in New Jersey, these small bugs never caused trouble on vegetation in their native Japan. Grow plant species or varieties that tend to not satisfy their hungry curiosities.
Japanese beetles are 7/16 inch long, about the size of an index finger's nail, metallic green-black beetles with copper-brown wing covers. They may glisten in sunlight, and the wing covers can sometimes look more like rusty-orange smears of nail polish.
Adult Japanese beetles emerge from the ground and begin feeding on plants in June across North America, for a period of about 4 to 6 weeks. These adults live for about 30 to 45 days, during which they eat, mate and lay eggs in the soil. Female beetles leave plants in the afternoon and burrow about 2 to 3 inches into the ground in grassy areas like lawns; they lay around 50 eggs over their lifetime. The eggs hatch and live statically as white larvae eating grass roots. By late summer they are plump white grubs about 1 inch in length. The following late spring, the white larvae pupate and then shortly later emerge as hungry, sexually mature adults.
Japanese beetles munch on about 300 different plants, eating leaves, flowers and soft or damaged fruits. They usually feed in groups, starting at the top of a plant and working downward. A single beetle doesn't do much damage. The beetles are most active on warm, sunny days and migrate to plants situated in direct sunlight. The University of Kentucky mentions that odors emitted from plants with damaged leaves tends to lure groupings of beetle on many plants. The damage centers on chewed-out areas in between leaf veins, creating a lacy silhouette. Moreover, adult Japanese beetles are highly mobile and infest new areas from several miles away. They make only short flights as they move about to feed or lay eggs.
Hand-pick small numbers of the pests and crush them or drown them in soapy water. Chemicals kill Japanese beetles, but also destroy populations of beneficial insects at the same time. The pyrethroid-based pesticide products generally provide 2 to 3 weeks' protection of plant foliage, while those with carbaryl provide 1 to 2 weeks' protection. Organic pesticides like neem oil and pyrethrin afford 3 to 4 weeks of protection. Insecticidal soap, extracts of garlic, hot pepper, or orange peels, and companion planting, however, are generally ineffective, according to the University of Kentucky.
Japanese beetle traps act to lure the pests and can increase the physical numbers of insects to your garden. If you choose to use traps, place them far away from the desirable plants you want protected so incoming flying beetles flock to the trap, rather than finding your plants. Controlling the white grubs in lawns does not diminish adult beetle problems since they fly in from good distances when they sense their favored foliage plants.
Some ornamental plants common in gardens tend to attract the destruction of Japanese beetles, so avoid growing them. A few examples include Japanese maples, hollyhocks, rose-of-Sharon, roses, grapes, basswoods and crab apples. Some varieties of these species do not taste as good to the insects and make good choices. Interestingly, most evergreen trees and shrubs, whether with needles, scaled or broadleaf foliage, do not get eaten by Japanese beetles.