Japanese beetles pose a serious threat to gardens because they voraciously consume the leaves, flowers or fruit of more than 250 different plants. Damage from Japanese beetle amounts to hundreds of millions of dollars annually to the agricultural and ornamental plant industry in the eastern United States. Fortunately, the Japanese beetle has many natural predators.
Many birds will feed on beetle grubs (larvae), and some will attack adult beetles, as well. You can attract insectivorous birds to your yard by putting out birdbaths, nest boxes and feeders, and by turning the soil in late summer or early autumn to expose the grubs. Starlings are the best-known beetle killers; they eat both the grubs and the adult Japanese beetle. Blackbirds, crows, grackles, robins, cardinals, catbirds, sparrows, bobwhites, blue jays, eastern kingbirds, woodpeckers and purple martins also eat beetle grubs, possibly taking adult beetles, as well. Pet chickens and guinea fowl will also eat Japanese beetles.
Moles, skunks, raccoons and shrews will feed on Japanese beetle grubs. Unfortunately, they may dig up your yard to do so.
Ants, spiders and other predatory insects eat beetle eggs in the soil even before they hatch. Assassin bugs, spined soldier bugs, ground beetles, wheel bugs and the larvae of tachinid flies attack the grubs in the soil. You can buy a bucketful of "beneficial nematodes" to add to your soil. Imported from China specifically to control Japanese beetles in the United States, the spring tiphia wasp burrows into the soil to lay its eggs on top of Japanese beetle grubs. The eggs hatch, and the wasps eat the beetle grubs. Likewise, Istocheta aldrichi, a tachinid fly, locates small beetles and glues its eggs on them. These look like common houseflies, but nearly always remain outdoors and feed on nectar and pollen.