Since Japanese Beetles were introduced to the United States, they do not have natural predators in this continent, but there are plenty of animals which have fit them into their regular feeding plans. Things that will attack Japanese Beetles are not limited to the animal world however.
Since Japanese Beetles spend much of their adult life in plants off the ground and subsequently flying from plant to ground and back again, birds such as Purple Martins and other swallows are a perfect fit for eating them. Starlings and Grackles are significant predators of both the adults and the larvae. Meadowlarks, Cardinals and Catbirds are known for targeting the adult beetles as a food source. Other Japanese beetle grub consuming birds include Crows and gulls.
Other Vertebrate Predators
Animals such as skunks, moles, and raccoons are known for their voracious consumption of grubs and other insects. Fortunately for those that want Japanese Beetles to go away, these vertebrates will gobble up Japanese Beetle larva in their path as well.
While spiders, predatory flies, mantids and other predatory invertebrates could eat a Japanese Beetle in its path, they are not known to have a significant effect on adult populations, however, insects such as ants and ground beetles that feed on the immature stages of insects have a more significant impact on Japanese Beetle populations.
Wasps are known for being predatory insects, but in the case of Japanese Beetles, wasps that parasitize other insects are more important. The genus of wasp known as Tiphia is being strategically introduced to target Japanese Beetle populations. Flies (specifically I. Aldrichi) in the Istocheta genus are being introduced for the same reason. The female flies attach an egg on the thorax of the beetles shortly after they emerge. After the egg hatches the next day, it burrows into the body cavity of the beetle to feed.
A bacterium native to the US known as Bacillus popilliae causes milky disease in Japanese Beetle populations. Another bacterium commonly known as BT is also an effective pathogen of Japanese Beetle larva. Other pathogens that affect Japanese Beetles include nematodes in the families Steinernematidae and Heterorhabditidae.