Although Japanese beetles (Coleoptera: Scarabaeidae: Popillia japonica) are native to Japan, over the last century they have been accidentally introduced to many other parts of the world. They were first discovered in the United States in 1916 and since then have spread across much of the country.
Part of what has made them so successful in the US is that these insects are generalists and aren’t picky about what they eat. They have over 300 known host plants, leaving no shortage of food for them wherever they go. Various trees, crops, cultivated fruits, and wildflowers are all on the menu for these insects. They will happily eat leaves, flowers, and fruit, depending on what’s available. The individual shown here appeared to be eating pollen from a hairy vetch (Vicia villosa, Fabaceae).
Another factor that has helped Japanese beetles thrive here is that their subterranean larval grubs feed predominantly on the roots of grasses. Between our abundant lawns, parks, and golf courses we have no shortage of grasses here. As of 2002 Japanese beetle larvae were the single most destructive pest of turf grass in the US, with pest management costing about $450 million.
A final factor in the success of Japanese beetles within the US has been the lack of predators. Since Japanese beetles were historically absent from North America, there aren’t really any other insects here that recognize them as food. Because of this, three natural predators were introduced from Japan in the 1920s as biocontrol agents. Two tiphid wasps (Hymenoptera: Tiphiidae: Tiphia vernalis and Tiphia popilliavora) and the winsome fly (Diptera: Tachinidae: Istocheta aldrichi) are parasitoids of these beetles and have had some success in controlling them here.