Japanese beetles emerge in late June in Illinois after over-wintering in the ground below the frost line as grubs. In the spring they move upwards feeding on the roots of your lawn and by the summer they are fully grown adults. When they emerge as adults, Japanese beetles feed on the leaves and flowers of over 250 species of plants and your garden is like a Mac Kelly's to them.
Roses are said to be a favorite of Japanese beetles and in my garden in the summer that is where I will usually find one or two having lunch. Fortunately, I'm not much of a rose gardener so I only have a couple of generic roses but I can imagine the frustration of rose lovers when these beetles emerge in the summer. They can eat entire rose petals and even entire rose leaves and shoots when they're young and soft. If you're wondering what is eating leaves on your plants and leaving them looking like skeletons, the culprit is probably a Japanese beetle.