They are the most devastating of insects munching on whole flowers and reducing leaves to mere lacy ghosts.
A variety of techniques can be used for controlling Japanese beetles. For the less squeamish, smacking them with a tennis racket then stomping on them makes for an afternoon of fun.
Or, in the early morning spread a light colored cloth or tarp beneath a rose bush and gently shake the plant. When the groggy pests fall onto the cloth put them in a jar then nuke in the microwave for a couple of minutes or fill the jar with water - they're not good swimmers.
Pheromone traps are a popular method for Japanese beetle control. You can get these from your local nursery.