The best way to control Japanese beetles on roses is with a product called neem oil, according to David Grist with Gardener's Supply Company. Neem oil not only controls Japanese beetles but also a long list of other soft-bodied insects, such as aphids, earwigs and mole crickets.
With the range of insects that neem oil controls, it is the best solution for keeping a garden of roses safe. Many individuals use products such as Milky Spore or Grub Guard, but depending on the size of the garden, one could still get an infestation of Japanese beetles while using these.
Japanese beetles move slowly, but they have been shown to fly a mile for a source of food. Neem oil helps control these outbreaks of Japanese beetles, and rids them from the garden for good, instead of temporarily. Neem oil is used in combination with another product called Rose Rx, which helps prevent against diseases that often plague roses. Examples of these types of diseases are black spot, powdery mildew, rust, scab and anthracnose. With neem oil and Rose Rx, these diseases are often avoided by avid rose gardeners. Other products such as pyrethrin-based insecticides also work well to control Japanese beetles.