Knockout roses, cherry trees, and many other trees and shrubs are very susceptible to Japanese beetles and also aphids. The best thing to do to prevent any insect damage is to apply a systemic insecticide early in the spring. a systemic insecticide will be absorbed by the root system of the plant and prevent any insect damage. This has to be done early in the spring because it will take time for the product to get absorbed into the root system and up through the plant.
If you were unable to get a systemic insecticide applied in the spring, you can spray a contact insecticide product like Sevin.
Do not apply any systemic insecticides to any edible plants or fruits.