I live in the country so fighting damaging insects is never ending. Because I try to use "natural" ways of getting rid of them, I have found going out early in the morning, while the beetles are sluggish, is best. They are clustered on my rugosa rose buds, I hold a zip lock bag under the buds, gently shake the buds, and the beetles drop inside the bag. When I have visited all the bushes, I zip the bag, lay it in the sun, and do it again the next day. When it is full or just plain icky, I throw in the trash and get a new bag. If I wait till later in the day, they are more likely to fly when you shake the bush. I've never totally eliminated the pests but have made a big enough dent in the population to allow my roses time to bloom. They seem to like the rugosa roses the best.
I do not grow roses as I am very allergic to them. My father on the other hand has several 'Rose of Sharons' and uses SD-40 on the Japanese beetles and it kills them and doesn't harm the plant. Just a thought if you don't like pesticides.