When I had a half acre lot in NJ, with six traps I was able to trap six quarts of Japanese beetles in half a day on peak days. Not fun disposing of them... I think if I had a larger property I would trap at the same rate of 12 traps per acre but I'd probably put most of the traps near the perimeter of the property so that any JBs that came onto the property stopped near the edge. My belief was (based on only a few very uncontrolled experiments) that JB's decomposed slowly, but made for great fertilizer.
(I still have fond memories of walking through the garden beating all my roses, vines, and shrubs with a stick to scare the beetles off the plants in hopes they would go into the traps. Ahh .. those were the days.)
Since they look so much alike, and their life cycles are so similar, I'm guessing that some of the same ideas will work for rose chafers.
On second thought, it might make sense to place the traps near areas where roses are being grown, but not directly in the same beds.
