Stop the beetles
Japanese beetles feed on almost 300 plant species, but roses are one of their favorites, as you've seen.
The least toxic way of handling them is to go out in the morning or evening and brush them into a container of soapy water. When disturbed, they drop straight down before flying, and are drowned in the soapy water. Doing this can make a remarkable dent in their populations.
Peak feeding is early-mid July in central Maryland, afterwards adult beetle populations usually decline. Their numbers vary from year to year, often based on how hot/dry the ground was the summer before when they tried to lay their eggs.
If additional control is necessary, use a labeled insecticide.
We don't generally recommend the use of traps, because they have a very powerful attractant that will lure more beetles into your garden than if you did not use traps.
Another common, though largely un-noticed rose pest in our area is the rose slug. The rose slug is not a slug but the larvae of a species of sawfly. Sawfly adults are very small flying insects related to wasps. The larva is small and “slug-like” in appearance. It feeds mostly from the underside of the leaf eating many holes and causing considerable damage. Severe rose slug feeding will stunt the rose’s growth and flowering. Apply insecticidal soap or horticultural oil, or another insecticide labeled for rose slugs.