One of the best ways to stop your rose bushes from being eaten is to use a gallon of water that is at a reasonable temperature and put a couple of squirts of liquid detergent that is ivory coloured into the water. The detergent will work as a surfactant and this will clog up the breathing system of the creatures that are eating the leaves. It is very important that all debris and fallen leaves are cleared from around your rose bush. The solution should be washed away about an hour later using clean water. If this idea does not work it may be worth considering cutting the bushes back a bit and disposing of the cuttings.
Soapy water is a good way of getting rid of the pests that are eating the leaves on your plants. This is an organic method that can be used for treating this problem with rose bushes. This is actually quite a common problem.
If you feel that Japanese beetles are causing the problem then you will need to tackle the problem during the early hours of the morning or last thing in the evening as this is when they are least active. In order to treat this problem you will need to shake the insects off the blooms and place them into a bucket of very hot water which is soapy, as this is the only way to actually kill them.