Make a solution of one part milk to nine parts water. Put the 10% solution in a spray bottle and spray liberally once a week. That took care of that problem on my roses. Unfortunately, I could not find a good non toxic solution to the white fly infestations.

I might add that this solution is good for strawberries and veggies too. The drawback is that your roses will smell like a milking barn.

I'd just add, it has to be cow's milk, not goat, or almond or soya or anything else, but it doesn't matter whether its full fat, skimmed or half fat. In fact, if its fully skim milk, it's less likely to smell like a milking barn...