Here are what my rose leaves look like…
Brown spots and holes throughout. I don't ever see pests on them, but they look like this every year.
You are correct. These little green things are rose slugs. Actually, they can be the larvae of several different insects but the result is the same. The rose will bounce back but several years of severe infestation may seriously harm it. Expect another generation is about 6 weeks. Damage starts at the bottom of the rose as they eat their way up the plant.
I believe I have figured out it's sawfly larvae.