I wouldn't recommend using Sevin. It often triggers spider mite outbreaks that are a real nuisance to deal with, and it is deadly to beneficial pollinators.

Does the pest create translucent windows in the leaf, eventually skeletonizing, but without big, rough chomps? If so it is rose slugs. These are tiny to medium-sized green worms on the underside of the leaves that just need to be wiped off.

Here's one of the types of Rose Slug we have here (Curled Rose Slug). You can find these guys on the underside of leaves, on the upper part of leaves, on the canes, and even inside blooms.

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Before you use any type of insecticide you should always ID what insect you're dealing with. Otherwise you could be using a product that won't do anything, wasting time and money, and possibly killing good insects that help keep the bad guys under control.

Thanks for the responses. I haven't been able to locate any insects on my knockouts, although I will be doing a more thorough inspection later today. The description of the damage to my leaves matches what is described above. I'm just afraid I might be too late because on bush looks to be 75% eaten!!!! Not giving up!

Possibly the reason why you can't locate the insects (rose slugs) is because the good insects have finally come along and eaten them up. In that case, the damage you are observing is "old" damage, not on-going current damage.

I never spray rose slugs because the good insects always come along after a couple weeks and eliminate the problem for me.

The rose bush will put out new leaves that should stay nice-looking once the rose slugs have been eliminated. You might pull off some of the worst looking leaves so that the bush doesn't look so bad until the new leaves come along.

Kate

Thanks for the response Kate. I now know I should have done a little more research before I sprayed with Sevin. I never considered that I would be eliminating the good insects along with the harmful ones. I just observed some pretty nice new growth starting on most of the bushes, including the most severely damaged one. It will be a good opportunity to see if the slugs are indeed gone. Fingers crossed! Thanks!