Hello Novelangel, in my opinion Rose sawflies can cause considerable damage to roses, including Knockouts. It is important to be aware of this little critter and take corrective action.
Rose sawfly adults look like a tiny fly and are rarely noticeable in the landscape. However, when their eggs hatch on the underside of your rose leaves as larvae, they are easily identified. Look for several green, small, usually a half inch in length, caterpillars on the underside of leaves where damage is seen. Some people have described their look similar to a tiny green slug which gives them the common name of Rose Slug.
Control:
Start by removing the leaves where damage is found. These larvae cannot travel far, so removing infected leaves from the area is sufficient. Next, use a forceful spray from a garden hose to knock off any remaining rose slugs. Be sure to get the undersides of the leaves thoroughly.