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 is typically noticed first because of the damage they cause as larvae. As the larvae feast, the leaves take on a mosaic or skeleton look because they eat the fleshy parts and leave all the little veins.
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.
While there are actually several similar sawflies, control remains the same no matter which kind you have. 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.
Often times chemical control is not needed if you can stay on top of things with the garden hose. This will allow natural populations of beneficial predators such as parasitic wasps and birds to increase, lowering your chance of outbreak in subsequent years. If the sawfly population is out of control, there are several chemicals that can be used but are not often needed.
For some excellent photos and more description of the insect lifecycle, check out Missouri Botanical Garden’s reference.