Q: My Knockout roses are pretty hardy, but something is decimating them, right down to the veins. There are little green worms, maybe caterpillars? They are coiled into a C shape, and there are several on each branch.
A: You have rose sawflies. The leaf damage looks a bit like Japanese beetle feeding but if they were the culprit you’d see lots of them feeding on your roses.
Sawfly larvae are much more inconspicuous. The feed from the bottom of leaves and they are only out in early morning…so gardeners have a hard time seeing them. You’ve done a good job catching a picture.
The sawfly larvae LOOK like caterpillars to the uneducated eye but they are not. A sawfly is a primitive wasp-like insect. The females have a saw-like blade at the tip of the abdomen that is used to cut slits into plant tissue into which they deposit eggs. The resulting larvae feed voraciously. But since they are not caterpillars, they are not affected by the organic caterpillar insecticide, B.t. (Dipel, Caterpillar Attack, etc)
Organic insecticides such as insecticidal soap (click for sources) or those containing pyrethrin (click for sources)l are effective as long as you apply them under the leaves when the larvae are present.
The contact insecticide carbaryl (click for sources)) offers good control if sprayed on the whole rose. Imidacloprid (click for sources)) is a systemic which can be applied to the soil around the roses in spring before feeding activity is noticed.
European rose slug