Q: Any idea what might be wrong with this rose bush? The leaves are very brown and look dead.
A: You have rose sawfly damage. Your damage is pretty severe but I can’t think of anything else that would give this blotching and window pane effect.
Sawfly larvae eat the bottom layer of a leaf. Afterward, the top layer dries out and becomes brown.
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.