Answer: It's hard to identify the culprit without seeing the plant, but my first guess is an infestation of roseslugs. These aren't true slugs, but rather the larvae of the sawfly. The wasplike adults lay eggs between the leaf layers, and the larvae emerge to feed, leaving translucent "windows" on the leaves. As they grow, the larvae consume entire leaves. Then they drop to the soil to pupate. In some places there may be multiple generations during a summer.
Some people report good luck using a neem-based spray as a repellant. If the tiny larvae do emerge and start eating, hose them off with a blast of water daily or handpick and destroy. Natural enemies often keep them in check. If the infestation continues, a spray of neem or insecticidal soap should help. Be sure to spray the entire plant, including the undersides of leaves.
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