Answer: I think culprits are roseslugs. They aren't slugs at all -- they are sawfly larvae. The adults, which are wasplike, lay eggs between the leaf layers, and the larvae emerge to feed, leaving "windows" on the leaves. As they grow, they consume entire leaves. The larvae drop to the soil to pupate, and in some places there may be multiple generations during a summer.
You can help control the problem by spraying a neem-based insecticide /repellent may repel the adults. If the tiny larvae do emerge and start eating, a dose of neem should alter their appetite. Insecticidal soap is another option.
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