Answer: Spittlebugs are generally not damaging to plants, unless the population gets overwhelmingly large. They don't make your plants look very nice, though. The foamy covering on these tiny insects protects them from hot sunshine. If you simply hose your plant off you'll wash the spit away and the insects will be vulnerable to predatory insects and sunshine. Do this often enough and they'll find another home. I use this method in my garden. I hose the plants off daily for about a week and that seems to take care of them for the season.
Best wishes with your rosemary!
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