Answer: If you've tried using a gnat control product and it isn't working, perhaps you're not dealing with gnats but with black bean aphids. They look the same: they congregate on stems and leaves. But gnats fly away when you touch the plant and aphids do not. In fact, aphid feeding will cause the leaves to yellow but the presence of gnats won't have any affect on the leaves. So, try spraying with insecticidal soap. It is deadly to aphids.
Good luck with your salvia!
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