Answer: Whether or not to treat the problem depends on whether the gnats are simply gathering on the stems (they do this sometimes!), or whether the pests are not gnats but black bean aphids. Aphids can suck the life out of plant leaves and stems and their presence should be discouraged. Insecticidal soap is usually effective against soft bodied insects such as aphids. I'd try hosing them off with water first to see if that doesn't discourage them. If they persist, use insecticidal soap, according to label directions.
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