Aphids. Look closer and see if there are any Garden Patrol handling the situation. Ladybugs/ladybeetles and their larvae! green lacewings and their larvae. Do you see any aphid mummies -- dried tan shells/husks of bloated aphids? Do you see any teeny tiny brown and yellow bee-like flying insects called hover flies?
If you need to deal with them on your own, start by just spraying them off with strong streams of water.
If you see ants marching up and down the plants, then the ants are shepherding the aphids and need to be controlled.
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