They are aphids, not necessarily bad. Less bad on a thistle than a prized plant. If you watch them I bet you will see them disappear, birds eat aphids and wasps use them either to take to their nests to use as hosts for their eggs, or some parasitize them in situ by injecting an egg into the body which will be food for the developing young. If you destroy aphids you could be destroying an important predator which will increase and keep pests at bay.
