Some species of ants are "aphid farmers" -- they tend aphids in a way similar to human farmers tending livestock. The ants protect the aphids from predators by patrolling the area where the aphids are feeding; if they encounter other insects, they sting or bite them until they leave or fall off the branch. As the aphids feed, they suck the plant juices out of the plant leaves and stems. Eventually, the plant wilts and the juice no longer flows. The ants will then pick up the aphids and carry them to a new juicy stem where they can continue to feed and produce the honeydew that the ants eat. At night, the ants will often carry the aphids to a safe place for the night and bring them back to a plant to feed in the morning!