Some species of ants “farm” aphids, protecting them on the plants they eat, eating the honeydew that the aphids release from the terminations of their alimentary canals. This is a “mutualistic relationship“.
Some farming ant species gather and store the aphid eggs in their nests over the winter. In the spring, the ants carry the newly hatched aphids back to the plants. Some species of dairying ants (such as the European yellow meadow ant, Lasius flavus) manage large “herds” of aphids that feed on roots of plants in the ant colony. Queens that are leaving to start a new colony take an aphid egg to found a new herd of underground aphids in the new colony. These farming ants protect the aphids by fighting off aphid predators. From Aphids wikipedia