The ant is guarding the aphids, protecting them from predators. The aphids are drinking the plant sap, which contains a lot of sugar, but not much nitrogen. As a result, the aphids have to consume a large amount of sap to get enough nitrogen. The aphids absorb the nitrogen, and some of the sugar, but most of the sugar and water pass on through, and are excreted as a liquid called honeydew.

Besides protecting the aphids from predators, the ants will also carry the aphids to new plants if the old plant runs low on sap. In the autumn, the ants will gather aphid eggs, and store them for the winter in their nest. In the spring, they place the newly hatched aphids on juicy plants. Adult aphids have wings, but the ants often cut them off to prevent the aphids from flying away. Some aphids have become so dependent on ants that they cannot survive without them.