I was recently examining a young roughleaf dogwood (Cornus drummondii, Cornaceae) and I noticed there were a lot of ants (Hymenoptera: Formicidae) congregating in clusters all over the plant. A closer look revealed the ants were gathered around groups of aphid nymphs (Hemiptera: Aphididae) that were feeding on the shrub:
The ants weren’t feeding on the aphids. Instead, they were feeding on a waste product aphids produce called honeydew. This substance is excreted from the anuses of the aphids, and it contains not only wastes but also sugary plant sap. Aphids ingest more sap than they can digest, and the excess is excreted along with their body wastes.
Many ants love honeydew and some will tend to aphids to protect this food source. Some herd aphids like cattle, defending them from predators, protecting eggs in their nests, and carrying nymphs and adults to more productive plant hosts. The ants can even rub the aphids with their antennae to make them release honeydew, effectively “milking” them. The individuals shown here seemed to be engaged in that behavior.
The relationship between many ants and aphids is a common example of a mutualism. This type of symbiosis is to the benefit of both parties. Ants gain a steady food supply, and aphids gain protection from harm.