Why do ants collect honeydew from aphids?

Ants collect honeydew from aphids because it serves as a valuable source of food. Honeydew is a sweet, sugary excretory product produced by aphids as they ingest large quantities of plant sap to extract necessary nutrients.123

This relationship is a form of mutualism in which ants provide care and protection to the aphids in exchange for this resource. Specifically, ants perform the following roles:32

• Protection: Ants guard aphids against weather conditions and natural enemies, such as predators and parasites.32
• Management: Ants may transfer aphids from wilted plants to healthy ones to ensure a consistent supply of honeydew. Some species, such as the corn root ant, even store aphid eggs in their nests during the winter and place the young on host plant roots in the spring.31
• Harvesting: Ants obtain the honeydew by stroking or "milking" the aphids with their antennae, which stimulates the insects to release the secretion.13

Some species, known as honey ants, have even evolved specialized workers that store this collected honeydew in their abdomens to feed the colony during times of food scarcity.42