Why are aphids sometimes called ants' cows?

Aphids are often called "ants' cows" because of a mutualistic relationship in which ants tend to and protect aphids in exchange for a sugary, nutrient-rich liquid called honeydew.

This interaction functions much like human dairy farming:

• Milking: Ants obtain the honeydew by stroking the aphids with their antennae, a behavior often referred to as "milking" the insects.12
• Protection and Care: Ants guard the aphids from predators and natural enemies, such as ladybird beetles and lacewings, and may even move them from wilted plants to healthier ones to ensure a steady food supply.23
• Active Management: In some species, such as the corn root aphid, the ants' involvement is extensive. They collect aphid eggs in the autumn, store them in their nests throughout the winter, and carry the newly hatched aphids to the roots of host plants in the spring.12

Because the aphids are often dependent on this care for their survival and provide a consistent resource for the colony, the term "ant cow" (or "ant cattle") has become a common descriptor for this unique biological association.145