How Ants and Aphids Help Each Other

Ants and Aphids Have a Strong Relationship

Ant tending aphids.
Ants and aphids have a mutualistic relationship. The ants feed on honeydew left behind by the aphids, and in return, they protect the aphids from predators. Flickr user Stuart Williams (CC license)

Ants and aphids share a well-documented symbiotic relationship, which means they both benefit mutually from their working relationship. Aphids produce a sugary food for the ants, in exchange, ants care for and protect the aphids from predators and parasites.

Aphids Produce a Sugary Meal

Aphids are also known as plant lice, they are very small sap-sucking insects that collect the sugar-rich fluids from host plants.

Aphids are also the bane of farmers the whole world over. Aphids are known crop destroyers. The aphids must consume large quantities of a plant to gain adequate nutrition. The aphids then excrete equally large quantities of waste, called honeydew, which in turn becomes a sugar-rich meal for ants.

Ants Turn Into Dairy Farmers

As most people know, where there is sugar, there is bound to be ants. Some ants are so hungry for the aphid honeydew, that they will "milk" the aphids to make them excrete the sugary substance. The ants stroke the aphids with their antennae, stimulating them to release the honeydew. Some aphid species have lost the ability to excrete waste on their own and depend entirely on caretaker ants to milk them.

Aphids in an Ant's Care

Aphid-herding ants make sure aphids stay well-fed and safe. When the host plant is depleted of nutrients, the ants carry their aphids to a new food source.

If predatory insects or parasites attempt to harm the aphids, the ants will defend them aggressively. Some ants even go so far as to destroy the eggs of known aphid predators like ladybugs.

Some species of ants continue to care for aphids during winter. The ants carry the aphid eggs to their nests for the winter months.

They store the precious aphids where temperatures and humidity are optimal, and move them as needed when conditions in the nest change. In spring, when the aphids hatch, the ants carry them to a host plant to feed.

A well-documented example of the extraordinary mutualistic relationship of a corn root aphid, from the species Aphis middletoniiand their caretaker cornfield ants, Lasius. Corn root aphids, as their name suggests, live and feed on the roots of corn plants. At the end of the growing season, the aphids deposit eggs in the soil where the corn plants have withered. The cornfield ants collect the aphid eggs and store them for the winter. Smartweed is a fast-growing weed that can grow in the spring in the cornfields. Cornfield ants carry the newly hatched aphids to the field and deposit them on the temporary host smartweed plants so they can begin feeding. Once the corn plants are growing, the ants move their honeydew-producing partners to the corn plants, their preferred host plant.

Aphids Appear to Be Slaves to the Ants

While it appears the ants are generous caretakers of aphids, ants are more concerned about maintaining their steady honeydew source than anything else.

Aphids are almost always wingless, but certain environmental conditions will trigger them to develop wings.

If the aphid population becomes too dense, or food sources decline, aphids can grow wings to fly to a new location. Ants, however, do not look favorably upon losing their food source.

Ants can prevent aphids from dispersing. Ants have been observed tearing the wings from aphids before they can become airborne. Also, a recent study has shown that ants can use semiochemicals to stop the aphids from developing wings and to impede their ability to walk away.

Sources:

Whitney Cranshaw and Richard Redak, Bugs Rule! An Introduction to the World of Insects, Princeton University Press, Princeton, 2013.