Enlarge / Ants from the species Pheidole megacephala tending aphids. They protect the tiny insects from predators and milk them for a sugary fluid called honeydew. (credit: Alex Wild)

One of the worst effects of ant infestation doesn’t happen inside your house or on your picnic blanket. It takes place in farms and gardens, where many species of ants tend to herds of aphids. These tiny insects are a bane to plants—aphids weaken plants by feeding on their sap, and they sometimes transmit deadly viruses while eating. This has earned aphids the nickname “plant lice.” But for ants, aphids are more like cows. They secrete a sugary fluid called honeydew that ants love to eat.

Aphid ranchers on the mugwort frontier

Just as humans have transformed millions of acres of open land into ranches for cattle, ants can easily transform fields into aphid ranches. A new study by a group of environmental scientists in Japan reveals that ant cultivation of aphids isn’t a haphazard process, either. Researchers in Science Advances describe an experiment where they discovered that ants in a Hokkaido field are often the only factor that prevents aphid colonies from going extinct. In addition, the ants prefer aphid colonies with a nice balance of green and red aphids, and they selectively breed aphids to exhibit these colors.

Even though many ant species tend aphids and similar sap-eating insects, the researchers focused exclusively on ants from the species L. japanocus. These ants tend colonies of the species M. yomogicola, a kind of aphid that feeds on the mugwort plant. The researchers identified three groups of eight aphid colonies for observation. They left one group alone to be tended by ants, while another group was isolated from their ant benefactors by painting the base of their mugwort homes with a sticky substance called Tanglefoot that prevents ants from climbing the stalks to their herds. The third group was a control, tended by ants, but exposed to a minimal amount of Tanglefoot. This was just to measure the effect of Tanglefoot on aphids (it was negligible).