Raising cattle for milk and beef has been an important part of the activities on Owl Acres for over 150 years. Another kind of animal husbandry has been going on here for millions of years. We noticed it recently when we were inspecting a Jerusalem artichoke plant in our prairie garden. Ants were tending a herd of aphids. As it turns out, this is an ancient and productive activity for ants, and aphids tolerate it for their own reasons.
Ants herd their aphid livestock, keeping them together, protecting them from predators, milking them routinely, and eating them. It’s not much different than tending a herd of dairy cattle but on a vastly different scale. Like our cattle produce a food that we eat, aphids produce a sticky, sugar-rich substance called honeydew that the ants eat. The honeydew is essentially the waste product the aphid excretes after it has taken the nutrients it needs from the sap of the leaves. The ants actually “milk” the aphids by stroking the aphid’s abdomen with the ant’s antennae. This causes the aphid to excrete a drop of honeydew which the ant then picks up and carries back to the ant’s nest.
Whereas we use fences to keep cattle contained, ants take a different, more chemical approach. According to some research, the ants exude chemicals on their feet that subdue the aphids and make them lethargic and slow-moving. That makes them easier for the ants to manage. Aphids have a peculiar lifecycle, and are able to grow wings when they need to move from one feeding ground to another. The ant herding them may not approve of these wings and either stunt their growth with chemicals or actually bite them off, leaving the aphids stranded.
Aphids are not beloved by gardeners and soybean farmers. They damage plants by piercing the surface of a leaf and sucking out the sap from the phloem in the leaf. They also introduce chemicals that further damage the leaf. The leaves turn yellow and if enough aphids are present on the plant, the plant may die. A secondary problem arises if the honeydew isn’t cleaned up. The sticky, sugar-rich substance can attract a sooty mold fungus that disfigures the leaves. Our little herdsmen want all the honeydew, so they collect all of it and keep the area clean. They stay busy milking their livestock and transporting the honeydew to the ant nest.
This arrangement with aphids is obviously beneficial to the ants. They get a continuous source of food. It’s good for the aphids, too. They get protection from predators like ladybird beetles, hover flies and other birds and insects who are more than happy to gobble up a juicy aphid snack. Ants also protect the aphids from parasitic wasps that try to lay their eggs inside the aphids. The ants will fight off the wasps, protecting the aphids from this unseemly invasion.
Like human livestock farmers, the ants are thought to cull their herds of aphids, eating the poor producers as well as dead aphids and excess young. And, like human livestock managers, if their livestock is threatened, or the “pasture” is getting thin, the ants will pick up their aphids and physically move them to a new and more suitable location. In some cases, the ants build “stables” around the roots of the plants that the aphids prefer, near to the ants’ underground nests. By tending the aphids, the ants reduce the likelihood of disease and parasites in the aphid population.
Fossil records indicate that ants have been herding aphids for at least 35 million years. Of the 4,000 or so species of aphids, about a quarter of them are farmed by ants. And the ants that do the tending are generally from the three largest ant subfamilies, the Myrmicinae, Dolichoderinae and Formicinae. There’s a wide variety of approaches that ants and aphids have evolved to maintain these relationships. Sometimes the relationship is seasonal where ants take up their herding chores when they find the aphids on leaves aboveground. And sometimes they set up aphid pens in or near their underground nests and keep the aphids reproducing asexually year-round.
Just as human livestock farmers may herd goats or sheep instead of cattle, ants do not restrict themselves to aphids. Some species will also tend herds of certain butterflies, treehoppers and mealybugs. In our prairie garden, though, it’s all aphids. The Jerusalem artichoke plant is robust and healthy, so in this case our little farmers aren’t doing any harm. It may well be a different story in a vegetable garden though.
Photo by Author. Alt text: Dozens of aphids, all head down, stud the stem of a Jerusalem Artichoke in the prairie garden. A lone black ant patrols the herd.