One common site I see while surveying Field Ant, or Formica colonies are clusters of goldenrod covered with aphids. Hovering over these aphids were Allegheny Mound Ants, or Formica exsectoides, not preying on them but instead carefully tending to them. Ants and aphids are engaged in a relationship called mutualism. If these goldenrod stems were ant-pastures, then the aphids are more like ant-cows, raised and milked for a sugary substance called honeydew.
Formica propinqua (Mound Ant) farming aphids, courtesy of Jake Nitta
How Ants Benefit from Aphid “Farming”
Aphids are sap-feeding insects, and as they digest this plant sap, they excrete honeydew, a liquid rich in sugars. Many ant species have adapted to use honeydew as a primary food source. In this relationship, ants protect the aphids from predators like ladybugs, keep them clean, and even transport them to fresh plants for more sap. To gather honeydew, worker ants “milk” the aphids by gently stroking them with their antennae, encouraging the aphids to release the sweet liquid, which ants eagerly lap up. The relationship is so valuable to both parties that ants sometimes shelter aphids in their nests at night or even during the winter months.
Argentine Ants (Linepithema humile) with aphids
Some recent research show that Formica ants will often look for specific sources of peroxides when sick to help fight fungal infections. It appears that aphid honeydew is a particularly strong source of these peroxides. It's possible that one of the reasons that ants protect aphids is that the honeydew secreted acts as a medicine for sick ants.
Ants as Skilled Caretakers
Within ant colonies, workers often specialize in different tasks such as nursing and foraging, and in ant species that farm aphids, some specialize in caring for and herding aphids. These “aphid caretakers” form a unique workforce that protects and moves aphid groups to ensure a steady honeydew supply. When colonies relocate, they sometimes carry aphid eggs with them to ensure they have a ready source of honeydew at their new nest.
Some species, like Acropyga, or Mealybug ants, feed nearly exclusively on honeydew produced by their ant-cows. In fact, when new queens set out to start new colonies, they bring a mealybug companion with them, a partnership which seems to have existed for millions of years.
Left, Acropyga princesses carrying mealybugs on the wing
Right, Acropyga princess with mealybug
One of the most common ants in the Northern Hemisphere, Lasius, or meadow ants, are evolving into more and more specialized aphid farmers. Likewise, their aphids are evolving to become honeydew producers. Some aphids farmed by Lasius flavus have a specialized “trophobiotic organ” that stores honeydew for the ants, an adaptation found only in aphids that have evolved to live exclusively within ant nests. In the fall, the ants collect aphid eggs and protect them throughout the winter, bringing them to new pastures when they hatch in the spring.
Lasius flavus with aphids, courtesy of Connor Cashman
Researchers have observed that ants manage their aphid populations much like farmers managing crops, ensuring only the most productive aphids are kept. Ants encourage honeydew production by maintaining the right conditions for the aphids, consuming excess young aphids for protein, and replacing adults when honeydew yields decline.
Aphids as Victims
More recent research has found that scientists are finding that while this arrangement is helpful for both, it comes with surprising costs for the aphids.
Some aphids, like Tuberculatus quercicola, have wings, which would normally let them fly to new places to start new colonies. To maintain their herds, ants will sometimes pull off the aphids’ wings or release chemicals that make the aphids stick around in one spot. These chemicals even slow the aphids’ walking speed, making it hard for them to leave. Keeping the aphids close together benefits the ants because they get more honeydew, but it can trap the aphids in areas that might not be good for them. Scientists think that this chemical control might sometimes signal safety to the aphids, but it could also hurt them if they actually need to move to survive.
While this partnership between ants and aphids does bring benefits, it also shows how ants' needs can limit the aphids in surprising ways. Instead of moving around and growing as they normally would, aphids in this partnership find themselves dedicating their resources—and even changing their bodies—to keep their ant protectors around.
A Relationship Studied Across the World
The ant-aphid symbiosis has been observed globally, from North America to Asia, with different ant species pairing with a variety of aphids on various plants. These partnerships can vary in intensity: some ants merely gather honeydew where they find it, seemingly without tending to the aphids, while others, like Formica podzolica, guard aphids against diseases, by grooming and removing dangerous spores which may cause fungal infections to ensure their honeydew supply remains healthy.
Through their close relationship with aphids, ants take on the role of tiny farmers. They maintain and protect their “herd,” revealing how these social insects impact ecosystems in unexpected and complex ways. From aphid caretakers to honeydew harvesters, ants demonstrate that even the smallest creatures are capable of remarkable, cooperative behavior.
Further Reading
Gull-E-Fareen, A., Bodlah, I., Rasheed, M. T., Niaz, Y., Bodlah, M. A., Asif, M., & Khokhar, N. M. (2020). Trophic associations of ants with aphid partners and new distribution records of some ants in Pothwar region of Pakistan. Pakistan Journal of Zoology, 53(1). https://doi.org/10.17582/journal.pjz/20190510120507
Nielsen C, Agrawal AA, Hajek AE. Ants defend aphids against lethal disease. Biol Lett. 2010 Apr 23;6(2):205-8. doi: 10.1098/rsbl.2009.0743. Epub 2009 Nov 18. PMID: 19923138; PMCID: PMC2865061.
Oliver TH, Mashanova A, Leather SR, Cook JM, Jansen VA. Ant semiochemicals limit apterous aphid dispersal. Proc Biol Sci. 2007 Dec 22;274(1629):3127-31. doi: 10.1098/rspb.2007.1251. PMID: 17925280; PMCID: PMC2293950.
Schifani, E., Peri, E., Giannetti, D., Colazza, S., & Grasso, D. A. (2023). Ant attendance does not necessarily imply protection of aphids from their arthropod natural enemies. Ecological Entomology, 48(3), 384–388. https://doi.org/10.1111/een.13226
Aphid-farming ants: Current biology. (n.d.). https://www.cell.com/current-biology/fulltext/S0960-9822(22)01077-6
Yao I. Ant attendance reduces flight muscle and wing size in the aphid Tuberculatus quercicola. Biol Lett. 2012 Aug 23;8(4):624-7. doi: 10.1098/rsbl.2012.0014. Epub 2012 Feb 29. PMID: 22378740; PMCID: PMC3391452.