Ants are culling sick livestock to stop pandemics from spreading through their herds.
Many ants are ranchers. They guard herds of aphids, sap-sucking insects that pay for their protection in honeydew, a sugary waste the ants drink. You can read about the basics in our aphid farming story. But any rancher knows that guarding a herd from wolves is only half the job. The other half is disease.
The fungus that wipes out herds
Aphids live packed together on the same stem, often as clones of one another. That makes them perfect targets for Pandora neoaphidis, a fungus that only infects aphids. Its spores land on an aphid, grow through its body and kill it. The corpse then becomes a launch pad, firing new spores onto the rest of the colony. A colony can be wiped out in a matter of days.
For an ant, that's a disaster twice over. It loses its food supply, and a sick herd can carry disease back toward the nest.
Ants run quarantines
In 2009, Charlotte Nielsen, Anurag Agrawal and Ann Hajek at Cornell watched Formica podzolica ants tending milkweed aphids, and found that the ants protected their herds from the fungus. In field experiments, the ants:
- quickly removed the bodies of aphids killed by the fungus
- removed those bodies more often than the bodies of aphids that died from freezing, so they could tell why an aphid died
- detected fungal spores on living aphids, then either groomed the spores off or carried the aphid away
The ants weren't just cleaning up. They were picking out the dangerous aphids and getting rid of them before the outbreak could spread. The researchers described it as focused sanitizing and quarantining behaviour.
Some ranchers are better than others
Later, Tatiana Novgorodova tested more ant species with a different fungus, Beauveria bassiana, which can infect ants as well as aphids.
- Wood ants (Formica rufa and Formica polyctena) removed over 80% of aphids dusted with spores, compared with under 8.3% of clean aphids. It didn't matter how many ants or aphids there were.
- Jet ants (Lasius fuliginosus) quarantined contaminated aphids too, behaving much like the wood ants.
- Black garden ants (Lasius niger) mostly didn't react. They touched the contaminated aphids with their antennae and kept collecting honeydew.
Removing a sick aphid is more effective than just grooming it, Novgorodova notes, because grooming can't always get rid of enough spores. Taking the aphid off the plant protects both the rest of the herd and the ants themselves.
The same rules ants use at home
Ants live in crowded colonies, so they've had millions of years to work out public health. They carry their dead out of the nest, groom each other to remove spores, and in some species, sick ants even leave the nest to die alone. Pulling a sick aphid off a stem may be the same instinct, applied to their livestock. Read more in Why ants have better healthcare than you.
A lesson from ant ranchers
Human farmers know this story well. Packing animals close together makes them easy to protect and easy to milk, but it also makes them perfect for spreading disease. Ants found the same answer we did: watch the herd closely, catch the sick early, and get them away from everyone else.
Some aphids don't rely on ants at all. Read about the soldier aphids that raise their own army.
Further reading
- Nielsen, C., Agrawal, A. A. & Hajek, A. E. (2010). Ants defend aphids against lethal disease. Biology Letters.
- Novgorodova, T. A. (2015). Are wood ants (Formica rufa group) able to protect their aphid partners from contamination by entomopathogenic fungi? Euroasian Entomological Journal.
- Novgorodova, T. A. & Kryukov, V. Y. (2017). Quarantining behaviour of ants towards infected aphids as an antifungal mechanism in ant–aphid interactions. Entomologia Experimentalis et Applicata.
- Novgorodova, T. A. (2021). Preventing transmission of lethal disease: removal behaviour of Lasius fuliginosus towards fungus contaminated aphids. Insects.
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