HMC Bee Lab: Ant MacDonald Had a Farm

Most of the ants I’m familiar with are scavengers- they feed on whatever they can find. They search for tasty food, but they aren’t really able to control how nutritious the things they find are. It turns out, this isn’t the case for all ant species. Some, like humans, farm their food!

I had heard of leafcutter ants, who famously maintain gardens of fungus inside their nests. However, researching what other species of ants eat, I was surprised by how often one particular food source popped up: honeydew. This isn’t the melon: these ants are farming and milking aphids for the sugary secretion they produce. In my mind, the fact ants are able to grow produce is impressive, but them being able to tend to livestock is incredible! The aphids, unlike the fungus, can walk away from the ants, and are living out in the open (instead of the controlled environment of the ant nest). Despite all of these challenges, aphids remain a primary food source for some ant species.

Ants tending to their flock of aphids [1]

When I think of communication, the first two things that come to mind are talking and pointing. But ants can’t really do either of these things: they primarily rely on pheromones, or chemical scents, to communicate with each other. Although different pheromones can signal different things, the ants still don’t really have a “language.” And yet, multiple species have discovered how to extract honeydew from aphids and tend to them on a colony-wide level.

Maybe this doesn’t sound all that incredible to you. Aphids produce honeydew on their own— how difficult is it for an ant to run into an aphid, notice the honeydew, lead other ants there, and begin to harvest the honeydew as a colony? Except, that’s not the end of the story. Not only do these ants harvest honeydew from the aphids, they actually alter the aphids’ behavior- that’s the most interesting part to me!

A different pair of ant and aphid species in a similar honeydew-farming setup [2]

There are a lot of ant species that tend to aphids, and a ton of aphid species that are tended to by ants. As we might expect, there’s a lot of variety in the farming techniques ant species use (especially when they’re farming different types of aphids). That being said, two particular tactics seem to be used by many aphid-tending ant species: herding and protecting the aphids, and causing them to produce more nutritious honeydew.

An ant confronts a hungry ladybug [3]

Herding and protecting aphids

One of the challenges ants face is that their aphids can crawl away. In contrast to fungus farming which takes place inside the ants’ nest, aphid-farming happens out in the open. Aphids will often wander off if the area they're in is too crowded or otherwise “unsuitable.” So, how do ants keep them in one place? In addition to physical herding, it appears ants are using pheromones! The presence of some ant pheromone has a “tranquillizing effect upon aphids,” limiting their dispersal from a particular area. However, this pheromone communication goes both ways.

Aphids sometimes secrete a chemical that signals other nearby aphids that there’s danger. It turns out that some ant species pay attention to this pheromone. The ant Lasius niger, for example, uses that aphid pheromone to locate aphid colonies. Another ant species, Linepithema humile (Argentine ants) shows “protective behavior” when it senses the alarm pheromone! Aphids are some of these ant species’ primary source of food: they have an interest in protecting their flock.

An ant sips a droplet of fresh honeydew [4]

Altering honeydew production

The impact of ants on honeydew production is a complicated one! There are a lot of questions of cause-and-effect. For example, ants that farm aphids usually don’t limit themselves to one species- they weigh both the quality and quantity of honeydew when they decide which type of aphids they want to tend. However, at the same time, some studies suggest that due to the protection ants offer aphids from predators, some aphid species (such as Chaitophorus populeti and Chaitophorus populialbae) might produce “better” honeydew in an attempt to retain the farming ants. Some aphids are able to manipulate the concentration of amino acids in the honeydew they produce. Even though those nutrients could be used by the aphids, they may find that the benefit of ant attendance may outweigh that cost. As a result, some aphids may deliberately produce a more amino-acid-rich honeydew to win the “competition for ant services” amongst local aphid species. So, the presence of ants seems to influence the honeydew production of aphids, because the aphids want to entice the ants!

Conclusion

This isn’t even the only example of ants farming their food! I’ve already mentioned fungus-growing ants. Some ants also harvest honeydew from other insects, like mealybugs. However, I think that the documented use of pheromone in the ant-aphid relationship makes this type of ant agriculture the most interesting. The two species have learned to communicate with each other! Part of what makes this partnership so incredible to me is how vastly different ant-aphid communication is from human-livestock interactions. Despite the differences in communication (smells vs. sound), these insects have been able to “domesticate” aphids in a way that really resembles human farming: they consistently tend their herd over long periods of time, working as a collective to milk honeydew and prevent the aphids from dispersing.

HMC Bee Lab

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Wednesday, November 3, 2021

Ant MacDonald Had a Farm

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