Have you ever seen aphids on (fruit) trees with ants running between them? While this is not a welcome view for gardeners, it’s an amazing example of ancient collaboration between two groups of insects. While many people know that ants defend aphids and aphids provide food for the ants, there is more to this relationship than meets the eye. I would like to tell you some more about ants farming aphids.
Exchanging food for services
Aphids feed on large volumes of plant sap, which is rich in sugar and amino acids (the building blocks of proteins). They eat more than they can ever use themselves, so these useful materials are still present in large quantities in their excrement, called honeydew. This is a great source of nutrients for ants. They tap the aphids’ abdomens with their antennae. In response, aphids produce a droplet of honeydew. But ants don’t just ‘milk’ aphids, they also eat some of them, especially young and less productive ones – it’s easy to see why we call it ‘farming’.
Ants also actively protect aphids, which are themselves rather defenceless against predators (e.g. ladybirds) and parasites (e.g. tiny wasps that would like to lay their eggs in aphids’ bodies). Sometimes ants even move ‘their’ aphids to a safe location in the face of danger. Additionally, ants protect aphids from mould and other diseases, by removing excess excrement and sick and dead insects. Sometimes, ants even help aphids with access to their favourite plants.
Aphid farming by ants is therefore an example of mutualism, a relationship in which both partners profit. This relationship is ancient, ants farmed aphids for at least 35 million years and maybe even 75 million. About a quarter of the roughly 4000 known aphid species are tended by ants.
Underground stables
When ants tend to aphids on the branches or leaves of plants, it’s usually because of a lucky encounter. Both aphids and ants can profit from the interaction but they can also live without their partner. However, some species can’t live without each other. Such obligate mutualism usually occurs for aphids that live underground, within or close to ants’ nests. Some ants even build ‘stables’ for their aphids around the roots of their preferred plants.
These aphids usually don’t reproduce sexually and don’t disperse. They often have short legs and long mouthparts that take a long time to retract from a plant – signs that they ‘gave up’ on their own anti-predator responses. The relationship between the species is long-lasting, possibly for the entire lifespan of the ant colony.
Communication
When aphids are in danger, they produce pheromones that alert other aphids, but also ‘call’ their ant farmers to come to the rescue. In turn, aphids can detect chemicals left in ants’ trails, which inform them that their protectors are around. In response, they disperse less and reproduce more.
Ants recognise their aphid partners (and each other) by the chemicals they present on their bodies (cuticular hydrocarbons). This is sometimes exploited by cheaters. One species of aphid (Paracletus cimiciformis) can produce two forms: mutualistic and parasitic. The parasitic one ‘pretends’ to be an ant larva by ‘smelling’ like them. It enters the ants’ nest and feeds on real ant larvae instead of plant roots.
Other farm animals
Ants do not only farm aphids. They can also tend to some butterflies and some scale insects, e.g. mealybugs. They do it mostly for honeydew but sometimes only for ‘meat’.
Some of these relationships are obligate and scale insects are passed from mother to daughter ant colony. Some virgin queens even carry the ‘cattle’ from their natal nest on their mating flight, either on their back or in their mouthparts.
Ants farming aphids can be seen as a nuisance as it allows aphids to flourish and reduces the effectiveness of biological pest control, like ladybirds or parasitic wasps. But I have to marvel at the mutualistic relationship between the species, also when it happens in my own garden.
You can read more about ants farming aphids in the article “Aphid-farming ants” by Aniek B.F. Ivens and Daniel J.C. Kronauer.
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Photo: Ilona Ilyés from Pixabay.