Two-faced ants

Ants (Hymenoptera: Formicidae) are social insects known for their incredible strength, complex colonies, and diverse feeding habits. Depending on their lifestyle, ants can play different roles in ecosystems, for example as predators, decomposers, or bodyguards of other species in mutualistic* relationships.

Examples for ants tending aphids are: Crematogaster auberti Emery (Myrmicinae), Camponotus foreli Emery and Camponotus aethiops Latreille (Formicinae). They all occur on the shrub Retama sphaerocarpa.

Help me and I help you

In nature, plants are attacked by a wide array of herbivorous insects. In response, plants have developed strategies to deal with them. Some of these strategies do not require plants to fight back themselves, e.g., by producing an insecticidal substance, instead plants cater to the needs of the herbivores’ natural enemies. Some plants have a close relationship with ants and have developed special structures such as food bodies (small structures e.g. on leaves that contain nutrients), extrafloral nectaries (nectar producing glands outside of flowers) or domatia (cavities in the stems, leaves, and spines of plants) that provide food or shelter for the insects. In return, ants protect their host* plant from unwanted visitors that try to colonize it. Thanks to their strength and large numbers, carnivorous ants are excellent bodyguards that will do everything in their power to keep their beloved plant free from hungry insects. They may kill and eat them, but already the presence of carnivorous ants on a plant can prevent other insects from even visiting the plant in the first place. 

Bad tenants

Aphids and other insects from the order Hemiptera are clear examples of visitors a plant does not want. These herbivorous insects extract nutrients from plants by piercing the plant leaves with their specialized, needle-like mouthparts and feeding on the sap. Aphids are very small, and the presence of a single aphid does not yet pose a problem for the plant. However, aphids have a remarkable reproduction capability and can clone themselves rapidly when they find a good host plant and the conditions are right. In this case, one aphid can produce dozens of new aphids in a matter of days, their offspring reproducing in the same rate, resulting in an incredibly rapid growth of the aphid population. As aphid numbers increase, the resources they consume add up greatly, which slows down or even inhibits plant growth and development. 

How ants betray their host plant for a sweet snack 

When aphids (or other honeydew producing insects) colonize a host plant of ants, an unexpected alliance forms. Instead of scaring away, fighting off or consuming the aphids, ants often choose to protect them. The reason is honeydew, a waste product produced by aphids and loved by ants (and other insects). Aphids produce this sticky sugary substance to dispose of excess sugar they take up when feeding on a plant.

To profit continuously from this source of sugar, ants protect the honeydew producers from their natural enemies like ladybugs and parasitoid wasps. In addition, ants may move aphids to plants that are still free of them to start new aphid colonies. On top of their protective actions, ants help aphid colonies to grow much larger than they usually would. By consuming honeydew, ants prevent aphid colonies from becoming a sticky mess due to an overflow of honeydew that can foster diseases. However, ants will also eat aphids, removing young aphids or those that do not hold their standards for honeydew production and thus aphids also serve as a protein source for the ants. This relationship resembles how humans farm cattle, with ants managing, nurturing, and defending their 'livestock' – the aphids. Both ants and aphids benefit from this relationship, a mutualistic* symbiosis.

For the host plant, however, this overload of farmed honeydew producers is harmfu as it can weaken the plant, reduce its productivity or even lead to its death. In addition, the presence of ants may scare away beneficial insects like pollinators, which can have a negative effect on the reproductive success of the plant.  To combat this, certain plants release ant-repellent chemicals when in bloom.

So, are ants friends or foes? The answer lies in the context of their complex ecological interactions.

*Glossary

Host: An organism that serves as a temporary or permanent habitat for another organism or provides it with food, protection or the opportunity to reproduce. Humans, for example, are a host for mosquitoes, which feed on their blood, or for malaria parasites, which reproduce in them.

Mutualism: A close relationship between two organisms of different species which is beneficial to both organisms.