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Mutualism is defined as an interaction between species showing themselves to be beneficial for the two protagonists. This is the case of the relations between aphids and ants.

Ants have developed a large number of associations with a wide range of organisms and particularly with other insects. In the temperate regions, the most frequent association involves the suborder Homoptera and mostly the aphids. A certain number of aphid species considered as myrmecophilous interact with ants.  

The species concerned respond to palpation by ants with the secretion of honeydew. For those ants it is a sugar- rich supplement to their diet. This behavioural relationship is called trophobiosis. It can be obligatory or facultative depending on the species. This kind of interaction exists since at least the Oligocene (50 million years BP). The ants, in exchange for this supply of food, provide the aphids with an aggressive defence force against their predator and parasitoid antagonists. This defence can sometimes be reinforced by release of a pheromone alarm signal produced by the aphids themselves. In consequence, the aphid populations prosper thanks to this protection service.

However, some predators (Coccinella magnifica, Hyperaspis sp.) or parasitoids (Paralipsis enervis, Lysiphlebus sp.) have adopted strategies to make themselves acceptable (through chemical camouflage, slow movements and so on) while continuing their antagonistic action. Others operate at night when the ants are less active (Forficula auricularia). Yet others simulate the presence of aphids by covering themselves with the remains of their prey (Chrysopa sp.). 

The ants most commonly encountered in these mutualistic associations belong to the genera Formica, Lasius and Myrmica.

Not all the aphids produce the same quantity of honeydew and those that make the most are taken care of best. In a similar way, the colonies visited by the ants have a better rate of reproduction. For the ants, the aphids indeed represent a source of carbohydrate supplied by the honeydew. Yet they are also a source of proteins –as prey which the ants also consume when the colony is too large or if some individuals show aggressive behaviour. The ants can also move the aphids to another plant if the plant resource is not of good enough quality (due to senescence or other factors) to feed the colony.

In Europe, the ant most frequently found in aphid colonies is Lasius niger. It constructs its nest in the ground but can extend it upwards, making an enclosure of earth which encircles the plant stems and thus keeps the aphid colonies shut inside, so sheltering them from predators and poor weather conditions.