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Monocolonialism holds clues for understanding the little fire ant

December 20, 2002

The presence of Wasmannia auropunctata in New Caledonia was first recorded in 1972. This small ant, with workers measuring only 1 or 2 mm, originates from tropical America. It was introduced by human activity, accidentally brought in, probably on plant material imported for use in forestry. The ant has colonized almost every ecological type of environment, spread with the movement of young tree cuttings, and is now a threat to several endemic species. Moreover, its highly painful sting disrupts coffee harvesting and can cause serious eye damage in domestic animals.

An IRD team from the service unit "Terrestrial biodiversity and environment in the tropical Pacific", is studying the ways in which Wasmannia auropunctata might have invaded and the repercussions of this for the functioning of ecosystems. One of the key strategies that some successfully proliferous invasive species adopt is monocolonialism. This might hold a valuable clue as to how the little fire ant spreads. Whereas most ants do not recognise or accept others from any nest different from their own, monocolonial species tolerate individuals from other nests. This tolerance enables them to boost their population considerably.

The team therefore focused their work on the recognition processes between ants from different nests. They assessed the degree of aggressiveness within the same species of Wasmannia, by putting together one by one ants taken from six nests between 60 and 410 km apart from each other, and from different ecological habitats. Tests were also performed by bringing two groups into confrontation, one sampled from Blue River Park, in the south of New Caledonia, the other from the Forest of Pinda'-, in the western part of the island. A further experiment involved placing the Wasmannia species face to face with individuals of Pheidole megacephala, another invasive species of ant present in New Caledonia.

During the confrontation experiments with individual ants, workers from different nests used neither their sting nor their venom on encountering ants of their own species. Moreover, when groups of such ants from distinct nests were brought into contact with each other, behaviour observed was not particularly aggressive. In fact the workers rapidly began to "fraternise" and, after 5 minutes, it was no longer possible to tell which ants originated from which nests. In contrast, confrontation with Pheidole megacephala elicited an extremely aggressive response in the little fire ants which often led to the death of their adversary.

A chemical recognition process enables New Caledonian Wasmannia to distinguish ants of their species and switches off all possible aggressive mechanisms between them. In the same nest, individuals continually exchange chemical substances from their cuticle surface (1). In this way they acquire a common chemical make-up. The ants characteristically have this "cuticular odour" which enables workers to recognise members of their particular colony and immediately to trigger an attack if an individual from outside their colony is present.

Colonies' cuticular odour is influenced by the kind of substrate in which they have settled and also by the ants' diet. However, acquiring a chemical identity under the particular environment's influence would have required longer than it has taken Wasmannia to spread in New-Caledonia. Scattered rapidly there by human activity, the separate Wasmannia populations have not had time each to develop their own chemical identity.

Monocolonialism can therefore partly explain such a spectacular expansion of the little fire ant. Future research, bringing together both genetic and chemical analyses, should help identify the process more finely by evaluating the degree of relationship between the different nests of little fire ants.

(1) The cuticle is a rigid impermeable layer covering the ant's body. It consists of cutin and proteins.

Institut de Recherche pour le Développement, Paris (IRD)