Competition for mutualists in an ant-homopteran interaction mediated by hierarchies of ant attendance

Workers of the ant species Lasius niger showed marked preferences when collecting honeydew from three aphid species living on tansy, Tanacetum vulgare. Colonies of Metoperum fuscoviride were visited most intensively. Brachycaudus cardui had an intermediate position, and colonies of Aphis. fabae were visited least intensively. L. niger abandoned colonies of the less preferred species as soon as colonies of a higher-ranked species were available on neighbouring tansy plants. As a consequence, the abandoned colonies suffered a higher predation by aphid predators and persisted for a shorter time than attended colonies of the same species. If two aphid species were present in a mixed colony on the same !;hoot, the preferred species increased in numbers, while the less preferred species decreased due to predation by L. niger. The results show that there is a competition for the mutualistic services of ants between aphid species exploiting the same resource. The outcome of this competition depends on the aphid's selective attractiveness to ants, which corresponded with already reported differences in honeydew quality and quantity.