A trail pheromone mediates the mutualism between ants and aphids

Mutualisms, like the ones between ants and aphids, evolve and persist when benefits outweigh the costs from the interactions between the partners. We show here that the trail pheromone of the red imported fire ant, Solenopsis invicta, can enhance these benefits by suppressing aphid walking dispersal and stimulating their reproduction. The cotton aphid, Aphis gossypii, was found to readily perceive and respond to two specific trail pheromonal components. Both Z,E-a-farnesene and E,E-a-farnesene suppressed walking dispersal of apterous aphids, whereas only Z,E-a-farnesene also increased aphid reproduction rate. This inter-species function of the trail pheromone increases and prolongs the availability of honeydew as a key food source for the ants. The aphid colony, meanwhile, benefits from faster population growth and continuous ant-provided protection. These findings reveal a novel mechanism by which ants and aphids both increase the benefits that they provide to each other, thereby likely enhancing the stability of their mutualistic relationship.