Bacteria may contribute to distant species recognition in ant–aphid mutualistic relationships

Mutualistic interactions between ant and aphid species have been the subject of considerable historical and contemporary investigations, the primary benefits being cleaning and protection for the aphids and carbohydrate-rich honeydew for the ants. Questions remained, however, as to the volatile semiochemical factor influencing this relationship. A recent study highlighted the role of bacterial honeydew volatile compounds in ant attraction. Here, ant’s ability to distantly discriminate two aphid species was investigated based on bacterial honeydew semiochemicals emissions using a two-way olfactometer. Both the mutualistic black bean aphid (Aphis fabae L.) and the non-myrmecophilous pea aphid (Acyrthosiphon pisum Harris) were found to be attractive for the black garden ant (Lasius niger L.). The level of attraction was similar in both assays (control versus one of the aphid species). However, when given a choice between these two aphid species, ants showed a significant preference for Aphis fabae. Honeydew volatiles, mostly from bacterial origins, are known to be a key element in ant attraction. Using the same olfactometry protocol, the relative attractiveness of volatiles emitted by honeydews collected from each aphid species and by bacteria isolated from each honeydew was in investigated. Again, ants significantly preferred volatiles released by Aphis fabae honeydew and bacteria. This information suggests that microbial honeydew volatiles enable ants to distantly discriminate aphid species. These results emphasize the importance of investigating the presence and potential effects of microbes in insect symbioses.