Ant attendance does not necessarily imply protection of aphids from their arthropod natural enemies

Many ants and hemipterans are bound by a mutualistic relationship (trophobiosis) which represents an ecological keystone: ants receive food (honeydew) providing different kinds of protection. Partner protection against arthropod natural enemies is considered to be frequent and to interfere with biological control strategies of hemipteran pests. We carried out an ant-exclusion field experiment in a fava bean (Vicia faba) plantation in Italy to monitor the abundance and behaviour of the ants (Plagiolepis pygmaea, Tetramorium semilaeve and the exotic Nylanderia jaegerskioeldi) and their relationship with facultative mutualist (Aphis fabae) and non-mutualist (Megoura viciae) aphids, the arthropod natural enemies of the aphids, and extrafloral nectaries. Ants concentrated their activity on the attendance of facultative mutualist aphids much more than on the extrafloral nectaries. The ant-exclusion treatment had no effect on the abundance of M. viciae and on the parasitization rate of A. fabae, while it reduced the abundance of A. fabae, aphid predators, and aphid parasitoids. Our results demonstrate that ant attendance does not imply the protection of aphids from arthropod natural enemies and suggest that the identity of the ant species involved is important for the outcome. As relatively few species have been studied in this regard, extending our knowledge to the role of more ant species is desirable to understand the ecology and evolution of ant-aphid mutualisms and to refine integrated control strategies of aphid pests.