First report of the acrobat ant Crematogaster scutellaris storing live aphids inside its oak-gall nests

This study provides new data about the role of ants in mutualistic interactions with aphids mediated by galls. We focused our investigation on galls induced by the cynipid Andricus kollari by conducting a survey and a subsequent experiment in an Italian oak forest. The ants Crematogaster scutellaris, Colobopsis truncata and Temnothorax italicus frequently used the galls as nests: Crematogaster scutellaris occupied galls located higher on oak trees, while C. truncata and T. italicus those at lower positions. In addition, galls occupied by C. scutellaris, varied their internal architecture in relation to the colony composition. Importantly, field surveys revealed for the first time that C. scutellaris nest galls also contained live individuals of the non-galligenous aphid Panaphis juglandis. Field experiments suggested that the ants actively seek, collect and stock the live aphids. No signs of predation and injuries were detected on the stored aphids that were probably kept for safe overwintering, although we cannot exclude a possible occasional use as food. This report reveals a possible novel relationship which could have important consequences on the phenology and presence of aphids on the host plant, highlighting once again the impact of ants on insect-plant multitrophic dynamics in both natural and agricultural ecosystems.