| Summary: | 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 <i>Andricus kollari</i> by conducting a survey and a subsequent experiment in an Italian oak forest. The ants <i>Crematogaster scutellaris</i>, <i>Colobopsis truncata</i> and <i>Temnothorax italicus</i> frequently used the galls as nests: <i>Crematogaster scutellaris</i> occupied galls which were located higher on the oak trees, while <i>C. truncata</i> and <i>T. italicus</i> were located at lower positions. In addition, galls occupied by <i>C. scutellaris</i> showed varied internal architecture in relation to the colony composition. Importantly, field surveys revealed for the first time that <i>C. scutellaris</i> nest galls also contained live individuals of the non-galligenous aphid <i>Panaphis juglandis</i>. Field experiments suggested that the ants actively seek, collect and stock live aphids. No signs of predation and injuries were detected on the stored aphids, which were probably kept for safe overwintering, though 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.
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