First Report on the Acrobat Ant Crematogaster scutellaris Storing Live Aphids in Its Oak-Gall Nests.

Abstract

Simple SummaryGalls represent an amazing microcosm which contains a variety of multiple interactions among different actors, and therefore, offers the opportunity to observe and investigate phenomena belonging to different areas of biology: from the development process, connected to the interaction between the galligenous agent and the host plant, to the moment of their colonization by different species, since some ants may provide defense against pathogens, certain phytophagous insects or favor mutualists. In the present work we describe some aspects of oak-gall colonization by different ant species, highlighting how the gall’s height on the plant influences ant colonization and how different ant species produce different nest architectures. The most relevant aspect, however, is the discovery of a novel ant-aphid relationship: the transport of living aphids into oak-gall nests. We found no evidence of immediate predation of these aphids inside the galls, so they are likely stored to overwinter due to a mutualistic relationship and/or serve as food storage. This is not only an interesting report on the mutualisms involving ants and their insect partners, but it may also have important consequences on the aphids’ phenology with the host plants. Once more, ants show their relevant impact on multitrophic interactions and ecosystem dynamics.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 which were located higher on the oak trees, while C. truncata and T. italicus were located at lower positions. In addition, galls occupied by C. scutellaris showed varied 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 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.