Towards more intimacy: moderate elevation of temperature drives increases in foraging and mutualistic interactions between Lasius niger and Aphis fabae

1. Climate change will likely affect the association between species interacting at different trophic levels. However, studies focusing on the impact of an elevation of temperature on ant-hemipteran mutualism remain scarce. 2. In the present study we investigated, in laboratory conditions, the foraging behaviour of the ant Lasius niger and its mutualistic interaction with the aphid Aphis fabae under three conditions of temperatures (i.e. 20°C, 23°C, and 26°C), as predicted by climatic scenarios. 3. Aphids were more mobile but as likely to release honeydew droplets at higher temperatures. As regards the ants, a moderate 3°C increase of temperature positively impacted their mutualistic interaction with aphids. Such reinforcement was achieved through an increase in the walking speed of ant foragers, in the number of mobilised ants as well as in the total amount of honeydew harvested by the colony. 4. A further elevation of temperature to 26°C reduced the benefits gained by the aphid-tending ants, in terms of lower amount of collected honeydew. 5. Based on our results, we hypothesise that, in temperate regions, a moderate increase of ambient atmospheric temperature by 3°C will benefit to the L. niger - A. fabae mutualistic interaction. A more marked elevation of 6°C may represent a threshold thermal value above which a switch of partners or a disruption of the interaction may occur under the temperatures predicted for the end of the century. These results demonstrate the fragility of mutualistic interactions, and more particularly their sensitivity to temperature increases predicted by realistic forecast models.