Most of my work focuses on parasitic or commensalistic ant associates. However, ants also engage with different partners that provide mutualistic services. Well known are aphids on plants and trees that secrete sugary honeydew in return for protection and cleaning services. Also mutualistic fungi and plants that provide shelter or food bodies are classical examples of mutualistic ant partners. Poorly known is that belowground on the roots of different plant species aphids form also very intimate mutualistic associations with their ant partner. Ants often tend these root aphids on roots that grow in or near their nest. In temperate Europe, the yellow meadow ant Lasius flavus is almost completely dependent on root aphids which are kept in high numbers in nest chambers built around herbaceous and grass roots. The root aphids gregariously feed on the sap in the roots and secrete droplets of sugary honeydew. This honeydew appears to be the main food source of a L. flavus colony. Different species of obligatory ant-associated root aphids co-occur in a L. flavus nest. These obligatory ant-associated root aphids evolved to a life in strict association with their ant host which was accompanied by behavioral (e.g., retracting appendages before transport) and morphological adaptations. The association between root aphids and L. flavus is extremely intimate. The root aphids are licked, cleaned, and are also carried around by the L. flavus host when the nest is opened.
Several studies already compared the ant visitation preference for different co-occurring aboveground aphids. Highly visited aphids are typically better protected against enemies. Carried to safety or to food plants by the host may confer much higher benefits for the aphids than mere visitation and enemy deterrence, I compared for the first time this strong mutualistic behavior, by comparing the rate of transport to the nest of 6 root aphid species (5 obligatory myrmecophilous, one loosely assofciated) by the host ant L. flavus. In addition, I compared their retrieval rate with those of the host’s own larvae, enabling us to assess whether they prefer some aphid partners over their own kin.
All associated root aphids were carried back to the nest, but in a clear preferential hierarchy. Geoica utricularia, Forda Formicaria, and Trama rara were rapidly transported, but slower than the own larvae. Tetraneura ulmi and Geoica setulosa were collected at a moderate rate and the loosely associated Aploneura lentisci was slowly retrieved. In contrast, different species of unassociated aphids were not transported and even provoked aggressive behavior in L. flavus. This study revealed that co-occurring symbionts may induce different degrees of host attraction, which ultimately may affect the coexistence and assembly of ant-symbiont communities.
Choice experiment with 6 root aphids and ant larvae
read more:
PARMENTIER T (2023) Differential transport of a guild of mutualistic root aphids by the ant Lasius flavus. Current Zoology 69: 409–417. https://doi.org/10.1093/cz/zoac060.
Many aphids lay eggs on plants and in crevices when temperatures drop. However, I occassionally find black aphid eggs inside L. flavus nests during winter. These eggs are carefully stored in piles near the ant brood piles. The aphid eggs are, like the root aphids, transported and brought into safety upon disturbance. So the winter storage of aphid eggs within the protective core of the Lasius nest, appears to be a very intimate mutualistic interaction.
When eggs hatch in spring, Anoecia aphid nymphs emerge. Currently, we try to unravel the chemical cues that elicit the transport behavior. I was able to transfer the cues to glass beads (see transported glass beads).