Lasius flavus ants protect root aphid eggs from predators and pathogens during winter hibernation

Cooperative brood care is key to the ecology and evolution of social
insects. Interestingly, social insects may also care for the brood of
other species that dwell in their nests. This study explores how the
yellow meadow ant Lasius flavus cares for the eggs of the root aphid
Anoecia zirnitsi and how this service affects the resistance of aphid eggs
to predators and pathogens. In winter, A. zirnitsi eggs were found
exclusively in L. flavus nest chambers near the ant brood. Laboratory
experiments showed that L. flavus detects, transports, piles, and grooms
the aphid eggs. We could recapitulate these caring behaviours in L. flavus
using glass dummies coated with chemical cues extracted from the aphid egg
surface. Other ant species did not collect nor nurse the eggs, suggesting
a specific interaction between L. flavus and A. zirnitsi. We further
demonstrated that L. flavus strongly increased the aphid eggs'
protection against predators and fungal pathogens. Ants, however, were not
essential for the eggs to hatch and aphid nymphs were capable of
independently colonizing grass roots. Our research highlights the crucial
protection services ants provide to root aphids, while the potential
benefits of these heterospecific interactions to ants remain unclear.