<i>Lasius flavus</i> 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 <i>Lasius flavus</i> cares for the eggs of the root aphid <i>Anoecia zirnitsi</i> and how this service affects the resistance of aphid eggs to predators and pathogens. In winter, <i>A. zirnitsi</i> eggs were found exclusively in <i>L. flavus</i> nest chambers near the ant brood. Laboratory experiments showed that <i>L. flavus</i> detects, transports, piles and grooms the aphid eggs. We could recapitulate these caring behaviours in <i>L. flavus</i> using glass beads coated with chemical cues extracted from the aphid egg surface. Other ant species did not collect or nurse the eggs, suggesting a specific interaction between <i>L. flavus</i> and the eggs of <i>A. zirnitsi</i>. We further demonstrated that <i>L. flavus</i> 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 <i>L. flavus</i> ants provide to root aphids in winter, while the potential costs and delayed benefits (honeydew provision) of this protection for the ants should be further explored.