Nematode infections induce distinct chemical signatures and provoke aggression in ants.

Maintaining group integrity by excluding outsiders and pathogens is a fundamental requirement and challenge of social living. In social insects, these defences rely heavily on chemical communication, with cuticular hydrocarbons (CHCs) mediating both nestmate recognition and infection-related social responses. In ants, CHCs are stored and homogenized in the pharyngeal gland, contributing to the formation of a shared colony odour that enables nestmate recognition. We investigated the behavioural and chemical responses of the clonal raider ant Ooceraea biroi to infections by the nematode Diploscapter sp., which specifically infects the pharyngeal gland. Using behavioural and chemical analyses, we show that: (i) infected ants elicit increased aggression from uninfected nestmates and non-nestmates, consistent with a social immune defence that may limit parasite entry in colonies; (ii) aggression correlates with infection-specific changes in CHC profiles; and (iii) infections do not compromise nestmate recognition, which mostly acts through CHCs different from those altered by infection. Thus, while many parasitic nematodes can evade host immune recognition, Diploscapter fails to evade the social recognition system of its host. Instead, efficient detection of and aggression towards infected individuals likely reduce parasite introduction and transmission both within and between colonies.