Social contact with fungus-exposed ants leads to pathogen transfer to healthy nest-mates, causing low-level infections. These micro-infections promote pathogen-specific immune gene expression and protective immunization of nest-mates.

To fight infectious diseases, host immune defenses are employed at multiple levels. Sanitary behavior, such as pathogen avoidance and removal, acts as a first line of defense to prevent infection before activation of the physiological immune system. Insect societies have evolved a wide range of collective hygiene measures and intensive health care toward pathogen-exposed group members. One of the most common behaviors is allogrooming, in which nestmates remove infectious particles from the body surfaces of exposed individuals. Here we show that, in invasive garden ants, grooming of fungus-exposed brood is effective beyond the sheer mechanical removal of fungal conidiospores; it also includes chemical disinfection through the application of poison produced by the ants themselves. Formic acid is the main active component of the poison. It inhibits fungal growth of conidiospores remaining on the brood surface after grooming and also those collected in the mouth of the grooming ant. This dual function is achieved by uptake of the poison droplet into the mouth through acidopore self-grooming and subsequent application onto the infectious brood via brood grooming. This extraordinary behavior extends the current understanding of grooming and the establishment of social immunity in insect societies.

In social groups, infections have the potential to spread rapidly and cause disease outbreaks. Here, we show that in a social insect, the ant Lasius neglectus, the negative consequences of fungal infections (Metarhizium brunneum) can be mitigated by employing an efficient multicomponent behaviour, termed destructive disinfection, which prevents further spread of the disease through the colony. Ants specifically target infected pupae during the pathogen’s non-contagious incubation period, utilising chemical ‘sickness cues’ emitted by pupae. They then remove the pupal cocoon, perforate its cuticle and administer antimicrobial poison, which enters the body and prevents pathogen replication from the inside out. Like the immune system of a metazoan body that specifically targets and eliminates infected cells, ants destroy infected brood to stop the pathogen completing its lifecycle, thus protecting the rest of the colony. Hence, in an analogous fashion, the same principles of disease defence apply at different levels of biological organisation.