Effects of the ectoparasitic fungus Rickia wasmannii on its ant host Myrmica scabrinodis: changes in host mortality and behavior

Social animals like ants, although seemingly well-armed against parasites because of the protective nature of the social system, are nevertheless exposed to several parasitic organisms, among them fungi of the order Laboulbeniales (Ascomycetes). Rickia wasmannii is one such ectoparasitic fungus that lives on different species of the ant genus Myrmica in Europe. The fungus attaches to the outer layer of the cuticle, while appearing on the surface of the hosts as a setae-like structure. There is little information on the effects of the fungi on the host. Usually it is regarded as neutral, in spite of its high abundance on the ants. We studied the impact of a R. wasmannii infection on the lifespan and grooming behavior of its main host M. scabrinodis in artificial nests in laboratory conditions for 2 months. The lifespan of infected individuals was significantly reduced in comparison with the lifespan of uninfected ants. The frequency of allogrooming was significantly higher in infected nests, while only a marginally significant positive effect could be detected in the case of autogrooming. Increased grooming may either be an effective aspect of host defense or, alternatively, a pathogen-induced behavioral change that enhances pathogen transmission. This study thus provides the first documented evidence of the potential effects of R. wasmannii on its ant hosts.