Prevalence and impact of a virulent parasite on a tripartite mutualism

The prevalence and impact of a specialized microfungal parasite (Escovopsis ) that infects the fungus gardens of leaf-cutting ants was examined in the laboratory and in the field in Panama. Escovopsis is a common parasite of l eaf-cutting ant colonies and is apparently more frequent in Acromyrmex spp. gardens than in gar dens of the more phylogenetically derived genus Atra s pp. In addition, larger colonies of Atta spp. appear to be less frequently infected with the parasite. In this study, the parasite Escovopsis had a ma jor impact on the success of this mutualism among ants, fungi, and bacteria . Infected colonies had a significantly lower rate of fungus garden accumul ation and produced substantially fewer workers. In addition, the extent of the reduction in colony growth rate depended on the isolate, with one isola te having a significantly larger impact than two others, suggesting that Es covopsis has different levels of virulence. Escovopsis is also spatially co ncentrated within parts of ant fungus gardens, with the younger regions hav ing significantly lower rates of infection as compared to the older regions . The discovery that gardens of fungus-growing ants are host to a virulent pathogen that is not related to any of the three mutualists suggests that u nrelated organisms may be important but primarily over-looked components of other mutualistic associations.