We study fungi that live in association with insects. We use observational and experiment work to understand these interactions. One particular model that we study is the fungus-growing (attine) ants, that cultivate fungi for food. Colonies of these ants are threaten by parasitic fungi in the genus Escovopsis, which consumes the fungus cultivated by the ants. We are unravelling the diversity of this parasite (and other fungi) as well as understanding its evolutionary adaptations to parasitize the fungal cultivar.