Apicystis bombi, a neogregarine generalist parasite, has a different virulence in different bee genera This differential impact is linked to the bee host defence mechanisms These defence mechanisms range from resistance (keeping parasite loads in balance) to tolerance (reducing the negative impact of the infection) A tolerant host could unbalance the host-parasite dynamics of a sensitive host In extreme cases this can lead to local or total extinction of a host species Hence, in this project, we aim to use A bombi as a model for generalist parasites to study host defence mechanisms in different bee species The bee hosts are an example of a threatened population, and their biodiversity is essential as insect pollination contributes to the seed set of 87% of all plant species This project is divided into three work packages (WP) In WP1, biological assays will be developed to measure bee tolerance and resistance to parasite infection The mechanisms will be studied on an individual level for mason bees (Genus Osmia, harbouring solitary species), honey bees and bumble bees (Genus Apis and Bombus, harbouring social species) WP2 will investigate, at the colony level, how social behaviour and associated immunity will influence tolerance and resistance strategies Finally, WP3 will study the foraging behaviour of sensitive and tolerant bees, and also verify if sensitive bees can avoid parasite contaminated flowers