The general topic of my thesis concerns the world of invisible microorganisms or, more specifically, that of endophytic fungi. Endophytic fungi live asymptomatically within the tissues of many plant species. In association with cool-season grasses, these fungi produce herbivore toxic alkaloids. Originally, this symbiosis was assumed to be truly mutualistic, with the fungus obtaining nutrients and shelter from the plant whereas the plant gains
protection from herbivores. This concept of a mutualistic relationship was mainly based on studies focusing on the performance of single herbivore species in the presence of
endophytes. Natural systems, however, consist of more than plants and herbivores. Thus, including higher trophic levels and using a multitrophic level approach allows us to
estimate the effects endophytes may have on complex consumer interaction webs of natural systems. Within multi-species interactions, there are not only direct but also
indirect effects to consider. To advance knowledge of the mechanisms by which endophytes and, more generally, variation in plant quality, influence multitrophic interactions and thus community interactions and structure, I investigated in detail the impacts of the interplay of these microorganisms with species at higher trophic levels in an aphid-parasitoid model system.