Honey bees and wild bees are important pollinators and contribute to higher yields in agriculture and to the biodiversity of the natural fauna. However, globalization and industrialization of bee colonies lead to increasing infection pressure. Global trade and transport of colonies for maximizing the yield of a specific crop (e.g. almonds in the USA) leads to stress: Quick change of environments and environmental influences, no time to adapt, one-sided nutrition and high population densities of bee colonies from different parts of the world resulting in an interchange of diseases and parasites between the beehives, but also with the local pollinators. The most severe example is the mite Varroa destructor, which originates from Asia and is now found worldwide. Likewise, the pathogens that it vectors, e.g. deformed wing virus (DWV) affect the health of the bee colonies. Also from Asia is the originally Apis cerana parasitising honeybee pathogenic microsporidium N. ceranae, which parasitises Western honey bees Apis mellifera worldwide. Although it is one of the most common infections, publications on its epidemiology still seem to be contradictory. On the one hand N. ceranae is considered to be more virulent than N. apis and an emergent and deadly pathogen, on the other hand it has been described that there is no significant correlation between N. ceranae infection and colony losses. Within my dissertation I addressed several of these contradictions and tried to shed light on ambiguities. I investigated the prevalence of N. ceranae in Northeast Germany and was able to describe a seasonality, which has been ascribed only to N. apis before. Furthermore, I investigated the influence of N. ceranae infections occurring in autumn on the colony status of the spring of the following year and calculated its biological effect size. Finally, I wanted to investigate the so far unknown parameters of an N. ceranae infection in bumblebees (Bombus terrestris), which led to the conclusion that, contrary to numerous publications, bumblebees are not true hosts of N. ceranae.
