Pollinators are declining globally and emerging pathogens are among the causative factors. Pathogens are shared between domesticated honey bees and wild pollinators with flowers as transmission hubs. In contrast to honey bees, pathogen impacts in wild bees remain mostly unstudied. This thesis aimed to investigate associations between honey bees infected with Nosema ceranae and wild bees in the field as well as direct effects in a solitary wild bee experimentally inoculated with N. ceranae. Chapter 1 used a field survey approach to test how the presence of honey bees infected with N. ceranae affects the community composition of wild bees sharing the habitat. The results demonstrate a negative correlation between functional diversity of wild bees and the presence of infected honey bees. However, this relationship differs between functional groups. The occurrence of the rare parasitic, the cavity-nesting ones and also the Red List species is negatively associated with pathogen presence in the surrounding honey bee hives whereas the opposite is shown for common social-polylectic species. Furthermore, a habitat analysis revealed that both the highly specialized solitary-oligolectic and the threatened Red List species were particularly dependent on high coverage of suitable foraging and nesting habitat. In conclusion, sensitivity to N. ceranae associated stressors has to be considered differentially for particular functional groups which argue for the support of a functionally diverse pollinator community and special arrangements for the most severely affected groups. In Chapter 2, the agriculturally managed wild bee Osmia bicornis was used as model for solitary bees in an inoculation experiment with N. ceranae. The results revealed that inoculation had negligible impacts only on male survival. Several different species of wild bees that shared floral resources with infected honey bees in the environment were screened for the pathogen and were shown to be widely positive for N. ceranae whereas the pathogen was not detected in wild bees collected at another field site with uninfected honey bees. O. bicornis could therefore be acting as a pathogen reservoir and vector enhancing the circulation of N. ceranae within the pollinator network which affects both managed and wild bees. Larvae of O. bicornis were exposed to spores of N. ceranae during development in Chapter 3. Exposed individuals were characterized by a greater mortality and a delay in the onset of pupation even though very few spores were detected in a few individuals. Exposure was also associated with a reduction in body size in males as well as an increase in head capsule width in both sexes. The individuals which were exposed to the pathogen as larvae did not show any fitness costs after hatching. The results indicate that even without proliferation, N. ceranae can have sublethal effects on particular life history stages of O. bicornis and affect development in this solitary bee model. Collectively, the results suggest indirect and direct impacts of N. ceranae on solitary wild bees but have to be extrapolated with caution regarding species-specific differences in pathogen impacts. Furthermore, in the environment, multiple stressors are afflicting wild bees and can interact synergistically. The present thesis delivers useful starting points for further inoculation experiments with a suitable solitary wild bee model and important information regarding pathogen reservoir species within the pollinator community. In conservation management, special support for particularly sensitive functional groups and life stages is required.
