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Ecological Factors Shaping Nosema and Crithidia Prevalence in Coastal Sage Scrub Bee and Ant Communities


Pathogens are one of the major factors attributed to bee species declines. Flowers can serve as hubs for bee pathogens and shared floral resources mediate the acquisition of pathogens from infected to healthy hosts. While previous research places emphasis on pathogen spillover from managed bees to wild bee species, there is a paucity of knowledge as to the role of non-bee floral visitors, such as ants, in shaping pathogen prevalence for bees. Here, we document the prevalence of two common bee pathogens, Nosema spp. and Crithidia spp., among honey bees, native bees, and ants at an ecological reserve in Southern California encompassing threatened coastal sage scrub habitat to understand how ecological factors, such as space and time, shape host-parasite interactions for these insect taxa. Nosema and Crithidia were detected in honey bees, native bees, and ants. Surprisingly, both pathogens were highly prevalent in ants. The interaction of sampling period x insect taxa had the most pronounced effects in influencing prevalence for both pathogens. Plant-host associations did not appear to be important in shaping pathogen prevalence for honey bees and native bees. This is the first study, to our knowledge, to detect Nosema and Crithidia in ants and several native bee species. We discuss two potential mechanisms, shared floral resources and consumption of infected prey, that may explain pathogen transmission between bees and ants. Ants may serve as a previously undescribed reservoir for Nosema and Crithidia and future research should define the potential for pathogen spillback from ants back into bee populations.

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