Seasonally increasing parasite load is associated with microbiota dysbiosis in wild bumblebees.

Gut microbiota often influence host defense against infection, but this relationship is incompletely understood in wild bumblebees. These critical pollinators host a characteristic core gut microbiota, yet field studies have offered conflicting insights into its association with Crithidia bombi, a prevalent trypanosomid parasite. To address this gap in our knowledge, we conducted an 3 year field survey, profiling the gut microbiota of 638 bumblebees from 9 sympatric species across diverse sites in Maine using 16S rRNA amplicon sequencing and qPCR for C. bombi detection. We confirmed a robust core bumblebee microbiota, identifying novel host-specific phylogenetic associations of bacterial amplicon sequence variants even among closely related host species. C. bombi infection was common and showed significant seasonal increases. We also found spatial variation, with higher prevalence in coastal regions. Crucially, increasing C. bombi infection load was consistently associated with microbiome dysbiosis. This dysbiosis was characterized by a depletion of core bumblebee-associated microbial taxa, notably Apibacter and Gilliamella (previously shown to be protective), and a corresponding increase in opportunistic, environmentally derived microbes like Entomomonas. While the core microbiota's association with initial pathogen transmission appears minor, its depletion in severe infections strongly supports a correlation to host health in wild bumblebees. IMPORTANCE: The community of microorganisms in close association with an animal, its microbiota, can be important to its health. Understanding how microbiota composition relates to health and disease is an important goal with broad potential implications. Like most animals, bumblebees have a characteristic core gut microbiota. We have conducted a broad survey of bumblebees over 3 years to examine the interactions of microbiota composition with infection by an endemic trypanosomatid parasite. We found that the relative abundances of core microbes were inversely related to infection load, and that increased pathogen load was associated with the prevalence of novel microbes. These results are evidence of strong associations between bumblebees and their core microbiota and suggest a role in providing resistance to severe parasitism.