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1. Pollen diet, more than geographic distance, shapes provision microbiome composition in two species of cavity-nesting bees

   https://doi.org/10.1093/femsec/fiaf067  

   Vannette, Rachel_L; Williams, Neal_M; Peterson, Stephen_S; Martin, Alexia_N (July 2025, FEMS Microbiology Ecology)

   Abstract The microbial composition of stored food can influence its stability and the microbial species consumed by the organism feeding on it. Many bee species store nectar and pollen in provisions constructed to feed developing offspring. Yet, whether microbial composition is determined by the pollen types within provisions, variation between bee species at the same nesting sites, or geographic distance was unclear. Here, we sampled two species of cooccurring cavity nesting bees in the genus Osmia at 13 sites in California and examined the composition of pollen, fungi, and bacteria in provisions. Pollen composition explained 15% of variation in bacterial composition and ∼30% of variation in fungal composition, whereas spatial distance among sites explained minimal additional variation. Symbiotic microbe genera Ascosphaera, Sodalis, and Wolbachia showed contrasting patterns of association with pollen composition, suggesting distinct acquisition and transmission routes for each. Comparing provisions from both bee species comprised of the same pollens points to environmental acquisition rather than bee species as a key factor shaping the early stages of the bee microbiome in Osmia. The patterns we observed also contrast with Apilactobacillus-dominated provision microbiome in other solitary bee species, suggesting variable mechanisms of microbial assembly in stored food among bee species. 

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2. Diet Breadth Affects Bacterial Identity but Not Diversity in the Pollen Provisions of Closely Related Polylectic and Oligolectic Bees

   https://doi.org/10.3390/insects11090645  

   Rothman, Jason A.; Cox-Foster, Diana L.; Andrikopoulos, Corey; McFrederick, Quinn S. (September 2020, Insects)

   null (Ed.)

   Mounting evidence suggests that microbes found in the pollen provisions of wild and solitary bees are important drivers of larval development. As these microbes are also known to be transmitted via the environment, most likely from flowers, the diet breadth of a bee may affect the diversity and identity of the microbes that occur in its pollen provisions. Here, we tested the hypothesis that, due to the importance of floral transmission of microbes, diet breadth affects pollen provision microbial community composition. We collected pollen provisions at four sites from the polylectic bee Osmia lignaria and the oligolectic bee Osmia ribifloris. We used high-throughput sequencing of the bacterial 16S rRNA gene to characterize the bacteria found in these provisions. We found minimal overlap in the specific bacterial variants in pollen provisions across the host species, even when the bees were constrained to foraging from the same flowers in cages at one site. Similarly, there was minimal overlap in the specific bacterial variants across sites, even within the same host species. Together, these findings highlight the importance of environmental transmission and host specific sorting influenced by diet breadth for microbes found in pollen provisions. Future studies addressing the functional consequences of this filtering, along with tests for differences between more species of oligoletic and polylectic bees will provide rich insights into the microbial ecology of solitary bees. 

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3. Solitary bee larvae modify bacterial diversity of pollen provisions in the stem-nesting bee, Osmia cornifrons (Megachilidae)

   https://doi.org/10.3389/fmicb.2022.1057626  

   Kueneman, Jordan G.; Gillung, Jessica; Van Dyke, Maria T.; Fordyce, Rachel F.; Danforth, Bryan N. (January 2023, Frontiers in Microbiology)

   Microbes, including diverse bacteria and fungi, play an important role in the health of both solitary and social bees. Among solitary bee species, in which larvae remain in a closed brood cell throughout development, experiments that modified or eliminated the brood cell microbiome through sterilization indicated that microbes contribute substantially to larval nutrition and are in some cases essential for larval development. To better understand how feeding larvae impact the microbial community of their pollen/nectar provisions, we examine the temporal shift in the bacterial community in the presence and absence of actively feeding larvae of the solitary, stem-nesting bee, Osmia cornifrons (Megachilidae). Our results indicate that the O . cornifrons brood cell bacterial community is initially diverse. However, larval solitary bees modify the microbial community of their pollen/nectar provisions over time by suppressing or eliminating rare taxa while favoring bacterial endosymbionts of insects and diverse plant pathogens, perhaps through improved conditions or competitive release. We suspect that the proliferation of opportunistic plant pathogens may improve nutrient availability of developing larvae through degradation of pollen. Thus, the health and development of solitary bees may be interconnected with pollen bacterial diversity and perhaps with the propagation of plant pathogens. 

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4. Diverse Diets with Consistent Core Microbiome in Wild Bee Pollen Provisions

   https://doi.org/10.3390/insects11080499  

   Dew, Rebecca M.; McFrederick, Quinn S.; Rehan, Sandra M. (August 2020, Insects)

   null (Ed.)

   Bees collect pollen from flowers for their offspring, and by doing so contribute critical pollination services for our crops and ecosystems. Unlike many managed bee species, wild bees are thought to obtain much of their microbiome from the environment. However, we know surprisingly little about what plant species bees visit and the microbes associated with the collected pollen. Here, we addressed the hypothesis that the pollen and microbial components of bee diets would change across the range of the bee, by amplicon sequencing pollen provisions of a widespread small carpenter bee, Ceratina calcarata, across three populations. Ceratina calcarata was found to use a diversity of floral resources across its range, but the bacterial genera associated with pollen provisions were very consistent. Acinetobacter, Erwinia, Lactobacillus, Sodalis, Sphingomonas and Wolbachia were among the top ten bacterial genera across all sites. Ceratina calcarata uses both raspberry (Rubus) and sumac (Rhus) stems as nesting substrates, however nests within these plants showed no preference for host plant pollen. Significant correlations in plant and bacterial co-occurrence differed between sites, indicating that many of the most common bacterial genera have either regional or transitory floral associations. This range-wide study suggests microbes present in brood provisions are conserved within a bee species, rather than mediated by climate or pollen composition. Moving forward, this has important implications for how these core bacteria affect larval health and whether these functions vary across space and diet. These data increase our understanding of how pollinators interact with and adjust to their changing environment. 

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5. Environmentally acquired gut-associated bacteria are not critical for growth and survival in a solitary bee, Megachile rotundata

   https://doi.org/10.1128/aem.02076-23  

   Brar, Gagandeep; Floden, Madison; McFrederick, Quinn; Rajamohan, Arun; Yocum, George; Bowsher, Julia (September 2024, Applied and Environmental Microbiology)

   Tortosa, Pablo (Ed.)

   ABSTRACT Social bees have been extensively studied for their gut microbial functions, but the significance of the gut microbiota in solitary bees remains less explored. Solitary bee,Megachile rotundatafemales provision their offspring with pollen from various plant species, harboring a diverse microbial community that colonizes larvae guts. TheApilactobacillusis the most abundant microbe, but evidence concerning the effects ofApilactobacillusand other provision microbes on growth and survival are lacking. We hypothesized that the presence ofApilactobacillusin abundance would enhance larval and prepupal development, weight, and survival, while the absence of intact microbial communities was expected to have a negative impact on bee fitness. We reared larvae on pollen provisions with naturally collected microbial communities (Natural pollen) or devoid of microbial communities (Sterile pollen). We also assessed the impact of introducingApilactobacillus micheneriby adding it to both types of pollen provisions. Feeding larvae with sterile pollen +A. micheneriled to the highest mortality rate, followed by natural pollen +A. micheneri, and sterile pollen. Larval development was significantly delayed in groups fed with sterile pollen. Interestingly, larval and prepupal weights did not significantly differ across treatments compared to natural pollen-fed larvae. 16S rRNA gene sequencing found a dominance ofSodalis, whenA. micheneriwas introduced to natural pollen. The presence ofSodaliswith abundantA. micheneri suggests potential crosstalk between both, shaping bee nutrition and health. Hence, this study highlights that the reliance on nonhost-specific environmental bacteria may not impact fitness ofM. rotundata.IMPORTANCEThis study investigates the impact of environmentally acquired gut microbes of solitary bee fitness with insights into the microbial ecology of bee and their health. While the symbiotic microbiome is well-studied in social bees, the role of environmental acquired microbiota in solitary bees remains unclear. Assessing this relationship in a solitary pollinator, the leaf-cutting bee,Megachile rotundata, we discovered that this bee species does not depend on the diverse environmental bacteria found in pollen for either its larval growth or survival. Surprisingly, high concentrations of the most abundant pollen bacteria, Apilactobacillus micheneridid not consistently benefit bee fitness, but caused larval mortality. Our findings also suggest an interaction betweenApilactobacillusand theSodalisand perhaps their role in bee nutrition. Hence, this study provides significant insights that contribute to understanding the fitness, conservation, and pollination ecology of other solitary bee species in the future. 

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