For a small carpenter bee, a visit to a flower may involve more than collecting pollen and nectar. Research led by Faculty of Science Professor Sandra Rehan explores new implications for how the places pollinators visit may influence their health.
Pollinators play a vital role in ecosystems and agriculture, yet many populations are under pressure from habitat loss, land-use change, pesticides and climate change.
As scientists work to understand the factors that affect pollinator health, microbial communities are receiving growing attention. These communities, made up of bacteria and fungi that live in and on animals, are known collectively as a microbiome. They can play important roles in nutrition, development and defence against disease. Some microbes may help support bee health, while others can contribute to illness.
鈥淭here has been growing recognition that an animal鈥檚 microbiome can be an important part of its biology,鈥 says Rehan. Important questions remain, however. 鈥淲e still know relatively little about where wild bees acquire their microbial communities.鈥
Rehan explains that much of what scientists know about bee microbiomes comes from a relatively small number of well-studied species, particularly managed bees such as honey bees. Wild bees, by contrast, are far more diverse and may interact with microbes and their environments in very different ways.
Researchers already know that pollinators can share microbes with flowers and that soil is an important source of microorganisms for plants. However, very few studies have examined all three together: bees, flowers and soil. To help fill that gap, Rehan led a research team that set out to understand where small carpenter bees get the microbes that live in and on them, and whether urbanization influences those relationships.
In this work, in Science of the Total Environment, researchers collected bees from 12 sites across the Toronto region between May and September 2025. The sites ranged from highly urbanized areas to more rural landscapes. Whenever a bee was collected, the researchers also collected samples from the flower it was visiting and from nearby soil.
Back in the lab, the team extracted DNA from the bee, flower and soil samples and used genetic sequencing to identify the bacteria and fungi present in each one. By comparing those microbial communities, the researchers found which microbes were shared across bees, flowers and soil, and how factors such as urbanization and season influenced those relationships.
鈥淥ne of the most striking findings was just how much overlap there was between the microbes associated with the bees and those found on their host flowers,鈥 says Rehan. Nearly all of the most common bacterial and fungal groups found on the pollinators also appeared on the flowers they visited, suggesting that flowers as major microbial source. Soil, on the other hand, appeared to be much less connected.
Even so, bees were not simply collecting microbes from flowers. The microbes they carried differed from those found on flowers, suggesting other factors determine a bee's microbiome. Soil may still contribute indirectly, the researchers say, through plants aquiring microbes from the ground.
The study also found that urbanization appeared to influence bee microbiomes. Bees from more urban areas tended to carry a greater diversity of bacteria but a lower diversity of fungi, a pattern not seen in flowers or soil. One possible explanation is that bees in cities encounter a wider variety of plants in parks, gardens and other green spaces.
Microbial communities found on bees and flowers also changed over the course of the growing season, while soil communities remained relatively stable. The study additionally detected fungi that include known bee pathogens, although the researchers did not find evidence that they were harmful. Rehan says the findings highlight the potential for monitoring bee microbiomes as a way to detect emerging threats to pollinator health.
鈥淭he next major question is what those microbes actually do,鈥 says Rehan. 鈥淯nderstanding their functions, how stable these relationships are over time and how environmental change affects them will be important next steps.鈥
Rehan also points to several areas for future research. These include studying bees, flowers and their surrounding environments together rather than in isolation, paying closer attention to fungi, which have received far less research attention than bacteria, and improving understanding of how urbanization affects bee microbiomes.
Rehan says such work could ultimately help researchers develop a fuller picture of pollinator health.
鈥淯nderstanding these relationships could help us develop a more complete picture of what constitutes high-quality habitat for wild pollinators,鈥 she says. 鈥淚 hope this work helps broaden the way we think about pollinator health and conservation.鈥