Is your garden bee-friendly? – City gardens promote bee parasites

Featured Image Caption: City gardens impact parasite prevalence in the yellow-faced bumblebee, Bombus vosnesenskii (Image Source: “Yellow-faced Bumblebee (Bombus vosnesenskii)” by jvhigbee is marked with CC BY-NC-ND 2.0.)

Reference: Ivers, N. A., Jordan, Z., Cohen, H., Tripodi, A., Brown, M. J. F., Liere, H., Lin, B. B., Philpott, S., & Jha, S. (2022). Parasitism of urban bumble bees influenced by pollinator taxonomic richness, local garden management, and surrounding impervious cover. Urban Ecosystems, 1-11.

City Slicker or City Sicker?
Ground-nesting bees create or find nests in the soil. Those that find, often use old animal burrows. Mulch can cover and reduce nesting sites (Image Source: “oh, hi!” by Rob Cruickshank is marked with CC BY 2.0.).

An intense buzzing sound rises from the ground as bumblebees emerge from nests built in the soil one by one. These are the worker bees setting out to find pollen and nectar to nourish themselves and the growing colony. However, this mission can be harrowing, especially for bees that live in cities. Navigating city landscapes can be challenging for a little bumblebee as gardens are often small and far apart, forcing them to travel great distances to gather enough food. Ground-nesting bumblebees rely on animal burrows left behind in the soil to set up colonies. Some garden management strategies can destroy these nesting grounds, like covering the garden in mulch. Cities also make suitable breeding grounds and transmission zones for many parasites, which have played a role in the national decline of bumblebees.     

Bee parasites thrive in cities when bees are highly susceptible to infection and are in close contact with other bees. Traveling far distances between gardens can require a lot of energy and additional nutrients from bees. These stressors can compromise immune system strength, making bees more susceptible to infection. As an alternative, bees may only forage at a few nearby gardens when the garden is surrounded by buildings and pavement. However, increased density can increase the number of contacts between bees. Despite these challenges, there is a promising future for bumblebees if we can reduce parasite transmission in cities. A group of researchers with lead author Nicholas Ivers assessed what factors promote bumblebee parasites and what we could do to stop them.

The Buzz on City Parasites
Yellow-faced bumblebees (Bombus vosnesenskii) carry pollen from flower to flower. During these visits, parasites can be left behind for the next visitor to pick up (Image Source: “Bombus vosnesenskii Yellow-faced Bumblebee” by David A. Hofmann is marked with CC BY-NC-SA 2.0.)

The yellow-faced bumblebee (Bombus vosnesenskii) is the most common bumblebee native to cities along the western coast of the United States. These bumblebees are vital pollinators – providing pollination services for agricultural crops and community gardens. As parasites might interrupt these services, it is crucial to understand how these parasites spread and what we can do to slow the spread. The yellow-faced bumblebee has at least two harmful parasites that live in the bees’ gut: Crithidia bombi and Apicystis bombi. Collecting bumblebees from 20 urban gardens across the central coast of California, researcher Ivers and colleagues counted the number of parasites.

The two parasites were highly abundant, with C. bombi found in 18.6% of the bees and A. bombi found in 6.4%. Crithidia bombi was more prevalent in gardens with higher mulch coverage and when the garden was surrounded by dense buildings and pavement. On the other hand, Apicystis bombi was present in gardens with less diverse pollinators, like bees, wasps, butterflies, and flies. The researchers found many other parasites in the collected bees, but these pathogens were rare (about 1% or less). Ultimately, yellow-faced bumblebee parasites are common, and we are making matters worse.

Some Mulch Needed Improvement

We can help the pollinators resist parasites by planting gardens with native plants and leaving the ground bare for bee nests (Image Source: “Pollinator Garden” by U.S. Fish and Wildlife Service – Midwest Region is marked with CC PDM 1.0.).

Mulch covers animal burrows that bees could use for nests, forcing workers to travel further to find nectar and pollen. Tired, hungry bees may be more susceptible to parasites causing higher infection rates in gardens with more mulch cover. If gardens are spread out with buildings and pavement surrounding them, bees may choose to forage within a single garden. When more bees are approaching the same flowers, parasites can be left on a flower by one bee and picked up by the next visitor. Lastly, the two parasites studied infect more pollinators than the yellow-faced bumblebee. When pollinators are diverse, parasites with multiple hosts are less detrimental to a single species.

Human activity can make it challenging for native bees to survive in cities. Parasites especially threaten pollinators that are critical for our food security. Ground-nesting bees are common (more than 70% of North American bees), and parasites may be a widespread problem. We can help ease the parasite burden on bees by planting native flowers and leaving the ground bare. By increasing floral and nesting resources, we create habitats that support diverse pollinators, which can help diffuse parasite loads. So, plant some flowers, lay off the mulch, and save the bees!

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Brandi Pessman

I am a fifth-year Ph.D. student at the University of Nebraska-Lincoln in the School of Biological Sciences. Growing up on a farm in a small town in Illinois, I developed an early love for animals and a fascination with their behaviors. When I was younger, however, it never crossed my mind that I would be using spiders to investigate how human presence affects animal behavior, but I am loving every second of it. Studying the behaviors of animals can tell us a lot about the role that we play in their survival (or death), which is becoming increasingly important as human populations continue to grow. When I am not studying spiders, I enjoy playing with my cat or being outdoors!

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