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Struggling bumblebees can thrive in an unlikely place: The city

Struggling bumblebees can thrive in an unlikely place: The city
A new study suggests bumblebees are better off in the city than the country. Pictured here is a colony of Bombus terrestris bumblebees, with workers tending to the brood and filling wax pots with nectar and pollen. (Ash Samuelson)

Imagine you’re a bumblebee queen in England. It’s February, and you’ve just woken up from your winter hibernation. You’re hungry, pregnant and ready to find the most promising spot to start a new colony.

So, where do you build your nest? Should it be near rural farmland, in a suburban village or right in the city center?

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If you picked farmland, you picked wrong. According to research published Tuesday in Proceedings of the Royal Society B, bumblebee colonies are significantly more successful in cities and suburbs than they are in the country.

The scientists who conducted the study found that bee colonies in the city and suburbs enjoyed several advantages over those near farmland. They were bigger, had more food stores, had a longer life span and produced more sexually active offspring than those in rural areas.

“It sounds counterintuitive, because bees didn’t evolve to live in urban environments, but equally, they didn’t evolve in the modern agricultural landscape either,” said Ash Samuelson, a doctoral candidate at Royal Holloway, University of London, who led the work.

Ideally, bumblebees would prefer to live in prairies in the United States or in wildflower meadows in Europe, where a wide variety of flowering plants live alongside each other, she said.

However, as these natural areas have been converted to either farmland or cityscapes, Samuelson and her colleagues think that city parks and suburban backyard gardens can ultimately offer bumblebees a more diverse range of flowers from which to collect pollen and nectar than can agricultural fields.

“One of the issues with most commercial crops is that they flower for a very short amount of time and then there is absolutely nothing to sustain the colony,” Samuelson said.

To come to these conclusions, Samuelson and her team started by collecting 176 bumblebee queens from local parks and farmlands in March and April of 2016, as the insects foraged for food before laying their first batch of eggs.

“They are easy to spot,” Sameulson said. “If you go to flowering plants you’ll see these really large bumblebees with eggs in them. Those are the queens.”

The queens were brought back to the lab and put into clear plastic boxes, where they began rearing new colonies.

Once the colonies were established, the scientists situated the plastic boxes in the backyards and farms of volunteers across southeast England. A total of 38 colonies were placed in environments that the authors classified as either city, village or rural.

An insulated field nest box in which a bumblebee colony was placed with access to the outside environment via a tube.
An insulated field nest box in which a bumblebee colony was placed with access to the outside environment via a tube. (Ash Samuelson)

Throughout the summer, Samuelson and her colleagues checked in on each colony once a week, always at night.

“We did that because all the bees are inside at night, so we can get an accurate colony count,” Samuelson said.

Another benefit of the nighttime checks is that bees can’t fly in the dark because they can’t see, she said.

“Usually when I work with bees I wear a bee suit, but at night I don’t have to,” she said. “It was easy to open the lid and take the samples, because the bees can’t fly out.”

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The weekly checks included counting the bees in the colony; noting whether the queen was alive, dead or absent; and recording the amount of pollen and nectar in the nest.

Researchers also weighed the nest, collected weather data, counted the number of reproductive bees in the nest (including males and future queens) and brought up to three sterile worker bees from each colony back to the lab to test for any parasites that may have made their way into the colony.

Ten weeks into the experiment, all of the colonies had died out, which is the usual life expectancy for a bumblebee colony, Samuelson said.

Previous research had suggested that bumblebees are happier and more abundant in urban areas than rural ones, but Samuelson said she was still surprised by what she found after she tallied the data.

“It was the magnitude of the difference that surprised us,” she said.

For example, all of the queens in rural colonies died or left about five weeks into the study, while the majority of queens in urban and suburban areas were still alive by week six. In addition, all of the country colonies died out by week six, while the city and suburban colonies were still going strong.

The researchers offered a couple of potential explanations for their findings. In addition to urban bees having access to a more diverse array of flowers, Samuelson said, they may also benefit from having less exposure to pesticides. But that idea still needs to be tested in future work.

“We have records of what pesticides are used in agriculture, but we don’t know when the private gardener goes to the local hardware store to buy pesticides there,” she said.

Inside the field nest box, colonies were housed in wooden nest boxes on a set of scales to record their weight.
Inside the field nest box, colonies were housed in wooden nest boxes on a set of scales to record their weight. (Ash Samuelson)

Maj Rundlöf, an entomologist at UC Davis, said the study was novel in that it was the first to follow locally sourced bumblebees throughout their colony cycle. However, she was not shocked by the results.

“Given the lack of flower resources, potential for pesticide exposure and lack of nesting habitats for wild bees in intensively managed agricultural landscapes, it is not very surprising that bumblebee colonies are not doing particularly well in such landscapes,” she said. “This well-performed study supports that.”

Her one caveat was that not all landscapes were represented equally in the study. Colonies were placed in only five agricultural sites, compared with 17 in cities and 16 in villages. In addition, three of the five agricultural colonies were invaded by a parasitic bee.

She said she would also like to know how bumblebees would fare in seminatural grasslands.

“It would be interesting to see how less intensively managed rural landscapes compare to urban habitats,” she said.

In the meantime, Samuelson said the findings suggest that as rural bumblebee populations fall, city areas could serve as an unlikely refuge for these essential pollinators, making them one of the few species that may benefit from an urban environment.

June 28, 4:30 p.m.: This article was updated with comments from Maj Rundlöf, an entomologist at UC Davis.

This article was originally published June 26 at 5:15 p.m.

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