City bees hit the road to hunt for flowers

U. TEXAS-AUSTIN / UC BERKELEY (US) — Ground-nesting bumblebees fare best in areas with more flower diversity and less pavement, research shows.

Bumblebees are important native pollinators. Strategies that reduce local use of pavement and increase natural habitat within the landscape could improve nesting opportunities for wild bees and help protect global food supplies, the study suggests.


Increasing the number of species-rich flowering patches in suburban and urban gardens, farms, and restored habitats also could provide pathways for bees to forage and improve pollination services over larger areas.

“We are potentially in a pollinator crisis,” says Shalene Jha, assistant professor of biology at the University of Texas at Austin and lead author of the study published in Proceedings of the National Academy of Sciences.

“Honey bees are declining precipitously, and wild bees have also been exhibiting population declines across the globe. Native bees provide critical pollination services for fruit, nut, fiber, and forage crops. Understanding how bees move around the landscape can help us both preserve biodiversity and improve crop yields.”

Animal pollination is estimated to be worth over $200 billion in global crop yields.

For the study Jha and senior author Claire Kremen, a professor at UC Berkeley’s Department of Environmental Science, Policy and Management, studied a native California bumblebee, Bombus vosnesenskii, in habitats across exurban areas, farms, and nature reserves.

In addition to finding that pavement negatively affects the bees, the scientists discovered that:

  • Bees will move longer distances to find patches of flowers that are rich in species; it’s not floral density that determines how far a bumblebee will fly, but floral diversity.
  • Bees will also forage further away from their home nest if the surrounding landscape is less heterogeneous. “In some ways, it’s a bet-hedging strategy,” says Jha. “If the landscape is composed of consistently dense flowering patches, bees take a risk and forage farther afield to find species-rich patches.”

“In combination with earlier work showing that bumblebees have become rare in agricultural landscapes, our study suggests that farmers could promote these valuable pollinators by diversifying crop types and by planting cover crops and flowering hedgerows to enhance floral diversity,” Kremen says.

Though it may seem obvious that pavement and ground nesting don’t mix, understanding of the effects of pavement and urban growth on native bees has been largely anecdotal, says Jha, who began the work as a postdoctoral researcher at UC Berkeley.

“Using genetic tools, we can now estimate the number of colonies in an area. This is helping us better understand how wild pollinators live and move across large, diverse landscapes.”

Bumblebees nest in the ground, and each colony contains a queen and a force of workers. As with honeybees, all of the bumblebee workers are sisters who spend some of their time flying around searching for flowers from which to collect pollen and nectar to feed the larvae back in the hive.

Unlike honeybees, which are not native, bumblebees do not make harvestable honey. They do, however, provide important pollination services to plants.

“Bumblebees are among the most effective native pollinators,” says Jha. “They are large and can carry a lot of pollen. They also vibrate or ‘buzz’ flowers with their bodies and thus are excellent at extracting pollen and moving it from plant to plant.”

To study the bumblebees, Jha didn’t scour the landscape for nests in the ground, which has proved in the past to be very difficult, especially over large areas. Instead, she analyzed the genetic relatedness of bees foraging in the landscape.

If bees collected in an area were genetically identified as sisters, they came from the same colony. Unrelated bees came from different colonies. Jha used this information, plus the bees’ locations, to estimate the number of bee colonies in an area and determine how far afield the individual bees were foraging.

Source: U. Texas at Austin