An estimate of bumble bee population and distribution shows that half of the species studied have seen a more than 50 percent decline.

For the study, which appears in Ecology, researchers compared current distributions of bumble bee species across Michigan to information they gleaned from museum specimens collected as far back as the 1880s. While the findings are specific to the state of Michigan, they mirror what is happening across the Americas, and in Europe and Asia.

“Bumble bees are important pollinators of plants across natural habitats, where they help support the seeds and berries that birds and other animals depend on,” says lead author Thomas Wood, an entomology postdoctoral researcher at Michigan State University. “They also are effective pollinators of many fruits and vegetables that are important for healthy human diets.

 Thomas Wood shows that half of bumble bee species have declined by more than 50 percent.
Thomas Wood shows that half of bumble bee species have declined by more than 50 percent.
Image: Michigan State U.

Take blueberries, for example. Bumble bees are highly efficient pollinators of this leading Michigan fruit crop. To extract the pollen necessary for fertilization, the blossom needs to be shaken vigorously, and bumble bees are expert at vibrating the flowers to shake out the pollen.

“In Michigan, there are 19 species of bumble bees and around 445 species of other bees,” Wood says. “Many of Michigan’s key crops depend on them. In fact, about 50 percent of cherry pollination is carried out by wild bees.”

Wood’s team scoured the state of Michigan and compared the distribution of 12 different bumble bee species across the state’s 83 counties before and after the year 2000. Some of the biggest declines include:

  • Rusty patched bumble bee: 100 percent
  • American bumble bee: 98 percent
  • Yellow banded bumble bee: 71 percent
  • Yellow bumble bee: 65 percent

Researchers did find some success stories: The common eastern bumble bee and the brown belted bumble bee, increased by 31 percent and 10 percent, respectively.

Why did some species remain stable or increase their ranges while other species declined?

Researchers removed pollen from the specimens to investigate the kinds of plants each bumble bee species visits. Bumble bees carry their pollen on their back legs, which scientists can remove and identify under a microscope, even decades after the bee was alive.

When looking at these pollen loads, the researchers found some consistent patterns.

“Species that declined collected pollen from fewer species of plants and seem to have a narrower range of plants they visit for pollen,” Wood says. “In contrast, the stable species visit a much wider variety of plants. This suggests that picky eaters are less able to switch if a favorite plant isn’t available.”

Researchers will next study why bumble bee species with narrow diets don’t show more diet flexibility.

“Most of the declining bumble bee species in Michigan fly in the summer, feeding on prairie plants and other flowers of open countryside,” Wood says. “Fewer types of plants flower in summer compared to the spring.

“We also know that prairies have been lost over the last century so summer bumble bees may have been hit twice due to a loss of their preferred habitat and a reduced ability to adapt to this change.”

Additional researchers from Michigan State and the University of Manitoba contributed to the study. The USDA National Institute for Food and Agriculture funded the work.