Bee diversity keeps colonies healthy

A European honeybee (Apis mellifera) flying to a squash flower. Both managed honeybee colonies and wild native bees pollinate Michigan winter squash flowers. Honeybees were found at all 14 farms included in the study, along with a diverse array of native bees. (Credit: Michelle Fearon)

The most diverse bee communities have the lowest levels of three common viral pathogens, according to a new analysis of thousands of native and nonnative bees.

Researchers netted and trapped more than 4,000 bees from 60 species at winter squash farms across the state of Michigan, where both managed honey bee colonies and wild native bees pollinate the squash flowers. All but one species—Apis mellifera, the common European honey bee—are native bees. The number of bee species found at each farm ranged from seven to 49.

Consistently, lower virus levels were strongly linked to greater species richness among the local bee communities, according to the study in the journal Ecology.

“This result is exciting because it suggests that promoting diverse bee communities may be a win-win strategy to simultaneously reduce viral infections in managed honeybee colonies while helping to maintain native bee biodiversity,” says lead author Michelle Fearon, a postdoctoral researcher in the University of Michigan ecology and evolutionary biology department.

“In light of recent global pollinator population declines that are due in part to the spread of pathogens, these results offer hope that conservation efforts could also broadly benefit pollinator health,” says Fearon, who conducted the study for her doctoral dissertation. She is now pursuing a follow-up study that explores how natural areas keep pollinator communities healthy.

Honey bee colony loss

The study is the first to show that high levels of biodiversity within bee communities can help dilute the harmful effects of viral pathogens. Support for this “dilution effect” has been reported in other host—pathogen systems—such as tick-borne Lyme disease—but this is the first time it’s been seen with pollinator viruses. The idea of a dilution effect remains controversial among ecologists, however.

Fearon and her colleagues collected 4,349 bees at 14 Michigan winter squash farms over two summers. Michigan winter squashes include acorn squash, butternut squash, spaghetti squash, and pumpkins.

The team found honey bees at all of the sites, and a diverse array of native bees in the squash fields and along field edges. In fact, native pollinators were much more common visitors to the squash flowers than honey bees at most locations.

The most consistently abundant species among the sampled bee communities included four types of bees—the European honey bee, the eastern bumblebee (Bombus impatiens), the squash bee (Eucera pruinosa), and several species of sweat bee (genus Lasioglossum).

The researchers tested those four groups for the presence of three viruses that commonly infect managed honey bee colonies: deformed wing virus, black queen cell virus, and sacbrood virus.

These pathogens contribute to high rates of colony loss among honey bees, and there are no widely available treatments that beekeepers can use to control them. Previous studies suggested that native bees are less commonly infected and may be less likely to transmit the pathogens to other bees.

Native bees vs. honey bees

The viruses spread as bees move from flower to flower, gathering pollen and nectar and pollinating the plants in the process. Researchers believe consumption of virus-contaminated pollen is a primary mode of transmission.

For each of the four target bee groups, researchers found that lower viral prevalence was strongly linked to greater biodiversity of the local bee community: the more bee species present, the lower the percentage of bees infected.

Species-rich communities included many native bee species, which apparently helped to dilute the impact of the pathogens.

Native bees likely reduce the viral prevalence in pollinator communities because they are poorer viral hosts than honey bees. This means that some native bees don’t get as sick as honey bees and are less likely to spread the virus to other bees,” says coauthor Elizabeth Tibbetts, a professor in the ecology and evolutionary biology department who was Fearon’s dissertation adviser.

“So, bees from pollinator communities with lots of species are less likely to get sick because they are sharing flowers with many bee species that are less likely to spread the virus, while bees from communities dominated by honeybees are more likely to share flowers with honey bees that are good at spreading the virus,” Tibbetts says.

Bees are indispensable pollinators, supporting both agricultural productivity and the diversity of flowering plants worldwide. In recent decades, both native bees and managed honey bee colonies have seen population declines blamed on multiple interacting factors including habitat loss, parasites and disease, and pesticide use.

“We found encouraging evidence that pollinator conservation efforts can broadly benefit the health of both managed honey bee colonies and native bees,” Fearon says. “This management strategy could be especially crucial in agricultural areas where crop flowers are visited by both honey bees and native bees—places that may be hot spots for viral transmission among bee species.”

The National Science Foundation, the North American Pollinator Protection Campaign, the Pollinator Partnership, The Garden Club of America, and the University of Michigan’s Rackham Graduate School and ecology and evolutionary biology department funded the work.

Source: University of Michigan