The University of Michigan is studying how diversity affects disease pressure in bees. The study was carried out by Michelle Fearon, a postdoctoral fellow in the University of Michigan’s department of ecology and evolutionary biology.

Researchers collected 4,349 trapped and netted bees from 60 different species at 14 winter squash farms across Michigan over two summers. The farms produced pumpkins as well as acorn, butternut and spaghetti squashes.

The most abundant bees in the study were the European honeybee (Apis mellifera), the eastern bumblebee (Bombus impatiens) and the squash bee (Eucera pruinose). There were several sweat bee species of the genus Lasioglossum, and all bees, except the European honeybee, were native bees.  

Research findings, published in the journal Ecology, indicate that bee communities with more diversity have the lowest levels of three viral pathogens, which are the black queen cell virus, deformed wing virus, and sacbrood virus. Testing revealed that lower viral presence was linked to greater biodiversity in the local bee community.

According to lead author Michelle Fearon, this is an exciting discovery because it suggests that promoting diverse bee communities can be a win-win strategy to reduce viral infections in managed honeybee colonies while at the same time maintaining native bee biodiversity.

This 1:04-minute video by University of Michigan speaks to the results of this study:  



The research team was pleasantly surprised to find from the study a consistent pattern that pollinator biodiversity helps keep multiple types of viral infections low in honeybees and several native bee species. Prior to the study they thought that pollinator biodiversity would increase or have no effect on infections in native bees.

Squash flowers are large, with rich pollen and nectar resources. This attracts many types of native bees, especially solitary squash bees. Honeybees also visit these flowers, but on a more sporadic basis. The entire life cycle of the squash bee is tied to squash plants, with male squash bees often sleeping inside the closed flowers and their nests being located in the soil just below the plants. These flower-loyal specialist bees only visit squash flowers, so all the pollen they spread around comes only from that same species.

This was the first study showing that high levels of diversity in bee communities helps dilute the harmful effects of pathogens, thereby diminishing the pathogens’ impact. This is known as ‘dilution effect’ and is the first time it has been shown with pollinator viruses. Some ecologists do not agree that biodiversity always leads to reduced impact of pathogens, according to Fearon. 

Large bee colonies seem more likely to potentially transmit viruses compared to solitary bees with their own nests. Viruses infecting honeybees may be more likely to spill over into bumblebees because they are closely related but less likely to be transmitted to more distantly related native bee species.

Fearon has two recommendations for landowners and beekeepers. Plant large patches of diverse, native wildflowers in hedgerows and cover plantings. These will attract more native bee species and increase pollination of native plants and crops while benefitting bee health and keeping pest populations down. Also, reduce the use of toxic insecticides and fungicides as they kill bees. If they must be used, do so in late afternoon or evening when pollinators are less active.

These study results have encouraged squash farmers, who were pleased with pollination and crop yields but underestimated the diversity of pollinators that visited their fields, to learn more about the bee species on their farms and how they can further increase pollinator diversity.

Fearon is following up with another study that explores how natural areas and the quality of the bee habitat keep pollinator communities healthy.