Michigan, Washington study: Habitat quality, biodiversity both can affect bee health

Efforts to promote the health of wild bees and managed honeybee colonies should take into account specific habitat needs, such as the density of wildflowers, according to a recent study from the University of Michigan and the University of Washington.

The study found that increasing other habitat measures, such as the amount of natural habitat surrounding croplands, could improve bee diversity, but have mixed effects on overall bee health. Bees are critical pollinators that support agricultural productivity and the diversity of flowering plants worldwide.

“Future land management needs to consider that broadly improving habitat quality to benefit pollinator community diversity may not necessarily also benefit pollinator health,” said University of Michigan biologist Michelle Fearon, lead author of a study, which was published online in the journal Ecology. “To promote pollinator health, we need to focus on improving specific habitat quality features that are linked to reducing pathogen prevalence, such as planting greater density of flowers.”

Recent decades have seen population declines in both native bees and managed honeybee colonies. The declines are blamed on multiple interacting elements, including habitat loss, parasites and disease, and pesticide use. 

Researchers netted and trapped more than 4,900 bees at 14 winter squash farms in southeastern Michigan, where both honeybees and wild native bees pollinate the squash flowers. The bees were analyzed for the presence of three common viral pathogens. Lower virus levels were strongly linked to greater species richness, or biodiversity, among local bee communities.

The findings provided support for what ecologists call the dilution effect, which posits that increased biodiversity can decrease or dilute infectious disease transmission. 

However, an unresolved question lingered after that study was published. Was biodiversity truly responsible for the observed reductions in viral levels, or was there something about habitat quality that drove changes in both bee biodiversity and viral pathogen prevalence? 

The study found that habitat can have both positive and negative impacts on pathogen levels in bee communities. In general, a higher proportion of natural area and a greater richness of land cover types were associated with increased viral prevalence, while greater floral density was associated with reduced viral prevalence.