Kathy Keatley Garvey/UC Davis.

Just like humans, bees and other pollinators need to nourish their bodies to keep them going, requiring protein, carbohydrates, minerals, fats, vitamins and water. This is where their foliage foraging habits come in: flowers help bees to meet these nutritional needs by producing nectar and pollen, which the insects buzz around to collect as food.

But unfortunately for the hungry bees, it turns out that this mutually beneficial plant-pollinator relationship comes with grave consequences: flowers actually facilitate the transmission of pathogens to visitors. And according to new research, published in Proceedings of the Royal Society B, this isn’t a species-specific situation. As flowers are typically frequented by multiple pollinator species, a “complex web of interactions” is at play, whereby the flower serves as a previously underappreciated platform of pathogen dissemination in both the environment and between bees.

“Flowers are hotspots for parasite spread between and within pollinator populations,” first author Peter Graystock, from the University of California, Riverside, said in a statement. “Both the flower and bee species play a role in how likely parasite dispersal will occur.” And this is an important finding, given the significance of bees in agriculture and thus our food system today.

To draw this conclusion, researchers designed an experimental setup in which one species of bee, either a honeybee (Apis mellifera) or bumblebee (Bombus terrestris), was allowed to buzz around mixed groups of 80 flowers ad libitum for three hours. The colonies were used as a source of “parasite providers,” confirmed to be infected with two different pathogens. The honeybee colonies contained N. apis and N. ceranae parasites, both of which cause disease in the hosts, and the bumblebee colonies contained N. bombi, C. bombi and A. bombi. All of these parasites can lead to lethargy, colony collapse and queen death during heavy infection.

After the bees’ floral boots were filled, the insects were removed and a second group of mixed flowers was added to the foraging area which a different, previously unused bee species was then allowed to forage upon, once again for three hours. The bees and flowers were then collected and screened for the presence of parasites, which confirmed that the five species of parasite were effectively dispersed onto flowers by their hosts, and then spread both between flowers and non-host bee species.

“By showing that visits from parasite-carrying bees can turn flowers into parasite platforms, we can say that it is likely that heavily visited flowers may become more ‘dirty’ with parasites,” said Graystock. “Planting more flowers would provide bees with more options, and parasite spread may thus be reduced.”

Although at this stage the kind of scale required to potentially make a meaningful difference has not been tested, Graystock tells IFLScience that it would be a logical method to reduce the number of visits to a single flower, and hence the likelihood of an individual flower to possess infective material.

According to Graystock, the findings could have implications for both the flower trade and the transport of bees, which often travel vast distances both across and between countries. Sterilizing flowers and improved bee screening may therefore be beneficial to reduce parasite spread and thus potential harm to bee health. 

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