Pathogens seek opportunities. Both wild and domesticated species present potential hosts for many microbes, and they’re not fussy about hopping from one to the other. Jonathan Giacomini and colleagues examined how the common eastern bumble bee, Bombus impatiens, and its protozoan pathogen, Crithidia bombi (Trypanosomatida), interacted when bees were given pollen from the medicinal sunflower Helianthus annuus.
Crithidia bombi is a parasite that enjoys the gut of bumblebees. When they pass out of the gut in cysts, they’re in a position to get picked up by the next passing arthropod. Inside the bee, it can affect ovarian development and impact the reproductive success of the queen and the colony.
Work by Giacomini and others have found that good nutrition is vital for bumblebee health and that H. annuus can be a huge benefit when it is the diet. The authors write that more than two-thirds of bees that consumed sunflower pollen had no detectable infection after one week. The pollen reduced the intensity of infection 20- to 50-fold compared to other pollen diets. If it’s so good, why not just feed the bees sunflower pollen? The scientists write that that would be a bad idea. Sunflower pollen is low in protein and doesn’t contain everything bees need to develop, so wildflower pollen is still necessary.
This need for a mix is why Giacomini and colleagues set out to find what the minimum dose of sunflower pollen mixed with wildflower pollen provided medicinal benefits. Once they found this minimum dose, they then asked: What are the costs and benefits of sunflower pollen on colony-level infection and performance? Finally, they asked if C. bombi could develop resistance to sunflower pollen.
“Compared to a wildflower diet, we found that the sunflower diet significantly reduced negative effects of infection on queen and drone production,” write Giacomini and colleagues. “Infection significantly reduced the probability of queen production in wildflower but not sunflower colonies, while infection significantly reduced the probability of drone production independent of pollen diet. However, infected sunflower colonies that did produce drones yielded on average four times as many as infected wildflower colonies.”
Body size and mass of adults were similar for both diets. This similarity shows that what the bees lost in nutrition, they gained in reduced harm from the parasites. However, the results were not the same as the results from the earlier experiments on individual bees. The authors write, “At the end of the whole-colony experiment, we discovered that colonies inoculated with C. bombi and fed mixed sunflower pollen had lower mean parasite loads than colonies fed wildflower pollen, but infection was not reduced as dramatically as in previous individual bee experiments.”
As the sunflower pollen didn’t clear the pathogen, there was a worry. Pathogens can often mutate rapidly to overcome challenges. Could the colonies be helping create a sunflower-resistant pathogen? The team didn’t find any variants exposed to sunflower pollen increasing in prevalence, so over the ten weeks of the experiment, there was no indication that C. bombi was adapting.
In fact, it’s not exactly clear how the sunflower pollen fights the pathogen. The authors note that you would expect some factors, like sunflower pollen’s poor nutritional content, to increase a bee’s susceptibility to illness. In addition, they note that sunflower pollen extract increased rather than suppressed C. bombi growth in vitro. So while there is a measurable benefit to providing bumblebees with sunflower pollen, it would be helpful to know how it works.
In the future, just as companion planting can help direct herbivores away from valued crops, it might also become a valuable tool to keep pollinators healthy.