Sunflowers help bees vanquish parasites. Now we know how.

Story by Elizabeth Anne Brown • Yesterday -NAT GEO

Bumblebees and other pollinators face many threats, including pesticide exposure, climate change, habitat loss due to agriculture and development, and pathogens that ravage multiple species. But a recent finding may help lighten their load.


A common eastern bumblebee (Bombus impatiens) gathers pollen in a field of yellow sunflowers, plants whose pollen helps the insects expel parasites.© Photograph by Bill Berry, Getty

Previous studies have shown sunflower pollen can work like a medicine for bumblebees afflicted by a parasite called Crithidia bombi, a single-celled organism that takes up residence in the bee’s gut and harms their health. But scientists couldn’t explain how sunflower pollen vanquished C. bombi—did it boost the bees’ immune function, or perhaps poison the parasite directly?

New research, published in the Journal of Insect Physiology, shows the answer is deceptively simple. “Sunflower pollen makes bumblebees poo a whole lot,” says lead author Jonathan Giacomini, which flushes the parasite out.

Plant products like nectar and pollen are a treasure trove of potential insect medicines that scientists are just beginning to understand, he adds. “There are natural things out there that bees are interacting with that can be beneficial for them,” Giacomini says. And by making changes to the landscape, scientists hope we can help give bees a fighting chance.

Plant power


A carpenter bee covered in pollen of a viburnum. Planting native flowers such as this—which is indigenous to the eastern United States—helps bees in a number of ways.© Photograph by Krista Schlyer

If you happen upon a fuzzy, buzzing, flying creature in eastern North America, there’s a strong chance it’s a common eastern bumblebee (Bombus impatiens). Yellow and black striped with a rump covered in soft hairs, they’re social insects that live in colonies and love a good crevice—they build their homes in birdhouses, woodpiles, abandoned burrows, and dense grasses.



A green metallic bee on a meadow beauty in Florida.© Photograph by Krista Schlyer

They are important pollinators, both in the wild and in agriculture, where they are raised and used to pollinate crops including tomatoes and pumpkins. Like other pollinators, bumblebees face many threats, and C. bombi isn’t even the biggest bumblebee bugaboo. On its own, the parasite doesn’t have much of an effect on a bumblebee’s health. But when food is scarce, C. bombi can shorten a bee’s lifespan and even reduce the number of young queens a colony can produce.



A leaf-cutter bee photographed in Rocky Mountain National Park, Colorado. Bees provide the vital service of pollination—and plants produce countless chemical compounds that serve as food and medicine for many animals.© Photograph by Krista Schlyer

Lynn Adler is an evolutionary ecologist at the University of Massachusetts Amherst who studies interactions between plants and insects. For years, she and longtime collaborator Rebecca Irwin at North Carolina State University suspected pollinators might be getting dosed by flowers since plants often invest chemically active compounds into their nectar and pollen to help their genetic payload arrive at its destination.

“Many plant defensive compounds can be medicinal at certain doses,” Adler says. After all, “most of our human medicines come from plants.”

Giacomini discovered the effect of sunflower pollen as an undergraduate working in Adler’s lab in 2018. From the very first tests, sunflower pollen dramatically reduced C. bombi parasite load in common eastern bumblebees, often clearing infection completely. “We’ve been shocked at how consistent and effective sunflower pollen has been,” Adler says.

But they couldn’t figure out how—separate studies over the years ruled out immune function boosts and were unable to pin down any chemical compounds in sunflower pollen that would spell doom for C. bombi.


“I started noticing, man, every time we run these experiments, the bees that are fed sunflower pollen are so much dirtier than the bees that eat wildflower pollen,” Giacomini remembers. It was then his hypotheses turned to the scatological.

Bombus away

To try to suss out the mechanism behind sunflower pollen’s medicinal effect, Giacomini, by then a PhD student, set up a bumblebee buffet in a lab at North Carolina State University.

Giacomini fed sunflower pollen to healthy bumblebees and bumblebees infected with C. bombi, then compared their excretions to other bees that received only wildflower pollen. (Bumblebees don’t separate their solid and liquid waste like we do, so bumblebee poo is a thin slurry that’s often bright yellow from undigested pollen.)

“It turned out that bee poop naturally fluoresces under ultraviolet light,” which made distinguishing between poo and non-poo remarkably easy, Giacomini recalls. “It was very dazzling—it almost looked like a galaxy.”

Regardless of whether they were infected or not, bees that ate sunflower pollen pooped 68 percent more in volume and 66 percent more frequently than bees that ate wildflower pollen alone.

The natural next question was why sunflower pollen had this effect. There are lots of ways to get the bowels moving—osmotic laxatives soften stool with extra water, while stimulant laxatives prompt the muscles of the gut to massage digested food down and out.

But preliminary research from the Adler lab again suggests a surprisingly simple explanation. Sunflower pollen’s outer shell is very spikey, which may irritate the lining of the gut into producing a lubricating mucus or somehow dislodge the parasite. According to as-yet unpublished data, Adler says bees fed the outer shell alone experience the same bathroom disruptions and anti-parasite effects, while bees fed the core of sunflower pollen did not.

Robert Paxton, a zoology professor at Martin Luther University Halle-Wittenberg who researches bee parasites and wasn’t involved in any of the sunflower studies, says the poo-based mechanism would jive with other work on gut parasites in bumblebees. Paxton points to a German study that suggests parasites that wreak havoc with honeybees can’t take hold in bumblebees simply because their gut transit time—the time it takes food to traverse the whole digestive tract—is much shorter.

Paxton adds he’d be eager to see if bees infected with C. bombi self-medicate by choosing to eat sunflower pollen more often than healthy bees—a behavior documented in ailing honeybees.

Good for the colony?

Sunflowers are just one plant among untold thousands that likely have medicinal benefits for bees, scientists believe. Resin from plants like poplar or pitch-apple trees can help honeybees fight fungal infection, while a compound derived from thyme is used by colony managers to help ward off varroa mites.

And since bees have been around since before the dinosaurs, researchers also track the plants pollinators use to identify potential candidates for human drugs—they have a 120-million-year head start on pollen research, after all.

Peter Graystock, a lecturer at Imperial College London who studies bee parasites and wasn’t involved in the sunflower research, praised the study as “elegant.”

The prodigious poops from sunflower pollen definitely seem to be “good for the individual because it reduces their parasite load,” he explains. But since C. bombi is transmitted via feces, having widespread bumblebee diarrhea may not be such a good thing on a community level. “They’re essentially shedding transmittable spores at a higher rate,” Graystock says. “Does this lead to faster transmission of the parasite around the community?”

There’s also the element of nutrition. Sunflower pollen is lower on protein compared to some other flowers and lacks two essential amino acids, so bumblebees can’t subsist on sunflower pollen alone. Paxton and Graystock are both concerned the nutritional downsides of sunflower pollen might outweigh the benefits of ditching the parasites.

But another study from the Adler lab currently undergoing peer review suggests sunflower pollen is worth the gamble. As a postdoctoral student, Rosemary Malfi set up healthy bumblebee colonies at 20 farms in New York state with variable amounts of sunflower plantings and tracked the colony’s progress over the course of a season.

“To our delight, the more sunflower at the farm, the lower the infection in those colonies—both the intensity of the infection” and the proportion of bees affected, Adler says. “And more than that, the colonies with more sunflowers actually made more queens,” a key metric of colony health that determines the next generation’s reproductive success.

Not a silver bullet

The authors emphasize we can’t save the bees by flooding our neighborhoods with sunflowers.

The team has found mixed results for sunflower pollen on C. bombi in other bee species—modest effects on two species closely related to the common eastern bumblebee and no effect on a third. Researchers plan to take it back to basics and look for anatomical differences in the bees’ guts that might explain why they respond differently to sunflower pollen.

In the meantime, if you’re buzzing to help the bees, planting “a diversity of flowers is a good idea,” Irwin explains. And sunflowers can certainly be part of a native wildflower smorgasbord, but make sure you get a pollen-bearing variety—the sunflowers raised for cut flowers are usually sterile.