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Don’t Poop Where You Eat: Bee Defecation on Flowers May Explain Disease Transmission

Bombus impatiens

Bombus impatiens. Photo credit: Scibadger via iNaturalist.

By Melissa Mayer

For most people, flowers call to mind many things—romance, appreciation, well wishes—but probably not … bee poop. Insect pollinators are crucial to maintaining biodiversity and crop yields but face global declines. Clues that may help save these important insects might come from an unexpected place: apian fecal matter.

Melissa Mayer

Melissa Mayer

It turns out that bees defecate while foraging pollen or nectar, and sick bees may defecate more than usual, possibly transmitting infection through their fecal matter. In a recent paper in the Journal of Insect Science, researchers set out to determine how important flower shape is to bee defecation patterns, with the hope that this data might help unravel the mysteries of disease transmission among bees.

“While it is common knowledge amongst bee biologists that bees frequently defecate, the fact that they will sometimes defecate on the flowers upon which they are feeding seems counterintuitive and deserves further study,” says Jenny Hazlehurst, PhD, assistant professor of biology at California State University East Bay and one of the authors of the study. “I am most excited about the potential applications of this research for understanding how to best protect pollinators from pathogens that are spread from the fecal to oral route.”

So Many Flowers, So Little Time

Jenny Hazlehurst, Ph.D.

Jenny Hazlehurst, Ph.D. Photo credit: Garvin Tso.

To figure out the importance of flower shape to the likelihood of bee defecation during foraging, the team performed 31 separate 4-hour foraging trials in a flight cage. For 12 hours before each flight, they fed the common eastern bumble bees (Bombus impatiens) a 30 percent sucrose solution mixed with fluorescent dye to make it easier to visualize their fecal matter.

It’s a strategy that Hazlehurst thought up while working on an earlier project, looking at another type of pollinator. “Sometimes the birds we captured as part of the [University of California Riverside] macrosystems project had been visiting hummingbird feeders in residential areas where people had put red dye in the nectar solution. The fecal samples we collected would come out as colorful red splashes as a result,” says Hazlehurst. “That gave me the idea to track where pollinators were defecating using fluorescent dye, which I had previously used as an analog for pollen to track the movement of pollen grains between flowers.”

During each flight trial, the bees had access to 12 individual flowers or inflorescences arranged in a random grid. These included four flower shapes: cup, tube, small composite flower (diameter of the disk is smaller than petal length), and large composite flower (diameter of the disk is larger than the petal length).

To standardize the foraging experience, the team emptied the flowers of naturally occurring nectar and deposited a sucrose solution in each flower prior to the trials. They also set up paper disks below each flower to simulate the way flowers pack together in natural foraging circumstances.

A Bouquet of Results

Bee poop

A photo of a bee poop spot (the bright pink) on flower petals. Photo credit: Jonah Bodden.

After each flight trial, the researchers examined the flowers and paper disks under ultraviolet light to detect bee poop. They reported 28 total fecal events with 46 percent of those taking place on flower parts and 54 percent occurring on the paper disks below the flowers.

The results confirmed that flower shape matters. Bees were significantly more likely to defecate on the seaside daisies (Erigeron glaucus), which served as the large composite flowers in the study. Statistical analysis revealed significant differences between large composite flowers and each of the other three flower shapes (cup-shaped, tube-shaped, and small composite flowers).

The researchers think floral morphology might explain these findings. The longer it takes a bee to forage on a flower, the more likely it is that that forager will defecate. Some factors that influence how long a bee spends at an individual flower include corolla depth, nectar concentration and viscosity, and whether the bee is gathering nectar or pollen. Composite flowers comprise many small florets, so foragers must move from floret to floret, which takes more time, increasing the likelihood that they will defecate and/or face exposure to fecal material left by a previous forager.

The Big Picture

Bee footprints

Bee footprints (little pink spots) on a mallow flower. Photo credit: Jenny Hazlehurst.

That exposure is particularly important to researchers since fecal matter may pass infection from bee to bee—and, crucially, from groups of managed bees to wild bee populations (or vice versa). “Emerging work is looking at the potential for the spillover of bee pathogens from managed bees that are used in agriculture such as the European honey bee and the eastern bumble bee into wild bee populations,” says Hazlehurst.

“Pathogen transmission at shared floral resources through defecation on the floral surface and subsequent consumption by the next pollinator to visit is one potential mechanism for how diseases could move between managed and wild bee populations, or even in the reverse direction. It is important that we know about this possibility so that we can come up with good management strategies to prevent this from happening.”

Hazlehurst hopes this study leads to more research into the factors, like infection, that play a role in bee defecation patterns and pathogen transmission on flowers. Future studies should also look at other large composite flowers to confirm the findings.

There’s no doubt that insect pollinators are vital links in the food chain. In fact, one out of every four bites of food needs bee participation to get to the fork—and bee pollination helps the agricultural industry to the tune of $15 billion in increased crop value every year. Which means bee poop might be a hot commodity, at least for researchers hoping to protect pollinators.

Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: melissa.j.mayer@gmail.com.

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