The buzz about bumblebee sensitivity

Bees, perhaps the world’s most essential pollinators, have seen better days. Rising temperatures, habitat loss, pesticides and disease are pushing some species to the brink of extinction. Larger bees, such as bumblebees, are particularly sensitive to rising temperatures. The American bumblebee (Bombus pensylvanicus) is a disturbing case in point: Its population has dropped a stunning 90% since the year 2000. It could soon join the U.S. government’s official endangered species list.

“Bees are really getting nailed from all sides,” said Jenny VanWyk, a visiting assistant lecturer in biology who studies bumblebees. “There’s not just one fire that we need to put out; it’s a combination of stressors.”

Throughout 2022 she conducted experiments with a research team of Mount Holyoke College students to better understand how two of those stressors — pathogens and heat — interact to impact bee behavior, immune functioning and survival rates. They did so using the College’s walk-in thermal chambers in Clapp Laboratory, which allow researchers to study the effects of specific controlled temperatures on experimental subjects.

Two questions at the core of VanWyk’s research are: How successfully can bees fight off infections under heat-stressed environments? And what are the physiological mechanisms that help bees survive exposure to pathogens at high temperatures? If scientists can better understand what helps bee populations fight off sickness, solutions for preventing “massive pathogen spillover events” that threaten the future of species can be created, VanWyk said.

These could help support the health of bee populations — not to mention protect high crop yields and global food security. “If you ate breakfast today, one in every three bites was pollinated by bees,” she said. “So from a human health perspective, we need bees.”

Hot and bothered

VanWyk’s research on campus involved the common eastern bumblebee (Bombus impatiens), which has been shown to act as a hub of pathogen transmission to other bee species. Her team experimentally infected bees with Crithida bombi, a gut pathogen that reduces bumblebees’ foraging abilities and reproductive rates. Up to 80% of wild bumblebees in New England are affected by it.

In a succession of thermal chamber experiments, the research team simultaneously exposed groups of both infected bees and healthy bees to temperatures ranging from 73 to 104 degrees Fahrenheit. Student team members closely observed and logged bee behavior, noting differences between the two groups. Then they dissected bees to measure the severity of infections that occurred at different temperature levels.

A major finding was that when infected bees were exposed to high temperatures, their pathogen loads declined precipitously. “Unfortunately, they’re still dying faster than infected bees did at lower temperatures,” VanWyck said. The heat level has “very big and very obvious consequences for survivorship rates.”

VanWyk is now preparing a manuscript for publication based on the research, which was supported by grants from the Mount Holyoke Faculty Fund and the USDA’s National Institute of Food and Agriculture. It builds on her earlier research describing how the length of flower petals plays a role in transmitting Crithida bombi between bumblebees.

The scientific method in action

Student researchers were crucial to the process of conducting the thermal chamber experiments and gathering bee behavior details, she noted. They participated in all the dissections and pathogen inoculations. One student used a 3D printer in the College’s Fimbel Maker & Innovation Lab to create small custom platforms where bees could congregate on the inside of each microcolony’s box.

“I could never have done this without them. What has been most delightful is how careful they are with close observation,” VanWyk said. “They really took the lead on noticing specific bee behavior at specific temperatures and reporting back to me. I learned so much from my students.”

Emilia Fallman ’23, who is co-authoring a paper with VanWyk, counted bees and captured behavioral data including food consumption levels. “It was laborious, but it was also really fun,” Fallman said.

Their biology major honors thesis was based on experiments conducted with VanWyk involving 24 different bumblebee microcolonies, 15 of them infected. The thesis analyzed how sucrose consumption, reproduction and survival rates varied at different temperatures. Beyond learning firsthand how vulnerable bumblebees are to temperature change, Fallman walked away having learned a larger lesson that had to do with the nature of scientific research.

“Science is very collaborative. You come to new understandings via a back-and-forth process,” they said. Originally from Humacao, Puerto Rico, Fallman began a job as a research assistant at Vanderbilt University in February.

VanWyk, who was Fallman’s thesis advisor, says the experiments offered up real-time examples of how the scientific method often involves disproving hypotheses — as happened during the thermal chamber research. “I hope the students left the lab realizing that this is how science works.”

Going forward, VanWyk plans to delve deeper into the chemical and physiological mechanisms in bees as they respond to heat and pathogens. It’s not hugely surprising that, in terms of bee mortality rates, while one stressor is bad, two are worse.

“But we need to understand exactly how it’s worse and why,” she said. “Because if we understand why, then perhaps we can come up with a solution for how to mitigate the impact of stressors.”