Pollen is an essential resource for bumble bee colony development, but a recent study published in Biology Letters found that pollen foraging may increase bees’ risk of heat stress. A group of researchers at North Carolina State University found that large pollen loads (as seen above) could increase the body temperature of bumble bees by about 2 degrees Celsius. (Photo by Elsa Youngsteadt, Ph.D.)

By Malia Naumchik

Malia Naumchik

With spring in full bloom, you might notice more bees buzzing outside and more pollen making you sneeze. Pollen makes big news as an aggravator of seasonal allergies, but, for bees and plants, pollen is essential for ensuring their reproductive success. Bees collect pollen as a source of protein to feed their larvae. During their foraging trips, they also move pollen from flower to flower, fertilizing the flowers and allowing plants to produce seeds and fruits.

With pollination being such an essential part of both bee and plant life cycles, the recent declines in bees associated with climate change are especially worrisome. Bumble bees are just one of several groups of pollinators seeing dramatic declines in their populations and ranges. Bumble bees are remarkably cold tolerant and well known for their ability to generate and maintain body heat, but it’s less clear how they will fare in the face of climate change or what factors might increase their risk of heat stress.

In a study I conducted with colleagues during my undergraduate studies at North Carolina State University, we sought to find out whether the strenuous task of foraging for pollen might put bumble bees at risk of overheating with warming temperatures. Our findings were published earlier this month in the journal Biology Letters.

To address this question, we wanted to determine if the body temperature of bumble bees increased with an increasing pollen load. I collected pollen-foraging common eastern bumble bee (Bombus impatiens) workers in the field in August and September 2021 at the JC Raulston Arboretum in Raleigh, North Carolina.

I caught bees with a range of pollen-load sizes off flowers, placed them into “bee-squeezers,” and measured their internal temperature using a thermocouple temperature probe. I then anesthetized the bees using carbon dioxide so I could remove the balls of pollen from their corbiculae (pollen baskets on their legs) and put the bees and pollen into vials to be weighed later in the lab. To account for the effects of microclimate on bumble bee body temperature, I also simultaneously logged the temperature of a dead, dried bee as an operative temperature model.

Common eastern bumble bee (Bombus impatiens) workers were captured off of flowers and placed into “bee squeezers” to measure their internal thoracic temperature using a thermocouple probe. (Photo by Melina Keighron)

We also wanted to measure B. impatiens critical thermal maximum, or CTmax, which is a measure of an organism’s upper sub-lethal thermal limit. We could then compare these CTmax values to the predicted body temperatures of bees with and without a full load of pollen from our model, to see if the increase in body temperature from carrying a full pollen load could push them to a body temperature within the range of their CTmax.

To determine B. impatiens CTmax, we collected more worker bees the following summer, in June and July 2022. We collected live bees from several gardens in Raleigh and Durham, North Carolina, as part of a larger ongoing study. We brought them back to the lab and placed them in vials in a dry bath set to 36 degrees Celsius. We increased the temperature of the dry bath by 1 C every four minutes until the bees reached their CTmax, which we described as the onset of muscle spasms. Afterward, the bees were placed into vials, frozen, and later weighed to account for any differences in CTmax due to variation in bee size.

The increase in body temperature common eastern bumble bees (Bombus impatiens) experience from carrying a full pollen load could put them at risk of entering the range of their critical thermal maximum—the point at which heat stress threatens an organism’s life. (Photo by Melina Keighron)

Our results indicated that bumble bee workers carrying a full pollen load, which could be up to almost a third of their body weight, were about 2 C hotter than unloaded bees. Our model predicted that, on a moderately hot day, this increase in body temperature could potentially push bees from a safe temperature to one within the range of their thermal limits that we measured. Bees probably won’t exert themselves to the point of heat exhaustion while foraging, but our study suggests that bumble bees might have to switch up their foraging habits in times of sustained heat.

Bees can adapt during extreme bouts of heat by shortening their foraging trip durations, distances, or time windows, shunting excess heat to their head or abdomen, or regurgitating nectar onto themselves to cool down. But, it’s still unknown how they will cope with consistently rising temperatures and what the implications of these adaptations are for the health of both individual foragers and entire colonies.

While we don’t know what bumble bees will actually do in the future to prevent heat stress while foraging in the field, less pollen foraging might mean fewer plants getting pollinated and less pollen brought in for growing bumble bee larvae. More research will be needed to determine the implications of heat stress in pollen-foraging bumble bees for bumble bee colony development and pollination services.

Malia Naumchik is a master’s student in biological sciences at Montana State University in Bozeman, Montana. Email: mmalia12@gmail.com.