New research finds that higher temperatures can actually benefit some bumble bee species - particularly those that make subterranean nests. However, periods of extreme heat appear to offset those benefits, and may contribute to declining bumble bee populations in the southeastern United States.
"A lot of research on how higher temperatures affect living things has been done on individual animals," says Clint Penick, corresponding author of a paper on the work and an assistant professor of insect ecology at Auburn University. "But when it comes to social animals, such as ants and bumble bees, you have to look at the entire society.
"For example, individuals within a bee colony have different, specialized roles," Penick says. "If any of those roles aren't fulfilled, it's difficult or impossible for the colony to thrive. If we're looking at systemic challenges, such as widespread increases in temperature, that could impact the well-being of species across large geographic areas."
"We previously found that individual bumble bees reach stressfully hot body temperatures while foraging on warmer days," says Elsa Youngsteadt, co-author of the paper and an associate professor of applied ecology at North Carolina State University. "But it's important to understand how that fits into the big picture for the entire colony. How do higher temperatures affect the nest, where the queen bee lives and where the next generation of bees is developing?"
It's well-established that nest temperature is important for bee well-being, because they have evolved sophisticated behaviors that regulate nest temperature.
"When it's too warm, bumble bees have a living air-conditioning system, with bees fanning their wings to circulate air through the nest," says Penick. "And when the air is cooler, worker bees 'incubate' the brood by vibrating their flight muscles. But there is very little work that has actively monitored bumble bee nest temperatures, so it's been difficult to know how changes in temperature may affect them."
"Given that there are many different bumble bee species, and that they are major pollinators in agricultural and natural systems, this was a question worth exploring," says Youngsteadt.
For the study, the researchers focused largely on the common eastern bumble bee (Bombus impatiens), which is widely used in commercial agriculture in the eastern United States. In the wild, B. impatiens makes its nests underground. However, when used commercially, it resides in aboveground nest boxes.
The study consisted of four parts. First, the researchers established baseline temperature of underground nests by creating simulated subterranean nests and monitoring their temperature without bees present. The researchers also monitored temperatures of aboveground nest boxes without bees present.
In the second part of the study, the researchers brought B. impatiens colonies into the lab and split them up into "microcolonies" that were placed in artificial nests. The researchers then manipulated the temperature to see how bee behavior changed as the nests were made warmer or cooler.
In the third part of the study, the researchers monitored the frequency with which insects visited cucumber flowers in six locations between Georgia and Michigan. The researchers also monitored temperature at each location. Researchers chose cucumbers because they are grown across a wide geographic region, have to be pollinated by insects, and are frequently pollinated by bumble bees.
"Cucumbers also grow great in pots, so we could use the same seeds and soil across all the sample sites," says Youngsteadt. "Presenting the same plants to pollinators across all the study sites helped us make sure that differences in bee visits were really due to the environment at the site-and especially the temperature-not something else about the plants."
Lastly, the researchers used modeling techniques to combine all four datasets and estimate how changes in ambient temperature would affect the behavior of bumble bee colonies.
"We know many bumble bee species are not doing well in the wild, and that they have been contracting from the southern end of their range in the U.S.," says Penick. "So we were surprised to see that - in many ways - bumble bee colonies should be doing better under increased temperatures.
"Bees with belowground nests don't seem to experience any negative effects, because the nests are insulated from higher heat aboveground. And the increase in temperatures means workers can spend less time incubating the brood and more time foraging, which is beneficial."
Aboveground nests appear to be a different story, with higher temperatures meaning workers have to spend more time fanning the nest to keep it cool - though even that cost is largely offset by the decreased time they would have to spend incubating larvae.
So, with the benefits appearing to outweigh the costs, why do higher temperatures still seem to be a problem for bumble bees?
"There is an upper limit to how much the bees can do to cool down a nest," says Youngsteadt. "In Georgia, where much of this study was done, above-ground nests only experience about 9 hours per year where the nest gets so hot that fanning doesn't help anymore. But in a warmer climate, that could go up to nearly 200 hours per year. Even though things look better for bumble bees during most hours of the year, just a few damaging hours could really set a colony back, for example if the larvae die or develop abnormally.
"We know this can happen, but we don't know how often it happens or how devastating it really is for a colony," says Youngsteadt. "We also need to know more about how the quality of pollen and nectar depend on temperature and what that means for bee health. These are questions we still want to explore in the future."
"One take-away from this is that we need to find ways to keep above-ground nest boxes cooler in order to improve bumble bee survival in agricultural settings," says Penick. "Ground-nesting bumble bees evolved over millions of years to live underground, and the commercial nest boxes they use in agriculture experience greater temperature shifts in a single day than an underground nest would experience over the course of an entire year."
"And there are things anyone can do to help," says Youngsteadt. "Planting flowering, native perennials in your yard or even in a pot on your windowsill can make a small difference. The shorter the distances that bees have to travel for forage, the better able they'll be to survive under challenging circumstances."
"Protecting forest land is also critical," notes Penick. "Forest cover is very important for bee populations, particularly for providing shady nesting habitat."
The paper, "Nesting biology shapes climate vulnerability of social bees (Bombus spp.)," is published in the Journal of Animal Ecology. First author of the paper is Francis Mullan, a former graduate student at Kennesaw State University. The paper was co-authored by Nicholas Green of Kennesaw State and Kevin McCluney of Bowling Green State University.
This work was done with support from the National Institute of Food and Agriculture under grant 2020-67013-31916; and from the Alabama Agricultural Experiment Station under award number 7008193.
