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| bumble bee (B. impatiens) Photo Credit: Eduardo Goody |
Scientific Frontline: Extended "At a Glance" Summary: Bumble Bee Climate Vulnerability
The Core Concept: Recent research demonstrates that while moderate ambient temperature increases can benefit bumble bees by expanding optimal foraging hours, extreme heat events severely threaten colony survival, particularly for populations utilizing above-ground nests.
Key Distinction/Mechanism: Unlike solitary insects, social bees rely on collective thermoregulatory behaviors. When temperatures drop, workers vibrate their flight muscles to incubate the brood; when it gets too hot, they fan their wings to act as a living air-conditioning system. Extreme heat diverts critical labor away from foraging to focus on cooling.
Major Frameworks/Components:
- Nesting Site Comparison: Evaluating thermal vulnerability by contrasting naturally insulated subterranean nests with highly exposed commercial above-ground nest boxes.
- Behavioral Resource Allocation: Quantifying the trade-offs in worker bee labor between essential tasks like brood incubation, nest fanning, and active foraging.
- Geographic Foraging Models: Monitoring bee visitation rates to pollinator-dependent crops (cucumbers) across varying environmental temperatures between Georgia and Michigan to model future climate scenarios.
Branch of Science: Entomology, Applied Ecology, Animal Behavior, and Climate Science.
Future Application: The findings will guide the engineering of specialized, temperature-regulated commercial nest boxes for agriculture, inform land conservation strategies to protect shaded forest habitats, and promote targeted native planting initiatives to reduce necessary foraging distances.
Why It Matters: Bumble bees are keystone pollinators vital to both natural ecosystems and commercial agriculture. Understanding the thermal thresholds of their nesting environments is essential for anticipating species decline and safeguarding global food security against progressive climate change.
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, the 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, a coauthor 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 is well established that nest temperature is important for bee well-being because bees 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 the baseline temperatures of underground nests by creating simulated subterranean nests and monitoring their temperature without bees present. The researchers also monitored the 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 the 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 well 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.”
Finally, the researchers used modeling techniques to combine all four data sets 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 US,” 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 that workers must 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.
Therefore, 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, aboveground nests only experience about nine 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 depends on temperature and what that means for bee health. These are questions we still want to explore in the future.”
“One takeaway from this is that we need to find ways to keep aboveground 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 forestland is also critical,” notes Penick. “Forest cover is very important for bee populations, particularly for providing shady nesting habitat.”
Previous research: Study Finds Carrying Pollen Heats Up Bumble Bees, Raising New Climate Change Questions (on Scientific Frontline)
Funding: 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.
Published in journal: Journal of Animal Ecology
Title: Nesting biology shapes climate vulnerability of social bees (Bombus spp.)
Authors: Francis R. Mullan, Nicholas S. Green, Elsa Youngsteadt, Kevin E. McCluney, and Clint A. Penick
Source/Credit: North Carolina State University | Matt Shipman
Reference Number: ent050626_01