Bumblebee Populations Threatened by Nests Overheating Due to Climate Change

Bumblebees are essential pollinators, but many species are in a downward spiral that is sometimes a mystery to scientists.

In a new study, researchers have found that the increasing temperatures of the human-caused climate crisis may be interfering with climate control in bumblebee nests, threatening future generations.

“The decline in populations and ranges of several species of bumblebees may be explained by issues of overheating of the nests and the brood,” said lead author of the study Dr. Peter Kevan of Canada’s University of Guelph, in a press release from Frontiers. “The constraints on the survival of the bumblebee brood indicate that heat is likely a major factor, with heating of the nest above about 35 degrees Celsius being lethal, despite the remarkable capacity of bumblebees to thermoregulate.”

There are more than 250 species of bumblebee on Earth, inhabiting a variety of environments. Many are in decline due to climate change, but the specific cause has been difficult to pinpoint.

Following a review of the scientific literature, the research team found that the optimal temperature window of bumblebee nests — roughly 28 to 32 degrees Celsius — was consistent between many species around the world.

“We can assume that the similarity reflects the evolutionary relatedness of the various species,” Kevan said in the press release.

The right temperature means minimal metabolic expenditure, while warmth in excess of that window can lead to dangerous heat stress. This means adaptation to higher temperatures could prove hard for bumblebees.

“Excessively high temperatures are more harmful to most animals and plants than cool temperatures. When conditions are cool, organisms that do not metabolically regulate their body temperatures simply slow down, but when temperatures get too high metabolic processes start to break down and cease,” Kevan explained. “Death ensues quickly.”

After a review of 180 years of literature, the team discovered that bumblebees seemed to survive at temperatures as high as 36 degrees Celsius, while 30 to 32 degrees was the optimal range for development, though that window could differ between biogeographical conditions and species.

“The similarity of the optimum temperature range in incubating nests is remarkable, about 28–32°C regardless of species from the cold High Arctic to tropical environments indicates that the optimal temperature for rearing of brood in Bombus spp. is a characteristic common to bumblebees (perhaps a synapomorphy) and with limited evolutionary plasticity,” the researchers wrote in the study.

The researchers said that, while bumblebees have a number of behavioral adaptations for thermoregulation, they may not be adequate to adapt to climate change. They called for more research on how the pollinators can survive the rising temperatures, as well as more studies into bumblebee ecology — temperature, thermoregulation, nest morphology and material properties.

A bumblebee colony acts as a “superorganism,” with reproductive fitness dependent on collective reproduction and survival rather than on individual bees, the press release said. Individual bees may be better able to cope with heat than others, but if the bees’ nest is too hot for raising healthy larvae, the entire colony will suffer.

“The effect of high nest temperatures has not been studied very much, which is surprising. We can surmise that nest temperatures above the mid-30s Celsius would likely be highly detrimental and that above about 35 Celsius death would occur, probably quite quickly,” Kevan said in the press release.

Honeybee studies have shown that hotter nest temperatures sap the strength of queen bees and weaken their ability to reproduce, leading to smaller worker bees and less optimum conditions. If heat affects bumblebees in a similar way, global heating could be a direct cause of their decline.

Some bumblebee colonies may be able to adapt the selection and form of nest site or their behavior to cool down their nests. Ground-penetrating radar might aid in the study of ground-nesting bee species and nest analysis using flow-through respirometry at varying temperatures could help researchers assess how much stress is being placed on bee colonies inside.

“We need both to understand how different colonies cope with the same conditions and how different species cope with different conditions, including whether some bumblebee species have broader thermal neutral zones, affording them more resilience,” the press release said.

“We hope that future scientists may take the ideas we present and apply them to their own research on bumblebee health and conversation,” Kevan concluded.

The study, “Thermodynamics, thermal performance and climate change: temperature regimes for bumblebee (Bombus spp.) colonies as examples of superorganisms,” was published in the journal Frontiers in Bee Science.