Scientists have been warning about this for years Bumblebee populations are decliningThis poses a risk to the crucial role pollinators play in helping sustain food and plants around the world. New research has found an “important factor” in why.
The answer? It’s getting way too hot.
Like humans, bumblebees’ bodies thrive within certain temperature thresholds. Based on 180 years of literature, University of Guelph environmental science professor Peter Kevan and his team found that bumblebees are most likely to survive up to 36 degrees Celsius, or 96.8 degrees Fahrenheit. Their optimal temperature range for development is between 30 and 32 degrees Celsius, or between 86 and 89.6 degrees Fahrenheit, depending on the species.
Bumblebees have the ability to be thermoregulatory, meaning they can somewhat control the heat throughout their bodies. But Kevan said natural skill doesn’t seem to be enough when it comes to their nests and the larvae that depend on them to grow.
The optimal temperature range for bumblebee nests is between 82.4 and 89.6 degrees Fahrenheit. And when the heat gets just above 35 degrees Celsius, Kevan says it “becomes deadly, despite bumblebees’ remarkable ability to regulate their temperature.”
“We can assume that nest temperatures above 30 degrees Celsius are likely to be very damaging and above 35 degrees Celsius death is likely to occur quite quickly,” Kevan said.
The average temperature on Earth has risen significantly in recent decades. Last year was the hottest ever recorded and 2024 has seen nothing but back-to-back heat extremes, with many cities around the world experiencing record high temperatures in winter. The situation is only expected to worsen because the continuous burning of fossil fuels releases gases that trap the sun’s heat, increasing temperatures.
“Extremely high temperatures are more harmful to most animals and plants than cool temperatures,” Kevan said. “When conditions are cool, organisms that do not metabolically regulate their body temperature simply slow down, but when temperatures become too high, metabolic processes begin to break down and stop. Death quickly follows.”
It’s not the only reason bumblebee populations are declining; habitat loss, pesticides, diseases and loss of wildflowers are also factors. There are more than 250 species of bumblebees worldwide, and there are 49 species of bumblebees in the US alone. According to the US Forest Service, two species have become extinct in recent years.
But Kevan’s team found that it appears to be difficult for bumblebee species to adapt to the heat, leading to the development of dangerous heat stress.
And with bumblebees, a disadvantage for one person can easily become a disadvantage for all. The research, published Friday in Frontiers in Bee Science, explains that bumblebee colonies are “superorganisms” in which overall survival is collective and depends on the ability of the colony to produce offspring as a whole. Essentially, even if some bumblebees can tolerate the heat, those that cannot pose a threat to the future of the colony. And when it’s so hot that the baby bees die before they’ve left their larvae stage, “the entire colony suffers,” the press release says.
“Excessive heat is clearly a major problem for perhaps most bumblebee species,” the study says, “and it is the factor considered in most detail for the effects of climate warming.”