The number of areas populated by bumblebees has fallen 46 percent in North America and 17 percent in Europe, and the new research found that regions with sharp bee declines also experienced strong variations in climate — and especially higher temperatures and worse heat waves.
“Where temperatures are getting more extreme, bees tend to be disappearing more often,” said Peter Soroye, a researcher at the University of Ottawa and one of the study’s authors.
The loss of bumblebee populations is alarming because they play a central role in pollinating many plants, including key crops such as tomatoes and cranberries.
“Unlike honeybees in North America, which have been brought over from Europe and kept in these colonies, bumblebees are native and evolved with these plants," Soroye said. "So when it comes to these natural landscapes, bumblebees are pretty irreplaceable.”
The study, which Soroye conducted with colleagues from the University of Ottawa and University College London, compared the observed locations for 66 species of bumblebees between 1901 and 1974 with places where they could be found between 2000 and 2014.
They found that nearly half of all regions in North America where bumblebees had been recorded in the earlier period no longer registered bees in the later period.
It’s unclear whether the bees might recover. Franklin’s bumblebee is a species once found in a narrow region where California and Oregon meet. The U.S. Fish and Wildlife Service recently proposed an endangered-species listing for the bee but noted that the listing may not actually happen because it’s unclear whether there are any bees left to protect.
“Based on the lack of observations of Franklin’s over the last 13 years it is possible that the species is extinct," the agency wrote.
The study combined observations (or lack thereof) of bumblebees, such as Franklin’s, with a map of changing temperatures and precipitation extremes over the same periods. And it found a strong link between the regions where bumblebees had declined or disappeared and those experiencing worsening heat waves or other types of weather extremes.
“Basically it was measuring how these extremes have pushed species beyond what they had to tolerate before,” Soroye said.
But in a comment on the new study, bee expert Sydney Cameron of the University of Illinois at Urbana-Champaign said there is a lot more work to do. Cameron argued that the data is “Western-centric” since it’s only focused on North American and Europe, and that beyond the large-scale correlations shown in the study between temperature and species declines, researchers need more-detailed studies of precisely what’s happening to bumblebees.
“I have no doubt that climate change is a likely factor in biodiversity decline in general, but this needs to be tested with experiments in both field and lab,” Cameron said.
Unlike many other insects, bumblebees are especially sensitive to temperature. Their large, hair-covered bodies give them an ability to internally heat up by flapping their wings at different speeds. But that also makes them vulnerable in hot weather.
“Relative to most other bees, bumblebees are exquisitely adapted to cold climate and live throughout the world in places that are seasonally cold,” said May Berenbaum, an expert on insects who is also a professor at the University of Illinois at Urbana-Champaign. “They’re effectively sewn into their winter underwear, as it were, so it’s a challenge for them to adjust behaviorally or physiologically to warming temperatures.”
“They can overheat pretty quickly,” added Soroye. “They are really big insects. They evolved in these more temperate regions. So when it gets really hot and they don’t have anywhere to shelter themselves, it can be pretty serious.”
In one 2013 scientific report, two Belgian researchers even proposed a concept called “bumblebee scarcity syndrome” to describe events in which after a heat wave struck, they could no longer find many — or any — bees where they had previously been abundant.
“This syndrome became so common during the last years that it considerably disturbs (or prevents) many field bumblebee experimentations,” they wrote.
This isn’t the first case in which insect declines have been pinned on worsening heat and therefore climate change.
In one striking 2018 study, researchers found massive declines in insect abundance in Puerto Rico’s Luquillo rainforest between the 1970s and the present, during which the average maximum temperature increased by 2 degrees Celsius (3.6 degrees Fahrenheit). Such a change — not in the average temperature overall but in the average high temperatures seen each day — means more pronounced heat extremes.
Granted, for bumblebees, it is not as though climate warming is the only danger. Landscape changes due to farming or other types of human disruption are also a threat, and the study took this into account. Pesticides are another problem, including a group known as neonicotinoids, which can have troubling effects on bee populations.
“What I suspect is that you wind up with this really terrible one-two punch,” Soroye said. “Climate change is making bees want to move to new places, and then you have things like pesticides and human land uses that are stopping them from moving.”