Bees are busiest in the spring. Their wingbeats quicken so they won't miss even a single flower blooming all around. Even so, they don't run into trouble from an overheated flight engine. That's because nature's cooling system kicks in.
A research team led by Jordan Glass of the Department of Animal Physiology at the University of Wyoming said on the 18th in the international journal Proceedings of the Royal Society B: Biological Sciences that "when bumblebees hover, they generate a downward airflow that directly cools the heat produced by the wing muscles."
Like honeybees, bumblebees are beneficial insects used to transfer pollen in crops. Honeybees, true to their name, gather honey, but bumblebees search only for pollen to feed their larvae. They were originally used to pollinate crops such as peppers and eggplants that produce pollen without nectar, but as honeybee numbers have plunged in recent years, they are now widely used in fruit farming as well.
◇Beating overheating during hovering, with wind
Glass focused on the fact that bumblebees generate tremendous heat when they are in front of flowers. With 130–240 rapid wingbeats per second, bumblebees can hover as if nearly motionless. This produces intense heat in the wing muscles, raising body temperature 30–35 Celsius degrees above the surrounding air. Through experiments, the team found how bumblebees overcome this critical situation: wind that cools the heat.
The team analyzed the flight of the eastern bumblebee (Bombus impatiens), native to North America, using a high-speed camera shooting 1,500 frames per second in a wind tunnel device that blows air artificially. Hovering bumblebees generated a downward airflow at a speed of 1.22 m per second. When dry ice vapor was introduced into the wind tunnel, they could see the air above the bumblebee narrow like a funnel before turbulence was pushed downward.
Next, they checked whether the downward airflow actually produced a cooling effect. The researchers placed 18 freshly euthanized bumblebees in the apparatus and heated them to 50 degrees with a heat gun. They then measured how quickly they cooled when air was blown at the same speed as when a live bumblebee hovers. The downward airflow lowered the bumblebees' body temperature by 5 degrees.
The findings go beyond satisfying mere curiosity. According to the Food and Agriculture Organization (FAO), 71 of the world's top 100 crops depend on bees for pollination. Bee safety is directly tied to human survival. "Without the cooling effect measured in the experiments, a hovering bumblebee would crash from overheating within two minutes," Glass said, adding, "Understanding how temperature affects bumblebee flight will let us predict insects' survival capacity more accurately in a warming world."
◇They also prevent overheating by changing their wingbeat style
This is the first precise measurement of the cooling effect generated by bumblebee wings. Still, questions remain. For example, the magnitude of the cooling effect during forward flight is harder to measure in the lab than during hovering. Sanjay Sane of the Tata Institute of Fundamental Research in India suspected that the cooling effect of the downward airflow is meaningful only during hovering. Glass found clues in desert-dwelling honeybees for how they cool themselves while moving.
In 2024, Glass reported in the Proceedings of the National Academy of Sciences (PNAS) that desert-dwelling honeybees adjust their flight style as temperatures rise to prevent body temperature from climbing. They sweep their wings more broadly to reduce the number of wingbeats needed to reach their destination. For humans, it's like lengthening one's stride. The team said this flight style helps them maintain body temperature even when ambient temperatures are high.
Bees also have a liquid-cooling system. They release moisture from the body to shed heat. This is the same principle as a dung beetle often climbing onto cow dung while crossing sun-baked ground at midday. As the moisture in the dung evaporates, it can lower body temperature.
But in the desert, they can't use moisture freely. If honeybees lose more than a certain amount of moisture, they become dehydrated or incapacitated. Even so, desert honeybees brought back the same amount of honey to the hive even as temperatures rose to 40 degrees. That suggests they have some other cooling technique.
The team filmed European honeybees (Apis mellifera), used in beekeeping, with a high-speed camera while varying the wind tunnel temperature between 25 degrees and 40 degrees. As temperatures rose, honeybees reduced wingbeat frequency and instead swept their wings larger and wider. "They lower wingbeat frequency and increase amplitude to improve flight efficiency," Glass said. "That reduces the need to evaporate moisture to cool the body."
◇Heat from wing muscles doubles as a weapon
The heat produced by bees' wing muscles is not always something to eliminate. Sometimes it becomes a weapon to defend the hive. The native East Asian honeybee (Apis cerana) faces the giant Asian hornet (Vespa mandarinia), which can reach 40 mm in body length, by surrounding it with dozens or hundreds of bees and cooking it with body heat. This is the so-called "heat balling" attack.
In 2012, a team led by Takeo Kubo of the Faculty of Science at the University of Tokyo reported in the international journal PLOS ONE that they had identified the neural circuit involved when honeybees surround giant hornets and launch a heat attack. According to the team, when more than 500 Japanese honeybees mass together and vibrate their flight muscles, the internal temperature rises to 47 degrees, killing the Japanese giant hornet within 30–60 minutes.
Honeybees' lethal temperature is 48–50 degrees, but the giant hornet dies at a lower 44–46 degrees. Thanks to this, honeybees survive heat-balling attacks while only the hornet dies. "The neural circuit that triggers heat balling is not observed in European honeybees," the team said. "It is behavior evolved to avoid being preyed upon by giant hornets that inhabit East Asia."
Ongoing damage to domestic beekeeping operations from giant hornets can be understood in the same context. Korea's native honeybee knows the heat-balling attack, but European honeybees used for beekeeping have not encountered giant hornets and have not developed such a weapon.
References
Proceedings of the Royal Society B: Biological Sciences (2026), DOI: https://doi.org/10.1098/rspb.2025.2166
PNAS (2024), DOI: https://doi.org/10.1073/pnas.2311025121
PLOS ONE (2012), DOI: https://doi.org/10.1371/journal.pone.0032902