1. How Japanese Honeybees Respond When a Giant Hornet Attacks Their Hive
The Asian giant hornet is a large wasp that attacks honeybee nests and catches worker bees. When Japanese honeybees are attacked, they leave the hive and gather around the hornet. Hundreds of bees surround it, forming a ball. This is called a "bee ball" [1].
A report from 1995 states that more than 500 bees surrounded the hornet at once [1]. However, the number varies between reports, ranging from about 400 to over 500 bees, and it seems to differ with each event [1]. It has also been reported that there are no sting marks. This suggests the bees fight by surrounding the enemy with their bodies rather than using their poison stingers [1]. What is happening inside the ball?
2. Why the Trapped Hornet Dies Within 10 Minutes Even Though Heat Alone Is Not Enough
The trapped Asian giant hornet dies within 10 minutes. When researchers examined the body, they found no sting marks. The highest temperature inside the bee ball was 45.9°C [2].
Researchers then conducted an experiment to test the effect of heat alone. They placed a hornet alone in a device that raised the temperature outside the ball. Even at 47°C, the hornet survived for 10 minutes [2]. This temperature is higher than the maximum inside the bee ball, yet the hornet lived. This means heat alone does not kill it.
Something else inside the ball seems to weaken the hornet’s ability to endure. What is it?
3. How Higher Carbon Dioxide Inside the Bee Ball Lowers the Hornet's Heat Tolerance
When scientists measured the air inside the ball, they found that the concentration of carbon dioxide (CO2) had risen. This happens because many bees gather and breathe. The CO2 level reached a maximum of 3.6 ± 0.2% within 0 to 5 minutes of formation [2].
Researchers then tested the temperature tolerance in air with about 3.7% CO2. According to reports, in normal air, the Asian giant hornet died in the range of 47–48°C during a 10-minute test. In air with increased CO2, it died in the range of 45–46°C. This is a drop of about 2°C [3]. On the other hand, the lethal temperature for honeybees is 50–51°C, leaving a gap of about 5°C between them and the hornet [2].
The temperature inside the ball is around 46°C, which is close to the limit in normal air (47–48°C). When CO2 increases and the limit drops to 45–46°C, the ball's temperature can reach that limit. For the bees, there is still a margin of 4–5°C. The study concluded that both temperature and CO2 are important [2]. So, what is CO2 doing to the hornet’s body?
4. How High Humidity in the Bee Ball Keeps the Hornet From Cooling Its Body
In a study published in 2012 by Tamagawa University and others, the bee ball reached 46°C within 5 minutes of capture. The CO2 level became about 4%, and the relative humidity exceeded 90% in 3 to 4 minutes [4].
When humidity is this high, it becomes difficult for moisture, like sweat, to evaporate. This makes it hard for the body to release heat. In experiments, the lethal temperature dropped by about 2°C in air with increased CO2. Researchers interpret this as high humidity possibly preventing cooling through evaporation [4]. The detailed mechanism is a subject for future research.
Meanwhile, the lethal temperatures for Japanese honeybees and Western honeybees were 50–51°C under the tested conditions, and were almost the same [4]. The bees’ bodies seem able to withstand the conditions inside the ball.
5. How the Bees Produce the Heat Inside the Ball
In winter, honeybees gather inside the hive to form a ball. They vibrate their flight muscles to produce heat. It is thought that the bee ball uses this method of generating heat for winter survival to defend against enemies [4].
In a research introduction from the University of Tokyo, it is stated that nerve cells in a part of the higher center called the "mushroom body" in the brain of Japanese honeybees forming the ball were active. Similar activity was seen when bees were simply exposed to high temperatures. This suggests that information about the high temperature of the ball is processed in the brain [5]. Can any bee use this technique?
6. Why Western Honeybees Do Not Use the Heat Ball Technique
Western honeybees, which originated in Europe, do not show this behavior. The bee ball is thought to have evolved in Asian honeybees, especially Japanese honeybees [5].
It is believed that Japanese honeybees, which have lived in the same regions as the Asian giant hornet for a long time, acquired the technique of defeating enemies by forming a ball with hundreds of bees. On the other hand, the lethal temperatures of Western honeybees and Japanese honeybees are almost the same [4]. One difference seems to be the "behavior" of gathering to increase heat, CO2, and humidity.
7. A Home Experiment: Feeling the Effect of Your Own Breath
If you want to investigate further, you can try this at home. Put your hands together in a bowl shape and blow your breath in front of your mouth. Your palms will become warm and humid. You can imagine that something similar to this is happening inside the bee ball due to the bees' breathing and body heat.
When you go out, observe honeybees on flowers in a park from a distance. It is a rule not to get too close.
You can also think about the numbers in the article. Write down the ball temperature (about 46°C), the hornet’s limit (45–46°C), and the bee’s limit (50–51°C) on paper and line them up. You can see the difference between their temperature limits.