How can honeybees kill a hornet bigger than themselves?
― Not with stings, but by "slow-cooking" it with body heat
A giant hornet weighs more than ten times as much as a Japanese honeybee, and stings won't beat it. So the bees use heat. Hundreds of them wrap around the intruder and raise the temperature to a knife-edge level: one the bees can survive and the hornet cannot.
It is autumn in the mountains. In front of a honeybee hive kept in a wooden box, a single large wasp arrives, as big as your thumb. It is a giant hornet.
A honeybee is a little over 1 cm long. Next to the hornet, its jaws and its armour are hopelessly weak. Against European honeybees, a few hornets are said to be able to wipe out a hive of tens of thousands.
But at the entrance of a hive of Japanese honeybees, native to Japan, something different happens. In an instant, the hornet vanishes inside a "ball" of bees.
There are just two reasons they win
A honeybee can disconnect its flight muscles from its wings and shiver them rapidly, producing heat and nothing else. When hundreds do this at once, the centre of the ball is said to reach about 46°C within minutes.
Hornets are said to die at around 44 to 46°C. Honeybees are said to survive up to about 48 to 50°C. The bees aim for the gap of a few degrees between the two, and hold the temperature there.
The bees lose on strength but win on how well their bodies handle heat. It is a fighting style that turns a difference in body design directly into a weapon.
Hundreds of bees become one "stove"
When a hornet nears the hive, the bees wait just inside the entrance. The moment the enemy starts to enter, hundreds of them leap on it at once and wrap it up. This ball is called a "hou-kyuu" (bee ball).
The bees in the ball keep shivering their chest muscles. It works much like the shivering we do when we are cold. One bee gives off very little heat. But hundreds of them are packed into one spot, and the bees on the outside trap the heat like a quilt.
Look at the left side of Figure 1. The hornet at the centre is walled in on every side by bee bodies, and the heat has nowhere to go. According to reports, the temperature at the centre reaches about 46°C within a few minutes and stays there for about 20 minutes.
A few degrees decide the fight
The key is not to overheat. At 50°C the hornet would certainly die. But the bees wrapped around it would die too.
On the scale on the right of Figure 1, the ball's temperature (the dotted arrow) sits at the top edge of the hornet's fatal band. It has only about 2 to 4°C to spare before the bees' own fatal band. It is like adjusting, with their own bodies, a sauna that only the enemy cannot stand.
More recent research suggests that carbon dioxide building up inside the ball also plays a part. The bees breathe hard, so the air in the ball becomes "stuffy". Some reports say this makes the hornet more likely to die even at a lower temperature than usual.
The European honeybees widely kept in beekeeping come from regions that never had giant hornets. They do form balls, but they are said to be less quick and less numerous at it than Japanese honeybees. The difference in fighting style is thought to come from whether a species has spent a long time in the same land facing the same enemy.
Honeybees on the island of Cyprus have been reported to ball up a different species of hornet and kill it mainly by blocking its breathing rather than by heat. The same "make a ball" tactic works in different ways against different enemies.
Summary
Japanese honeybees can match a giant hornet neither in strength nor in stings. So hundreds of them wrap around the enemy and shiver their flight muscles to make heat. Hold the centre at about 46°C, and only the hornet, which is weaker against heat, dies.
Small bodies, but hundreds together make a stove.
What decided the fight was not a difference in strength, but a difference of a few degrees in what each side can stand.
For how bees tell each other where flowers are, see "How can honeybees tell each other where the flowers are with a figure-eight dance?". For how body size relates to how easily heat escapes, see "Why do smaller animals have to keep eating to stay alive?".
- Set out five cups and pour the same amount of equally hot water into each.
- Leave one cup on its own. Line up the other four tightly against each other, and drape a towel over them.
- After 20 minutes, touch the water in the lone cup and in the grouped cups with your finger and compare (don't use boiling water; use a temperature you can touch).
The grouped cups should have kept more of their warmth. There is less surface for heat to escape, and the outer cups act as a "quilt". The bee ball works the same way, and on top of that every bee inside keeps making heat. If you come across a hornet nest outdoors, do not go near it.
For those who want to know more ― terms, formulas, and links to textbooksEach part is labelled with its level, from middle-school science to university courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Physics" or "Biology"
- High school+Advanced high-school material, or textbook sidebar content
- UniversityUniversity-level specialist courses (animal physiology, behavioural ecology) not taught in high school
- ResearchNot yet settled even at university level; researchers are still investigating
Middle schoolTerms: this phenomenon has a name
- Bee ball (hou-kyuu): a ball that honeybees form by wrapping hundreds of bodies around an enemy. One that kills with heat is also called a "heat-killing bee ball".
- Shivering thermogenesis: making heat only, by shivering muscles in small movements without moving the body. Honeybees do this with their wings held still, using the chest muscles they use for flying.
- Lethal temperature: the borderline temperature above which a living thing dies. It differs between species, and this difference is the bees' "weapon".
Middle schoolHigh schoolCheck with a formula: how many minutes until the ball reaches 46°C?
We estimate how long it takes for the heat from hundreds of honeybees to raise the ball's temperature from about 35°C to about 46°C. We consider the fastest case, where no heat escapes.
| In symbols | t ＝ m × c × ΔT ÷ P （P ＝ N × p） |
| In words | Time taken ＝ mass of the ball × specific heat × temperature rise wanted ÷ total heat output of all the bees |
| Where it comes from | Conservation of energy. The heat needed to raise the temperature (mass × specific heat × temperature change) is supplied by the heat the bees make each second |
What the symbols mean:
t is time (seconds), m is the mass of the whole ball (g), c is specific heat (the heat needed to raise 1 g by 1°C, in J/(g·K)), ΔT is the temperature rise (°C), P is the heat output of the whole ball (W, meaning J per second), N is the number of bees, and p is the heat output of one bee (W).
| Number of bees in the ball N | About 500 (said to be hundreds) |
| Mass of one honeybee | About 0.1 g |
| Heat made by one bee's shivering p | Said to be a few tens of milliwatts; here we use 0.03 W |
| Specific heat of an insect body c | A little less than water; said to be about 3.5 J/(g·K) |
| Starting temperature and target | From about 35°C to about 46°C |
| Heat output of the whole ball P | 500 × 0.03 ＝ 15 (W) |
| Mass of the whole ball m | 500 × 0.1 ＝ 50 (g) |
| Heat needed to raise it by 1°C | 50 × 3.5 ＝ 175 (J) |
| Temperature rise ΔT | 46 − 35 ＝ 11 (°C) |
| Total heat needed | 175 × 11 ＝ 1925 (J) |
| Time taken t | 1925 ÷ 15 ≒ 128 (seconds) |
| Convert to minutes | 128 ÷ 60 ≒ 2.1 (minutes) |
This estimate says that if no heat escaped at all, the ball would reach 46°C in about 2 minutes. In reality heat does escape, which fits well with the "few minutes" that have been observed. One bee's heat is tiny, but 500 together give off about as much as a small light bulb (15 W).
High schoolHigh school+Why can the temperature be held steady?
High schoolAn object's temperature stops changing when the heat coming in balances the heat going out. In a bee ball, the temperature is set by the balance between the heat the bees make and the heat escaping from the surface of the ball. If the bees shiver less, the temperature stops rising.
High school+The bigger the ball, the smaller its surface area (where heat escapes) is compared with its volume (where heat is made). So the more bees huddle into a round shape, the more easily they warm up. Penguins huddling together on a cold day work on the same principle.
UniversityHeat capacity and insect temperature control
The "heat needed to raise it by 1°C (175 J)" in ② is the heat capacity of the whole ball. Insects like honeybees, which can raise their own body temperature, are treated in animal physiology as examples of "heterothermy". They keep their chest warm by shivering thermogenesis in the flight muscles. The best-known study of the bee ball is the one Masato Ono and colleagues reported in a British science journal in 1995.
📖 Derivations of the formula, and further reading: Heat capacity (Wikipedia) ／ Japanese honeybee (Wikipedia)
ResearchWhat is still not clear
- How much does heat matter, and how much does carbon dioxide? Some reports say the carbon dioxide in the ball lowers the temperature a hornet can stand, but the relative size of the two effects has not been pinned down.
- Which bees join the ball? Some reports say bees exposed to high heat live shorter lives afterwards. One view is that many older worker bees join, and the division of roles within the colony is being studied.
- How do they "agree" on the temperature? How hundreds of bees stop making heat at about 46°C without any signal is still poorly understood.
In other words, what this article says is "an explanation of what is known so far". The figure of 46°C is also said to vary with the conditions of observation.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: "Animal life and the history of living things", "Quantity of heat" | The bee-ball behaviour, the idea of quantity of heat |
| High school | Basic Physics: "Heat and specific heat" | Check with a formula (mass × specific heat × temperature change) |
| High school+ | Biology: "Animal behaviour" | Surface area and volume, how a group warms itself |
| University | Animal physiology, behavioural ecology | Shivering thermogenesis, heat capacity, differences in lethal temperature between species |
| Research | Social and defensive behaviour of insects | The effect of carbon dioxide, how the bees that join are chosen |
| ― | Links to daily life | Beekeeping, why huddling keeps you warm |
- Ono, M., Igarashi, T., Ohno, E. (1995) Unusual thermal defence by a honeybee against mass attack by hornets. Nature 377
- Sugahara, M., Sakamoto, F. (2009) Heat and carbon dioxide generated by honeybees jointly act to kill hornets. Naturwissenschaften 96
- Papachristoforou, A. et al. (2007) Smothered to death: hornets asphyxiated by honeybees. Current Biology 17
- Heinrich, B. (1993) The Hot-Blooded Insects. Harvard University Press
- Wikipedia, "ニホンミツバチ" (Japanese honeybee; Japanese-language article)
※This article is a general-audience science explainer. The figures given are rough guides to help you understand how things work. Temperatures and numbers of bees vary with the report and the conditions. If you find a hornet nest, do not go near it; ask your local authority or a professional pest-control company to remove it.