Kamikaze bees kill invading hornets by forming ‘hot defensive balls’

Honeybees kill invading hornets by forming kamikaze ‘hot defensive balls’ that reach temperatures in excess of 46°C in a bid to defend their hive

  • Experts marked 350 Japanese honeybee workers to record their age in days
  • The bees – aged between 15 and 20 days old – typically live for several weeks
  • The control group lived for 16 days at a constant temperature of 32°C in the hive
  • The group that formed the ‘hot defensive bee balls’ were all dead within 10 days  
  • Bees that joined in with the first defence were more likely to join in with a second
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Honeybees that form kamikaze-style ‘hot defensive balls’ to protect the hive against intruding hornets shorten their lifespan during the process.

When honeybee hives are under attack, the insects will form ‘hot defensive bee balls’ which can reach temperatures in excess of 46°C (115°F).

Experts have been aware of the desperate defensive measure, which they can sustain for more than 30 minutes at a time, since 1995.

However, new research has shown that the defensive measure from the bees, which is enough to kill the hornets, also reduces their lifespan significantly.

Scientists have also found bees which have been involved in previous skirmishes are more likely to return when another attack occurs – despite the increased risk to their lives.

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 Kamikaze honeybees will return to kill invading hornets despite the fact the technique they use to assault them shortens their lifespan. This stock image shows honeybees using their ‘hot defensive ball’ defence against a hornet

The finding was made by entomologists at Tamagawa University near Tokyo, who were interested in how the ‘hot defensive ball’ manoeuvre affected honeybees.

They marked 350 Japanese honeybee workers with colours to record their age in days, according to reports in New Scientist.

The worker bees – aged between 15 and 20 days old and which typically live for several weeks – were then divided into two groups.

The group that formed the hot balls were all dead within 10 days. The control group, which was kept in the hive at a constant temperature of 32°C (90°F), lived for 16.

Writing in the paper, its authors said: ‘Our experiments revealed that bee ball participants died significantly earlier than nonparticipants of identical age.

‘These findings strongly suggest that the high temperature created in-side of the bee ball had a physiological effect on participating workers and resulted in reduced life expectancy.’

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When honeybee hives are under attack, the insects will form ‘hot defensive bee balls’ that can reach temperatures in excess of 46°C (115°F). This stock image shows a western honey bee carrying pollen back to the hive

HOW DOES THE HONEYBEES ’HOT DEFENSIVE BALLS’ DEFENCE STRATEGY WORK?

Honeybees should have no chance against the ferocious Japanese hornet – the predators are an inch (2.5 cm) long.

But the tiny creatures can actually triumph by swarming over their foes in such numbers that hornets are ‘cooked’ inside a ball of bees, a technique first discovered in 1995.

The defence mechanism is known as a ‘hot defensive bee ball’.

When hornets attack, they kill all the worker bees, before ‘looting’ a nest for larvae and food. To prevent this, Japanese honeybees developed the defence mechanism to stop the predators.

The bees swarm over the hornets in groups of up to 500, and start vibrating their wings until the temperature reaches 46°C (115°F). The heat is fatal for the hornets.

It’s vital that this happens quickly, or the hornets can release pheromones to call for assistance. 

In one study, conducted in 2012, researchers in Japan watched the bees as they assaulted an inch-long hornet – pulling them off the ball as they attacked and scanning their brains to see how they coordinated their attacks.

The scientists, Takeo Kubo of the University of Tokyo and Masato Ono of Tamagawa University sampled bees at different points during the assault – and found that bees engage higher brain functions as they swarm into the ball.

The bees coordinate their attacks, sharing information about heat in the ball – which could be a trigger for the bursts of brain activity.

When hornets attack, they kill all the worker bees, before ‘looting’ a nest for larvae and food. So the bees developed the defence mechanism to stop the predators.

The bees swarm over the hornets in groups of up to 500, and start vibrating their wings until the temperature reaches 46°C (115°F). The heat is fatal for the hornets.

It is vital that this happens quickly, or the hornets can release pheromones to call for assistance.

Hornets attack hives as many times as 30 times a week in the autumn, so honeybees have to be constantly on guard.

When the next attack occurs, bees that were part of the first defence are more likely to join in a second ‘hot defensive ball’, researchers found.

Randolf Menzel, who studies bees at the Free University of Berlin, told New Scientist that is isn’t exactly clear what drives this behaviour.

One suggestion is that the heat may damage their brains and reduce their capacity for self-preservation. 

The full findings were published in the journal Behavioral Ecology and Sociobiology.

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