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.
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.’
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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