Page last updated at 15:48 GMT, Friday, 3 July 2009 16:48 UK

Honeybee mobs overpower hornets

By Victoria Gill
Science reporter, BBC News


Honeybees mount a very effective mobbing defence

Honeybee hordes use two weapons - heat and carbon dioxide - to kill their natural enemies, giant hornets.

Japanese honeybees form "bee balls" - mobbing and smothering the predators.

This has previously been referred to as "heat-balling", but a study has now shown that carbon dioxide also plays a role in its lethal effectiveness.

In the journal Naturwissenschaften, the scientists describe how hornets are killed within 10 minutes when they are trapped inside a ball of bees.

Japanese giant hornets, which can be up to 5cm long, are voracious predators that can devastate bees' nests and consume their larvae.

But, if the bees spot their attacker in time, they mount a powerful defence in the form of a bee ball. This study found that the heat inside the bee ball alone was not enough to reliably kill the hornets.

Giant hornet
Giant hornets were taped to temperature and gas probes

"They can survive for 10 minutes at a temperature up to 47C, and the temperature inside the bee balls does not rise higher than 46C," said Fumio Sakamoto, a researcher from Kyoto Gakuen University in Japan, and one of the authors of the study.

His team recreated experimental bee balls and took direct measurements from inside them.

They anaesthetised giant hornets and fixed them to the tip either of a thermometer probe, or the inlet of a gas detector.

Once the hornets recovered from their anaesthesia, the probes were touched to the bees' nest.

"The bee ball formed (around the hornet) immediately," said Dr Sakamoto.

After 10 minutes the bees were packed solidly enough around the probe to be removed from the nest in a distinct ball.

As the temperature inside the ball increased to more than 45C, the carbon dioxide level also rose sharply.

In a parallel experiment, the scientists found that in an atmosphere relatively high in carbon dioxide, the temperature at which hornets could survive for 10 minutes was lowered.

"So we concluded that carbon dioxide produced inside the bee ball by the honeybees is a major factor, together with temperature, involved in the bees' defence."

Bees mobbing a hornet
The bee ball formed as a bump on the bottom of the nest

Dr Sakamoto is not sure, at this point, whether the bees were effectively "gassing" the hornets, or simply depriving them of oxygen.

"Either way, the carbon dioxide increase and/or the oxygen decrease lowered the temperature that was lethal to the hornets, " he told BBC News.

"We are going to do the additional experiments about this point using mixed air of various oxygen and carbon dioxide (concentrations)."

The mob of bees also appeared to operate in "two phases".

"The hornet may be killed during the first 0-5 minute period, in which the highest level of heat production and carbon dioxide emissions take place," said Dr Sakamoto.

This might suggest that the bees are aware of what physiological state the hornet is in.

Dr Sakamoto said: "The latter 5-10 min period may be free running to ensure their victim's death."

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