Honeybee hordes¹ use two weapons - heat and carbon dioxide - to kill their natural enemies, giant hornets.

蜜蜂部落使用两种武器-热量和二氧化碳-杀死了它们的天敌，大黄蜂。

Giant hornets were taped to temperature and gas probes

Japanese honeybees form "bee balls" - mobbing and smothering²（令人窒息的） the predators³（食肉动物）.

This has previously⁴ been referred to as "heat-balling", but a study has now shown that carbon dioxide also plays a role in its lethal⁵（致命的） 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 voracious⁶（狼吞虎咽的，贪婪的） predators that can devastate⁷（毁坏） bees' nests and consume their larvae⁸（幼虫）.

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（可靠地，确实地） kill the hornets.

"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（使…麻醉） giant hornets and fixed⁹ them to the tip either of a thermometer probe, or the inlet（插入物） of a gas detector¹⁰.

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 relatively¹¹ 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."

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（致命的） 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 emissions¹² take place," said Dr Sakamoto.

This might suggest that the bees are aware of what physiological¹³ state the hornet is in.

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