14 DAY TRIAL // For those of you not living in Japan, the giant hornet's name probably doesn't elicit too much panic. But the large insect is one of nature's most effective killers, with instances in which some 30 giant hornets killed as much as 30,000 honey bees in a few hours being reported more than once. In a new study however, researchers managed to discover a very peculiar defense mechanism that some bees employ in staving off the attack of single hornets – they simply pack themselves so tightly around the invader, that it cannot move, breathe or perform any actions. The bees remain in position until the hornet dies.
The researchers who carried out the study took some hornets and anesthetized them, after which they strapped them to the end of either a thermometer or a gas probe. The entire ensemble, with the hornet at the tip, was then touched to bee hives. Almost immediately, large numbers of insects exited the hive and attached themselves to the invader so tightly that when the probe was removed a ball of bees remained at the tip, still clenching the hornet.
Naturalists say that this is the only effective way bees have of getting rid of the hornets. And, they add, it only works if the hornets are seen in time. If they attack by surprise, the bees stand no chance whatsoever. Over the course of a regular attack (30 hornets versus 30,000 bees), only 2 to 5 hornets die on average, whereas the bees are all destroyed, their hives ransacked, and their larvae and pupae used as food for hornet larvae.
In a new study appearing in the latest issue of the journal Naturwissenschaften, researchers from the Kyoto Gakuen University, in Japan, say that, up until now, experts have believed that the bee-balls were killing the hornets through heat. After being exposed to temperatures higher than 46-47 degrees Celsius for more than 10 minutes, hornets perish, but the new research shows that the temperature inside the “ball” never exceeds 46 degrees Celsius. As such, the BBC News informs, another factor must be at work, the team hypothesized and proceeded towards measuring gas concentrations inside.
They learned to their amazement that the thing that killed the hornets in less than 10 minutes was a high concentration of carbon dioxide, which was also created when the invading insect was sealed from the world. “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' defense. Either way, the carbon dioxide increase and/or the oxygen decrease lowered the temperature that was lethal to the hornets,” explains researcher Fumio Sakamoto, one of the authors of the new study.