With loud alarms, honey bees alert their conspecifics in the beehive to an impending attack by giant hornets. The warning shouts were similar to the screams of fear or the screeching of primates and birds with which they responded to the appearance of enemies, the researchers write in the trade magazine Royal Society Open Science. The signal is having an effect: when the warning call sounded, the workers gathered and presumably prepared their defense.
Giant hornets are among the worst enemies of the eastern honey bee (Apis cerana). As a rule, several of the many times larger hornets attack a colony of bees after a single scout has identified a suitable hive. If the attack is successful, the hornets kill the workers assembled for defense, occupy the nest and feed the now defenseless brood to their own offspring.
In order to investigate the bees’ reaction to imminent danger more closely, the team around Heather Mattila from Wellesley College (USA) listened to colonies of various beekeepers in Vietnam. The researchers hung microphones between the individual frames of the beehives and recorded the noises before, during and after one or more Asian giant hornets (Vespa soror) approached the hives.
If an enemy approaches, a cacophony breaks out in the stick, the noise level swells
As the recordings showed, there is usually a lot going on in the beehive acoustically. The insects constantly produce noises. A special hissing sound stands out among the hum and hum, the function of which has not yet been finally clarified. An acoustic stop signal is also known, which brings conspecifics to interrupt their tasks, such as the waggle dance. This dance shows conspecifics the way to a source of food.
As soon as a hornet approached the beehive, the noise level increased eightfold and a real cacophony broke out, as the researchers report. They identified a humming sound signal in the noise, which the bees probably emitted specifically as a warning of the impending attack. “This sound has a lot in common with numerous known mammalian alarm signals that immediately signal that there is an impending danger,” said Mattila.
While the bees warned their conspecifics acoustically, they whirred their wings and lifted their abdomen together with the Nasonov gland located there. This gland produces fragrances that also serve for communication between the bees. Presumably, the animals warned each other of the danger in different ways, the researchers write.
How the honey bees form and perceive the warning sound is still open. Bees do not have ears; they sense sounds rather than hear them. In their antennae they have a sense organ that can register changes in pressure and sound, the so-called Johnston organ. They also have an organ on their legs with which they can perceive vibrations from the ground. The sound communication of bees is therefore called “vibroacoustic”.
According to the study, the emitted warning sounds seemed to have an effect: The researchers observed that the workers gathered in front of the entrance to the hive, possibly to initiate protective measures. It is known from previous studies that Asian honey bees place stinking dung from other animals at the entrance to ward off hornets – a behavior that is interpreted as the first documented use of tools by bees. In addition, the bees trap attacking hornets in a so-called heat ball and suffocate them in it.
Further research should corroborate the observations and conclusions, the researchers write. “This research shows how amazingly complex the signals produced by Asiatic bees can be,” said Gard Otis, one of the researchers involved. “We feel like we have only scratched the surface of the road to understanding their communication. There is still a lot to learn.”