It sounds shrill through the hive: Researchers have identified a special alarm signal that Asian honeybees generate when threatened by giant hornets. The distinctive tone has interesting similarities to alarm calls from vertebrates. Apparently the acoustic signal helps the bees to organize their defense against the rabid predatory insects. The discovery is thus another example of the amazingly complex abilities of state-building insects, say the researchers.
Blossom nectar and pollen form the basis of life for honey bees, but it is well known that there are also socially living insects with an appetite for meat: Wasps and hornets like to prey on insects – some species also target bees. The honey bee species Apis cerana, which is widespread in the Far East, is particularly affected by this: its colonies are afflicted by the Asian giant hornet Vespa soror, which can be up to four centimeters in size. After being scouted out by scouts, these predatory insects approach the squadron and attack bees and the brood of a hive. That is why they are feared by Asian beekeepers.
But as an international team of researchers has already shown in previous studies, the Far Eastern cousins of our honeybees are not completely helpless victims: They have developed astonishingly complex defense strategies against the giant hornets. For example, they set up stinking barricades from collected animal droppings at the entrance to the hive when scout hornets appear there. This enables them to better defend themselves against a possibly advancing invasion squadron. The scientists have also already demonstrated a special combat strategy: Since the bees can only do little with their stings, they take the enemies into a headlock. They form a cluster around a hornet and generate deadly heat through muscle vibrations.
On the trail of acoustic communication
As the researchers are now reporting, a special communicative element apparently also belongs to the defense system of the Asian honeybees. It is already known that bees also transmit information acoustically. They generate the tones by causing their bodies to vibrate. As part of their studies on Asian bee colonies in Vietnam, the scientists therefore also overheard the insects through microphones in the hives. As they report, a bee colony usually sounds calm and relaxed. But as they found, this changes in a characteristic way when a giant hornet appears: “I could hardly believe what I was hearing from a meter away from a beehive,” reports co-author Gard Otis from the Canadian University of Guelph.
More detailed investigations then showed: When the bees of a colony have detected a giant hornet on the hive, the background noise in the beehive increases to a level eight times higher than the normal state. A special acoustic element stands out in particular. This newly discovered signal sounds shrill and its frequencies change abruptly, acoustic analyzes have shown. It thus resembles the alarm, fear, and panic calls known from primates, birds, or meerkats in response to predators, the researchers say. “The signals are conspicuous and have acoustic properties that are apparently aimed at attracting the attention of colony members,” says lead author Heather Mattila of Wellesley College in the USA. “These sounds also convey a sense of urgency for our human perception,” says the researcher.
Hornet species are distinguished
Interestingly, the scientists were also able to show that the acoustic pattern apparently specifically means “giant hornet alarm”. Because the bees do not use it when a less dangerous hornet species appears on the hive: Vespa velutina only captures individual bees and does not approach the squadron like the giant hornets after being scouted by scouts. Apparently, the bees can differentiate between these two hornet species and then show correspondingly different reactions, the scientists explain.
The team also found that when the Giant Hornets alarm sounded, the bees lift their abdomen, buzz their wings, and frantically begin to run. In addition, they expose a gland that produces odorous substances. This indicates that in addition to the warning tone, they are using other means of conveying information in order to attract the attention of the stick members. Ultimately, all of this leads to the state preparing for an imminent attack, according to the observations. So the insects begin to build barricades from animal manure at the entrance to the hive.
Once again, the study illustrates the highly developed behavior and forms of communication that representatives of insects are capable of – a group of animals that has long been thought to be rather simple. Otis and his colleagues now want to investigate the astonishingly complex signal system of the bees further: “We have the feeling that we have only scratched the surface of the understanding of their communication. There is still a lot to learn, ”says the scientist.
Source: University of Guelph, Wellesley College. Articles: Royal Society Open Science, doi: 10.1098 / rsos.211215
Video: Asian honeybees react loudly to an attack by giant hornets in Vietnam. (Credit: Heather Mattila / Wellesley College)