Nov 11, 2021

When Murder Hornets Attack, Bees Start Heartbreaking 'Shrieks' to Warn The Colony


Bees can do a whole lot more than just buzz. When faced with a deadly 'murder' hornet, a new study has found Asian honeybees (Apis cerana) can respond with a complex repertoire of warning signals.

These signals are so sophisticated, researchers have compared them to the alarm shrieks, fear screams, and panic calls of primates, birds, and meerkats.

Some of the recorded bee calls – known as 'antipredator pipes' – have never been described in this species before, and seem to arise only when a particularly deadly predator is present. 

The Asian hornet (Vespa velutina) and especially the giant hornet (V. soror) certainly fit the bill. These voracious predators – closely related to the 'murder hornets' recently found in North America – frequently attack bee hives, and in unfortunate cases may even destroy an entire colony, slaughtering the bees one by one.

So, when a scouting hornet or even just its scent is detected near the hive, there's no time for the colony to lose.

When the presence of predatory hornets was sensed, researchers found worker bees at the hive entrance strung their antipredator pipes together into "longer, insistent messages, analogous to how individual clangs of a bell contribute to a sustained fire alarm."

Technically, these alarm bells are actually vibroacoustic signals, or short pulses of vibration and sound frequencies produced by a bee's wings and thorax.

Even when there isn't a threat around, Asian honeybees are always 'buzzing' to some extent, 'hissing' as a group or 'head-butting' each other with vibrational pulses (although we still don't really know what these signals convey).

According to the current research, which gathered nearly 30,000 honeybee signal recordings, when a murder hornet shows up, the usual hissing and head-butting increases by seven-fold.

At the same time, worker bees also begin making antipredatory pipes, which are harsher and more irregular in their frequencies.

Video recordings show the bees make this antipredatory signal by raising their abdomens, buzzing their wings and racing about "frantically".

In some cases, the signals seem to cause worker bees at the hive's entrance to go into defense mode, spreading animal dung on the hive to repel predators, or trying to form a ball of bees around the scouting hornet, which heats it to death.

It's hard to say if the antipredatory pipes carry messages about either of these specific defensive actions, but as a whole, the robust database of new recordings suggests bee pipes are a "rallying call for collective defense".

"These sophisticated defences require timely predator detection and swift activation of a defending workforce," the authors write.

"Vibroacoustic signals likely play an important role in organizing these responses because they are transmitted quickly between senders and receivers within nests."

Compared to a swarm of giant murder hornets, researchers found attacks by smaller hornet species that hunt on their own didn't cause quite as much chaos in the honeybee colony. The anti-predatory pipes were initiated less often as well. It's therefore possible this newly described call is specially evolved for murder hornets.

"This research shows how amazingly complex signals produced by Asian hive bees can be," says behavioral ecologist Gard Otis from the University of Guelph in Canada.

"We feel like we have only grazed the surface of understanding their communication. There's a lot more to be learned." 

The study was published in Royal Society Open Science.

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