The Asian giant hornet (Vespa mandarinia), also known as murder hornets, can brutally attack and wipe out entire honeybee colonies in hours. Once the hornets infiltrate a nest, they remove the hive's brood and take bee larvae and pupae back to their nests to feed their own young, reports Katie Hunt for CNN.
However, honeybees are not wholly defenseless to hornet attacks. Asian honeybees (Apis cerana) will warn hive mates of an impending attack with their bodies when hornets are near. The bees cock their abdomens up in the air and vibrate their wings to make a noise comparable to a scream for help, reports Sabrina Imbler for the New York Times.
The scream is called an antipredator pipe, and it's similar to the shrieks and panic calls that mammals, like primates and meerkats, use when they are afraid, according to a new study published in Royal Society Open Science this week.
"Our study showed that the bees didn't make the sound if there weren't any hornets. It was made very infrequently in response to smaller hornets, a bit more often if the bees smelled a giant hornet (but didn't see one), and they made them by far the most when a giant hornet was directly outside of their nest," the study's first author Heather Mattila, a biology professor at Wellesley College, tells CNN in an email.
Murder hornets are native to Asia but the invasive insects recently started a buzz in the United States after a nest was discovered in 2019 in Washington state. For this new study, researchers focused on how Asian honeybees react when the Vespa soror, another species of giant hornet, threatens a honeybee hive. Mattila and her team first took note of the Asian honeybee's alarming call after hearing it while documenting the honeybee's use of animal dung to ward off hornets in Vietnam, reports Jennifer Ouellette for Ars Technica. The practice is called fecal spotting, and researchers found that hornets were less likely to chew into honeybee hives if the entrances were lined with poop, Ars Technica reports.
As Mattila and her colleagues continued to observe bees, the team noticed whenever giant murder hornets were near, noise levels of the hives shot up.
"We could hear the bees' sounds from several feet away," Mattila explains to Ars Technica. "So, we started popping microphones into colonies so that we could eavesdrop on them."
The scientists also video recorded the bee's highly organized behavior in hives of local beekeepers. Researchers recorded the honeybee soundscapes and behaviors while hornets were near, when hornets were not present, and when paper soaked in hornet pheromones was placed near the hive, reports Kate Baggaley for Popular Science.
After analyzing over 30 hours of bee noises containing 25,000 instances of acoustic signaling, the team found that even if the nest was not under direct attack by hornets, the hive was bustling with activity if hornets were near. Bees would start to communicate with each other by vibrating their wings and thoraxes in a frantic motion, per Popular Science. The technique, known as piping, sends vibrations that fellow bees can sense with their legs. When Asian hornets or their scent is present, the hives are louder.
"They make them in rapid series, and so it sounds like a siren that's going on and on and repeating," Mattila says to Popular Science. "They change a lot in tone; they're really harsh and noisy."
Aside from making the eerie noise, the signal causes more bees to gather at the hive's entrance and begin other defense actions against the hornets. Like dung-smearing and forming bee balls to kill incoming hornets, a statement explains. Bee balling is when hundreds of bees swarm together around a hornet to squeeze and constrict it to a point where the hornet can't breathe, CNN reports.
Mattila and her team suspect the noise may function as an alarm signal since the noise peaked as hornets hovered outside the colony's entrance, but the data is correlative, so why bees' scream is not fully known, the New York Times reports. The researchers also noted that when bees create the sound with their thoraxes and wings, a pheromone-producing gland is exposed and may employ other communication strategies to rally more bees when undergoing an attack. Scientists are planning on researching the gland's purpose next, Ars Technica reports.