When two species of Australia's stingless bees go to battle, an extraordinary amount of carnage ensues, according to a new study.

During this extreme warfare, thousands of worker bees from both sides perish, and young from the losing side are dragged out of the nest to die—a previously unseen behavior described in the December issue of the American Naturalist.

These battles decimate the ranks of worker bees, the sterile females that hives rely on to collect nectar and farmers use to pollinate their crops. (Read more about pollinators in National Geographic magazine.)

What's more, they're the only ones waging war—male drones sit on the sidelines. The battles also leave entire colonies vulnerable to parasites and disease as they settle into their newly conquered nest.

Of course, fights within and between bee species are nothing new. Robber bees in South America, for instance, make their living raiding other colonies' food stores, Christoph Grüter, a bee biologist at the University of Lausanne in Switzerland, said via email. Nest takeovers can also occur.

Australia's stingless bees, however, take the fighting to a whole new level, which has puzzled scientists: Why risk so many of your workers in months of fighting?

"The scale of the fighting reported by Cunningham is remarkable," said Grüter, who was not involved in the new research.

"It's a very risky strategy to attack another colony, and there should be some mechanisms in place that help attacking colonies choose a weak adversary," he explained.

"But it's completely unknown how colonies select their victims."

View Images Tetragonula carbonaria, a stingless bee species native to Australia, gathers nectar. Photograph by Tobias Smith

Battered, Limbless, Yet Still Fighting

Many beekeepers in southeastern Australia keep hives of the stingless bee species Tetragonula carbonaria and Tetragonula hockingsi side by side.

Usually it's a peaceful coexistence, "and then one day, bang, a hive is under attack," said study leader Paul Cunningham, an insect ecologist at Queensland University of Technology in Brisbane, Australia.

Understanding what triggers one colony to go to war against another could help the industry prevent disruptions to their honey harvest, Cunningham said in an email. (See "The Plight of the Honeybee.")

So Cunningham and colleagues began observing bee battles from 2008 to 2013, during which both species played the part of aggressor and defender.

In the one four-month-long battle studied in close detail starting in July 2008, T. hockingsi attacked and took over a T. carbonaria nest.

The first fight lasted for three days, with about 50 pairs of combatants per day.

"It's all about biting," said Cunningham. "The bees grab each other with their mandibles and don't let go." Any body part is fair game, he explained, and legs are common targets.

Losing a limb doesn't stop the fighters. "If you're an insect and your leg breaks off, at least you have five more," Cunningham said. "So the fight goes on [with] legless bees fighting bees with their wings ripped."

View Images Female worker bees attack a hive in Australia. Photograph by Tobias Smith

Royal Treatment

The second battle lasted over two weeks, during which T. hockingsi made it to the front door of the hive and were even able to drag some of the young T. carbonaria out of the nest. They were unable to take over, though, and T. carbonaria remained in control of their hive.

The third and final battle raged over six days, with over a thousand workers involved. T. hockingsi wrested control of the hive and dragged about 1,400 immature male and female T. carbonaria out of the nest and left them to die. (Watch a video about Africa's killer bees.)

Male drones from several hives—including the ones involved in the battle—collected on nearby plants during the fights. None participated in the skirmishes.

"Most likely they were arriving because these attacks are associated with a mating event," said Cunningham.

The drones would periodically swarm around the outside of the hive, possibly waiting for the conquering queen to arrive, or for virgin queens from the losing side to escape, Cunningham explained.

View Images While the females battle for control of a hive, male drones perch on foliage, possibly waiting to mate with escaping queens. Photograph by Tobias Smith

Why Go to War?

Cunningham and colleagues wanted to make sure these interspecies battles weren't a fluke, so they kept tabs on 260 T. carbonaria hives across southeastern Queensland during the study period.

They found that hives switched hands 46 times, with the majority changing from T. carbonaria to T. hockingsi.

The researchers had not expected to find interspecies battles.

Because T. carbonaria and T. hockingsi look the same, the team analyzed the genetic makeup of the fighters—and were shocked to find that they had two different species on their hands. (See some intimate portraits of bees.)

Now Cunningham and colleagues are searching for answers. "How do the stages of the attack progress?" he wonders.

"What goes on inside the hive as the attack is taking place, [and] how is the new queen escorted to the usurped hive?"

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