A Game of Hives: native bees at war

The Iron Comb? Australian native bee hives seem to be worth going to war over. This is a top view of a Tetragonula carbonaria hive. Image: Tim Heard.

Who would have guessed that Australian backyards might be a battlefield for bees?

Or that these deadly skirmishes can involve aerial battles lasting days, thousands of fatalities on both the attacking and defending sides, ousting of the helpless from the hive by the attackers and eventual overthrow of the resident queen, with the victor’s queen then being installed in her place.

A cluster of dead native bees found on the ground in a Brisbane backyard was enough to entice a group of scientists into a deeper investigation of this unusual behaviour of the Australian native bee species, Tetragonula carbonaria.

Their study of the backyard colony, published in The American Naturalist, led to the surprising discovery that the T. carbonaria study colony was not only being attacked by neighbouring colonies of its own species but also by a closely related species, T. hockingsi.

While the T. carbonaria species was known to engage in battles between neighbouring colonies prior to this work, this study provided the first evidence of fatal fighting between different species.

Fighting to the death

Fighting to the death or ‘fatal fighting’ is relatively rare in nature. Evolutionary biologists propose that this is because species have evolved different ways to assess strength and fighting ability that doesn’t involve the loss of the individual.

In species where fighting does escalate to death, scientific theory predicts the risk of death is outweighed by the benefits being obtained, such as fighting for scarce food resources, mates or nest sites.

Fatal fighting has been well studied in ants with beneficial outcomes including slave-making, raiding of nest supplies and gaining access to new food sites.

In the case of the T. carbonaria, the researchers hypothesised that the fighting swarms were most likely attempts at taking over neighbouring hives—that is, the nest location, nesting materials and stored resources.

To test their hypothesis, they made regular observations on the ‘study’ hive in the backyard and collected the dead bees after fights for analysis. Using modern molecular techniques they were able to track which group of bees were attacking and which were defending. It was this analysis that led to the surprising discovery that the attacking bees were in fact a separate species.

Following a succession of attacks by the same T. hockingsi colony over a four-month period, the defending T. carbonaria colony was defeated and the hive usurped, with the winning colony installing a new queen.

The war rages on

To ensure that what had occurred at the study hive was not a one-off event, the research team monitored the colonies of over 260 commercial T. carbonaria hives over a five-year period, recording any changes in species through changes in hive architecture (see note).

[NOTE: T. carbonaria has a brood chamber, in which cells are even and connected by their walls to adjacent cells at the same height—see the spiral-like chamber in the image top of page. The T. hockingsi brood chamber takes on a less organised appearance, being an irregular lattice comprised of clumps of around ten cells connected by vertical pillars—see image below.]

A hive of the T. hockingsi species. Image: Tim Heard.

They found evidence of 46 interspecies hive changes (via the change in hive architecture) during the five year period, which were most likely to be usurpation events.

There is still much to be learnt about these small creatures, such as what instigates the attacks, how and when the invading queen enters the nest, and whether the young in the usurped hive are spared and reared as slaves, or killed outright.

For Australian native bees, it is thought that the capture of a fully provisioned nest (including ‘propolis’, pollen and honey stores) is a sufficiently large benefit that it outweighs the loss of so many lives.

As the researchers note, this is an excellent example of how little we actually know about small stingless bees, which can be excellent and resilient alternative pollinators to declining honey bee populations.

Reference

Cunningham JP, Hereward JP, Heard T, De Barro PJ, West SA. 2014. Bees at War: Interspecific Battles and Nest Usurpation in Stingless Bees. Am. Nat., 184(6).