Earlier today, I published a post on how Japanese honeybees defend themselves from hornets with a mass defence that relies on heat and carbon dioxide. This article was originally written two years ago, and describes the slightly different tactic of Cyprian honeybees.
When Oriental hornets attack, Cyprian honeybees mob them in a huge ball that targets the breathing apparatus in the hornet’s abdomen. The hornets can’t breathe without expanding their abdomens and with sheer numbers, the bees strangle the hornets to death.
Hornets are giant wasps that pack a powerful sting. To most people, they can be a painful nuisance, but to honeybees, they’re killing machines. Hornets greatly outsize and overpower honeybees and a few individuals can decimate entire colonies.
Asian honeybees have developed a remarkable defence called ‘heat-balling’ against their local hornet, Vespa velutina. A giant ball of bees piles onto the predator, weighing it down while vibrating their wing muscles. The frenetic activity greatly increases the temperature inside the ball to about 46C – hot enough to cook the hornet alive, but five degrees under the bees’ maximum tolerated temperature.
Cyprian honeybees face a different predator, the Oriental hornet Vespa orientalis and unlike its wimpier cousin V.velutina, this species can take the heat. The Oriental hornet lives in hot, dry climates ranging from Central Asia to the Mediterrenean and it tolerates temperatures just as high as honeybees.
Heat-balling shouldn’t work on them. And yet, Cyprian bees still encase Oriental hornets in large balls. Surprisingly, the strategy works – despite their heat tolerance, the hornets still die. The bees’ stings are useless against the hornet’s tough cuticle and they barely use them. What could they be doing instead?
Rising temperatures and high carbon dioxide emissions are the means through which humans are inadvertently causing the decline of several species. But one animal actively uses both heat and carbon dioxide as murderous weapons – the unassuming honeybee.
With their stings and numbers, bees already seem to be well-defended but they are completely outgunned by giant hornets (right). These two-inch long monsters are three times longer than several times heavier than tiny honeybees and raiding parties can decimate entire hives. European bees mount little in the way of an effective defence, but Japanese bees aren’t so helpless. When their hives are invaded, they launch a mass counterattack.
Swarms of workers dogpile the hornet, pinning it down while vibrating their wing muscles. At the centre of this “heatball”, the frenetic buzzing heats up the hornet to a roasting 45 degrees Celsius.
Scientists have long thought that this manoeuvre bakes the hornet alive, for the bees that surround it are more resistant to high temperatures. But Michio Sugahara and Fumio Sakamoto from Kyoto Gakuen University have found that this isn’t the whole story.