In Australia, the penalty for burglary is several years in prison. But that’s for humans. For the small hive beetle, breaking and entering into the hive of stingless bees carries a far harsher sentence – being mummified alive in a sticky tomb of wax, mud and resin.
Small hive beetles are notorious pests. They invade the hives of bees, eat their stored food, and kill their young. In Africa, where the beetles come from, honeybees know how to deal with these pests – they wait until the beetles are in a confined space, and then imprison them within tombs of resin. But the beetle has invaded other continents, where bees are less adept at getting rid of them. In North America and Australia, it has become a serious pest, and can wipe out entire colonies.
The worker bees don’t sit idly by. They vigorously attack the beetles, but to no avail. When the beetles are attacked, they adopt a “turtle defence posture” by lowering their heads and withdrawing their legs under their hard body armour. They create an impenetrable dome that no bee can bypass. Eventually, the workers abandon their assault, and the beetles scurry deeper into the hive.
Stingless bees (Trigona carbonaria) – a native of Australia – use a different tactic. Mark Greco from the Swiss Bee Research Centre found that some workers immediately start coating the beetles with batumen – a mixture of wax and resin that they use to build their nest entrances. All the while, their hive-mates attack continuously, trapping the beetles in their own defensive posture. They can’t expose themselves or they’d get torn apart. Instead, they are forced to stay stock still, while the bees mummify them in their own armoured shell.
To see whether this defence actually works, Greco marked ten beetles with barium sulphate and let them loose at the entrance of five bee hives. These hives weren’t in their usual setting – Greco had placed them inside a medical CT scanner.
When people go for CT scans, they drink a cocktail of barium sulphate so their digestive tract will be easier to see. Greco used the same tactic to track the beetles. Every five minutes, he used the CT scanner to create a three-dimensional reconstruction of the hives’ interiors. In these scans, the barium-marked beetles stood out like beacons. Greco found that even though some beetles managed to make it into the hive, all of them had been completely immobilised within 10 minutes. That’s a far more effective strategy than even the resin tombs of African honeybees.
How is it that stingless bees, that only share a brief evolutionary history with hive beetles, have such an effective defence? Greco thinks that the tactic evolved in response to other threats, other species of beetles that could harm the hive.
Reference: Greco, Hoffmann, Dollin, Duncan, Spooner-Hart & Neumann. 2011. The alternative Pharaoh approach: stingless bees mummify beetle parasites alive. Naturwissenschaften http://dx.doi.org/10.1007/s00114-009-0631-9
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