When a hornet attacks, bees know what to do. A few hundred workers swarm around the predator, roasting it alive with their body heat. The bees vibrate their wing muscles to generate temperatures of about 46 degrees centigrade (the normal hive temperature is 32 degrees centigrade). Hornets attack hives to carry off bees to eat. Unfortunately, the bees defence mechanism is a sacrificial mission for the bees involved.
These bee-balls have to be formed quickly around the attacking hornet to prevent it releasing pheromones to attract reinforcements. The bees that defend the hive by forming a ball will then die quicker than those that remain in the hive. (Within 10 days compared with 16 days for the remainers; Behavioural Ecology & Sociobiology).
Hornets can attack hives 30 time a week in the autumn. In experiments, it was also found that if you exposed the bees to a second hornet attack, the bees that had joined in the first ball were more likely to help out a second time.
Even the insects are displaying some serious survival instincts when defending themselves.