https://www.newscientist.com/article/2174097-honeybees-gang-up-to-roast-invading-hornets-alive-at-a-terrible-cost/

When hornets attack a hive to carry off bees to eat, a group of worker bees quickly surround the intruder. The bees vibrate their wing muscles to generate temperatures of about 46oC for more than 30 minutes, enough to kill the hornets. It’s crucial they deploy the balls quickly, otherwise the hornet releases pheromones that attracts reinforcements.

Entomologist Atsushi Ugajin at Tamagawa University near Tokyo began wondering about the costs to the honeybees. He wondered if heat exposure in the balls might reduce their life expectancy.