Why do honeybees swarm in summer?

Ben Hoare explains the biology of this imposing summer spectacle.


During a swarm, thousands of honeybees leave their hive en masse and circle before settling nearby. The resulting ball of bees clustered on a branch, fence or wall is one of the most imposing spectacles of early summer. But you’re unlikely to be stung, as swarms have no young or food stores to defend.


Swarming is the way honeybee colonies divide – it’s effectively a form of colony-level reproduction. The stage visible to us begins when a queen flies out of her hive or nest, taking over half of the workers with her to found a new colony, while the rest stay behind to raise replacement queens (only one of which survives).

So swarming is to be welcomed – a result of favourable weather and plentiful nectar and pollen supplies fuelling a rapid rise in numbers of worker bees. Since managing swarms can be tricky for beekeepers, a huge research effort has tried to identify the key triggers involved.

There appear to be several, including overcrowding in the ‘brood combs’ where larvae are reared, and new workers being produced too fast for the colony to feed them. Young workers are ‘house bees’ for their first three weeks, carrying out tasks within the nest, so there has to be a high enough proportion of older workers to forage for them all.

Another factor is the queen’s age. Older queens produce less of a pheromone called ‘queen substance’, which workers pass to each other by touch. If it becomes too dilute to be felt throughout a growing, crowded colony, the bees prepare to swarm.


  • Workers fill up with honey before swarming, so have full stomachs and can be drowsy.
  • A swarm may have 2,000–30,000 bees.
  • Swarms last a few hours or days, while ‘scout’ bees disperse to locate a permanent new site for the colony.