In beekeeping, a queen excluder is a selective barrier inside the beehive that allows worker bees but not the larger queens and drones to traverse the barrier. Queen excluders are also used with some queen breeding methods. Some beekeepers believe that excluders lead to less efficient hives as often worker bees, not used to travelling through the excluder, are intimidated and stay in the lower brood box. This can lead to rapid filling of the brood box and overcrowding resulting eventually in the hive swarming.