Crowding honeybee colonies in apiaries can increase their vulnerability to the deadly ectoparasite Varroa destructor

When humans switched from hunting honeybee colonies living scattered in the wild to keeping them in hives crowded in apiaries, they may have greatly increased disease transmission between colonies. The effects of clustering colonies were studied. Two groups of 12 colonies, with hives crowded or dispersed, were established in a common environment and left untreated for mites. Drones made many homing errors in the crowded group, but not in the dispersed group. In early summer, in both groups, the colonies that did not swarm developed high mite counts, but the colonies that swarmed maintained low mite counts. In late summer, in the crowded group but not in the dispersed group, the colonies that swarmed also developed high mite counts. All colonies with high mite counts in late summer died over winter; all colonies with low mite counts in late summer survived over winter. Evidently, swarming can reduce a colony's mite load, but when colonies are crowded in apiaries, this mite-load reduction is erased as mites are spread through drifting and robbing.