Over the past decades, millions of managed colonies of honeybees have died from varroa mites that transmit deadly viruses, yet wild colonies survive.
Without miticides used by beekeepers, managed colonies almost always die out within two or three years, often falling victim to mite-transmitted, wing-shriveling deformed wing virus (DWV). But miticides can contaminate honey and steady use can lead to resistance in mites.
The study reports that beekeepers can learn lessons from the ecology of wild colonies: their small nests and frequent swarming lead to lower varroa mite (Varroa destructor) infestations, less disease and better colony survival..
“Our study sheds light on the fact that, while we manage honeybees to maximize honey production and profits, we may not be managing the bees in the way that is best for the bees themselves,” said J. Carter Loftus ’14, who conducted the research as part of his undergraduate honors thesis with adviser and senior author Thomas Seeley, the Horace White Professor in Biology in the College of Arts and Sciences. Michael Smith, a graduate student in Seeley’s lab, was also a co-author.
In the study, the researchers set up two sets of 12 colonies, with one set in large hives (168 liters, typical for managed colonies) and one in small hives (42 liters, typical for wild colonies).
Generally, when a colony gets too large for its nest, one half of the colony departs with the old queen to establish a new colony. This is called “swarming.” The bees left behind rear a new queen. Beekeepers control swarming and maximize honey production by housing colonies in large hives.
In the study, when colonies in the small hives grew too large for the space, they swarmed. Over the two-year study period, 10 of the small-hive colonies swarmed, while only two of the 12 large-hive colonies swarmed.
The researchers found three times higher density of mites in the large-hive group, and 10 of the 12 colonies died. In the small-hive group, only four of the 12 colonies died, and of those, one collapsed because the queen began to lay only unfertilized (drone producing) eggs. The remaining three that died had heavy mite infestation. Loftus and colleagues suspect this occurred because when the large-hive colonies died, bees from the nearby small hives robbed the large hives of their honey. In doing so, they may have picked up mites and brought them back to their hives.
“If we had spread these colonies out further, we might have avoided that problem,” said Smith, who added that future work will explore the effects of hive proximity on mite infestations and disease.
Seven of the 12 colonies in large hives developed high levels of DWV, but the researchers observed no DWV in the small-hive colonies.
Overall, the researchers found the small nests of wild colonies promoted frequent swarming as colony populations grew, forcing half the adult bees to leave the nests, presumably taking mites with them and lowering mite densities in colonies. Mites also feed on bee larvae, but when bees swarm, and before a new queen gets established and lays eggs, there is no brood for a period, which could cull mites from a colony.
“Unfortunately, honeybees kept in small hives do not produce a huge surplus of honey, so keeping bees in small hives is not a viable option for commercial beekeepers,” said Loftus. The study suggests hobby beekeepers could manually split colonies to mimic the effects of swarming and maintain healthy colonies.