The Varroa mite, Varroa destructor, is an external parasite that lives on honey bees (both Apis mellifera and Apis cerana). The adult mites feed on the hemolymph of adult honey bees and lay their eggs in brood cells in the honey bee hive. This parasite is one of the main threats to honey bee hive health and is thought to play a role in colony collapse disorder. The concurrent transmission of viruses, such as deformed wing virus, causes the main mortality in the honey bees.
This is the time when you should begin to remove the sealed drone brood that harbor a proportionally higher number of Varroa mites than worker brood. You can diminish the mite population considerably by continuously having the bees rear drone brood and then removing them regularly. The drones are usually reared until mid-July, after which the drone removal can be stopped.
You can continue removing drone brood for as long as the colony is willing to rear them. The swarms and nucleus can be treated for Varroa with treatments like evaporating thymol or trickling oxalic acid in as early as midsummer, if no honey is harvested from these colonies during the same season.
The winter bees develop during August and September and Varroa mites might injure the developing bees. It is for this reason that the Varroa control in August is essential. The details of the techniques and protocols for Varroa treatment vary slightly between the Nordic countries, but the basic principles are the same in beekeeping regardless of bees or area.
Nordic brown bees have a shorter brood rearing period compared to other subspecies. Brood rearing begins later in the spring and stops earlier in the fall. The broodless period also begins earlier in the fall and gives a good chance for Varroa control with oxalic acid on adult bees.