This is the time when you should begin to remove the sealed drone brood that have an increased number of Varro mites. You can diminish the mite population considerably by continuously growing drone brood and 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 grow 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, although 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.