Towards integrated control of varroa: 5) monitoring honey bee brood rearing in winter, and the proportion of varroa in small patches of sealed brood cells

Oxalic acid is used by beekeepers to control varroa mites, Varroa destructor. Oxalic acid is effective against mites phoretic on adult bees but not against mites in sealed brood cells. In Britain and other temperate areas, brood rearing diminishes in winter and may even fall to zero, providing a natural window of opportunity for varroa control with oxalic acid. We monitored brood rearing from September to March for four years in hives in Sussex, southern Britain. We also sampled phoretic mites on adult bees and in brood cells to determine the proportion protected from oxalic acid in sealed brood cells. Our results showed that December is consistently the month with the least sealed brood, with 17 times less than November or January, on average. However, winter reduction in brood rearing varied among years, and even in December some colonies still had sealed brood (range 9–52%, n = 4 winters). Although the amounts were low, even a small patch of c. 500–600 sealed cells (300 cm2) contains approximately 14% of the mites in a colony, enough to halve the duration of control provided by an oxalic acid treatment. Our results indicate that beekeepers should check their hives in winter if they plan to control varroa with oxalic acid. This will show which month(s) have the least capped brood, whether there is period when all colonies are broodless, and if not which hives have patches of capped brood to remove.