Brood removal influences fall of Varroa destructor in honey bee colonies

The hygienic removal of Apis mellifera brood infested with Varroa destructor disrupts the reproduction of the infesting mites, and exposes the foundress mites to potential removal from the colony by grooming. Using brood deliberately infested with marked mites, we investigated the association between the removal of mite-infested brood and the removal of exposed foundress mites in Italian (IHB) and Russian honey bee (RHB) colonies. Our results showed that RHB colonies removed more mite-infested brood in significantly less time (average = 87.9 ± 2.0% for 2.6 ± 0.1 days) than IHB colonies (average = 61.9 ± 7.3% for 3.2 ± 0.1 days or 19.3% per day). For the inoculated brood that was not removed, RHB colonies had lower proportions of brood cells containing: (a) live marked mites regardless of reproductive status (RHB = 4.4 ± 1.3%; IHB = 17.7 ± 5.9%); (b) dead marked mites (RHB = 1.1 ± 0.5%; IHB = 7.1 ± 2.2%); (c) lost introduced marked mites (RHB = 6.6 ± 1.6%; IHB = 13.3 ± 2.8%); and (d) reproductive marked mites (RHB = 8.3 ± 6.3%; IHB = 23.8 ± 6.9%) than IHB colonies did. These observations suggest that RHB colonies indiscriminately remove mite-infested brood regardless of mite status. Regarding trapped mites (i.e., those found below a modified queen excluder), the number of mite-infested brood cells removed positively correlated with the number of mites that were trapped in both honey bee stocks. The majority of the trapped mites fell during the first three days post mite inoculation, which coincided with the highest rates of brood removal. The highest proportions of trapped gravid foundress mites were also recorded during this time, when host bees were early in their development. The comparatively strong and rapid hygienic response of RHB to mite-infested brood and the associated removal of infesting foundresses are probably parts of a suite of factors responsible for suppressing V. destructor populations in RHB colonies.