Suppressed mite reproduction explained by the behaviour of adult bees
John R Harbo and Jeffrey W Harris
Suppressed mite reproduction (SMR) is a heritable trait of the honey bee (Apis mellifera) that can control the parasitic mite, Varroa destructor. The purpose of this study was to determine whether or not adult bees with the SMR trait affect mites in brood after cells are capped. Colonies with or without the SMR trait were each given a comb of newly-capped worker brood that was naturally infested with varroa. Each of 7 source colonies provided a comb of brood to at least one SMR (n = 9) and one control colony (n = 8). These combs were removed from their host colonies 8 days later and mite populations were evaluated in cells with bee pupae that were > 8 days post-capping. Colonies with SMR bees averaged 2.2% of their cells infested with mites; controls averaged 9.0%. Therefore, bees with the SMR trait apparently removed mites from capped cells. Of the mites that remained, the SMR colonies had a much higher rate of nonreproductive mites, 80% vs. 29%. This suggests that bees with the SMR trait removed reproductive mites more often than they removed nonreproductive mites. When comparing only the number of nonreproducing mites that had no progeny, the groups were almost identical averaging 1.2 and 1.3 mites per 100 cells of brood. This suggests that the SMR bees did not remove mites from brood cells if the mites did not lay eggs. By targeting the reproductive mites, bees with the SMR trait give the illusion that nearly all of the mites are nonreproductive. Therefore, our selection for a high frequency of nonreproductive mites may have produced bees that primarily remove reproductive mites from capped brood.