C. Effect of Cell Size
Partly because mites reproduce better in drone brood than worker brood, people tend to think that smaller cells would decrease mite reproduction. However two recent studies show that there was either no difference in mite population between colonies (Ellis et al., 2009) using “small cells” (4.8 to 4.9 mm diameter) and regular foundations (5.2-5.4 mm), or small cells actually had a significantly higher mite population (Berry et al., 2010). Unfortunately, neither of these recent studies determined the fecundity or fertility of mites in the two types of cells.
Earlier studies were conflicting. Taylor et al. (2007) found that “foundation” cell size did not affect the reproductive success of V. destructor, but more mites invaded cells drawn from the 4.8mm foundation . However, Piccirillo and De Jong (2003) and Maggi et al. (2010) found that mite invasion rate increased positively, and linearly, with the width of worker and drone brood cells, probably because brood that develops in large cells receive more visits from nurses, increasing the invasion chance. Maggi et al. (2010) also found that the percentage of fertile mites was lower in smaller cells. An earlier study (Message and Goncalves, 1995) showed in Africanized bees, larger cells had a higher invasion rate, and also had higher effective fecundity in mites.
Our own study suggests that cells that are too large also reduce mite reproduction (Zhou et al., 2001). In a study trying to determine the mechanisms of why varroa mites do not reproduce on worker brood of A. cerana, we accidentally discovered that in both A. cerana and A. mellifera queens laid worker eggs in drone cells in the fall. We took advantage of this, and compared the reproductive output of mites on two hosts: workers reared in worker-cells (WW) or workers reared in drone-cells (WD). In 2001, both the fertility and fecundity of the two groups were significantly different (Fig. 4). It is not clear why mites would reproduce less on identical hosts that were housed in larger cells. One possibility is that workers reared in drone cells are fed a different diet by nurses (One study showed workers reared in drone cells were heavier and had more ovaries, suggesting a different diet or more nutrition). A second possibility is that workers spin larger cocoons in drone cells, and mites detect the extra space, and this affects their reproduction.