--snip--
Beekeeping
books list cerana drone cell size at 4.7mm and worker cells at 4.3mm.
According to "The Varroa Handbook" varroa has never been able to reproduce
in cerana worker brood.  Thats' really the key to why cerana and varroa
coexist.   If you are seeing varroa reproduce in Scut worker cells of 4.3 mm
then cell size is NOT the answer with cerana and the reason varroa doesn't
reproduce in cerana worker cells is another reason besides simply cell size.
--snip--

In Apis cerana varroa is found nearly only on drone brood (here too varroa likes our drones more than our workers).  The cerana workers pierce the drone brood caps with a small hole, and I believe that I have understood correctly that they use this hole to detect severe varroa infestation (including perhaps other problems) and clear the drone and varroa out when the varroa levels get too high.  The question is why is varroa only on the drone brood?  Because their cells are larger?  Probably not only, but because they have a longer development period.  So cell size may be incidental to varroa levels.  Has anyone noted a correlation between cell size and worker development time?  Do small cells not only make smaller workers, but make them faster - too fast for varroa buildup?  Maybe that's what leads to the reports of reduction in varroa in colonies with small cell sizes.  I'm sure someone with more experience than I can answer that question.

The other Asian cavity-nesting bee that I know of that has coevolved with varroa is Apis nigrocincta - it too pierces holes in its drone caps.  Whether it evolved that trait in parallel to cerana or that one species has evolved from the other and inherited the trait I do not know.

Martin
wannabe (soontobe?) beekeeper