Hi Peter
There are two separate issues here: what attracts mites to drone brood, and why. It's pretty clear that in A. mellifera mites will reproduce in both worker and drone cells, but they prefer drone brood. This is why drone trapping is successful in controlling mite build up in its early stages. They seem not to spill over into the worker brood until the levels are quite high. On the other hand, in A.

Peter Rosenkranz (et al) wrote in 2010:
In the original host A. cerana the reproduction of Varroa mites
(V. jacobsoni and V. destructor) is limited to drone brood for yet 
*unknown reasons*. This phenomenon is considered a crucial point for
the balanced host–parasite relationship in A. cerana.
Journal of Invertebrate Pathology 103 (2010) S96–S119

So, we don't really know why this relationship between drones and varroa came about, but in terms of evolutionary explanation, it would appear that varroa seek out drone larvae in either species, primarily by the odor. Some evidence points to their evaluating cell size, but it is hard to see how a tiny mite could "measure" the circumference of a honey bee cell, while the odor cue is much more well supported by data.

PLB

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