> Obviously, Apis cerana  has some form of
defense that it uses against the varroa. Perhaps this could be
transferred to Apis mellifera?

Apis cerana drills a hole in the cap of drone cells.  If they detect varroa, they haul the pupa out and dispose of it.  All GM 'products' currently available are simple insertions of a very small number (usually one) of genes.  To transpose such a behaviour from one species to another is, for now, truly beyond our ability, since it almost certainly involves a large number of genes.  Maybe in a million years Apis mellifera will evolutionarily 'learn' to do this too.  Maybe we can speed it up by selection.  Maybe we can select for a different defense, maybe our bees will evolve a novel defense.  I don't think we'll be able to 'engineer' them to.

Martin Damus