Tim Eisele asks:
   " ... Does anyone know what it is that attracts varroa to drone
     brood, or have ideas about possibilities?..."
 
This is one of the avenues that researchers are investigating.  It has
been established that the attraction is smell.  It's a hugh leap from
that realization to synthesizing a lure.  Just another topic competing
for limited research funds.
 
Aaron Morris - thinking something smells droney around here!