I had a visit from a German lady beekeper last year who had carried out some
work at University on using temeperature for mite control before becoming a
commercial beekeeper.
 
As I recall, they took sealed frames of brood out and heated them to
temperatures in the mid 40 degrees celcius.  The sealed brood could stand it
but the varroa were killed by the temperature.
 
Apparently it was not considered suitable for commercial use as there was only
a few degrees difference between the temperature needed to kill the varroa and
the temperature at which the sealed brood was killed.
 
If some our beekeeping friends in Germany are on line, they may be able to
provide more accurate details.
 
Trevor Weatherhead
AUSTRALIA