I read recently an article that evaluated this method of varroa control,
and if it wasn't on BEE-L then it was in a recent issue of the American
Bee Journal.  Study results indicated that although the dark hive (hence
high hive temperatures) WAS effective in controlling (keeping in check)
the varroa population, the high hive temperatures also resulted in a
significant reduction in honey production.  As usual, there's good news
and there's bad news.  Sorry that I can't better identify the source of
this information.
 
Happy Holidays!
 
Aaron Morris - I think, therefore I bee!