Lloyd,
 
Your comments make me wonder if the appartent natural resistance of some
hives may be due to a queen which continues to lay drone eggs up till the
end of the laying cylce for the year.  I only have 20 hives but I've noticed
this in at least one of the hives. when the mites first arrived here they
wiped out everyones apairies, 100%!
  Two years later while driving along a rural road,  I stumbled onto ten
hives, seven of which were alive. The owner kept them  in gums with no frame
never treating with any medication. To my surprise, he wanted to sell
because the State bee inspector was after him to transfer them into modern
equipment.
After buying them I did the tranferring and treated with strips but have
always wondered if they could have resistance.
 
Steve Davis
 
 
>However, in July/August the
>bees are raising relatively few drones so as the mite population increases
>and the drone larvae decrease, the mites start to attack worker larvae.
>Once they infect a large percent of worker larvae, the hive is doomed.