have you uncapped some of the drone cells and looked for little
spots on the larva?  pull out several frames of bees ( the more
bees the better) and look for rust colored spot on the bees.
these are mites.
 
do you know for sure that you are the only
beekeeper within 15 miles. unless you are absolutely sure of that
then you owe it to other beekeepers to control the mites.
 
pull your honey now and put apistan on now and you won't have
any chemicals in your honey next year.
 
you say that your news bees were certified mite free. my thought
would be that you are an awfully trusting.
 
another way to go is to try the essential plant oils that have
been discused of late on the list.
 
IMHO
 
dave alexander kg7zq