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