I've gotten several replys to my question which were informative but nobody that had actually had experiance with a mite resistant bee strain.Again I can only assume tis mites killing my swarms,I have'nt had any tests run since its such a hassle.But the facts are:I had no problems for 2 years,all colonies thrived & wintered well.The last 2 winters,all colonies were dead by shortly after X-mas.Starvation is out since all hives had ample stores.In every hive bees were still mostly in clusters on full combs & simply dead.If anyone has other theories on cause of death I'm all ears. Of course being a bee-keeper, I will order several more swarms & have at it again.Thanks for your help, Mike "I take my chances,I don't mind working without a net, I take my chances,I take my chances every chance I get" Mary Chapin Carpenter