My friends overwintered bees died. He treated them but never did a mite check to see if the treatment worked. Possibly the bees came out of winter with a high mite count. Even though he treated for Varroa, the hive still died. The treatment probably was too late to save the bees. He went to look at them a week ago and all of the bees were gone. This is a symptom of Varroa. One week the bees look great, the next week there are no bees in the hive. Sometimes there is a small cluster of bees with the queen left.
If you have an overwintered hive and you have not done a mite check, I strongly urge you to do so now. It might even be too late to save some of them. But by mid July, untreated overwintered colonies may be empty of bees.
One of the only mite treatments that can be used during a nectar flow is Mite Away Quick Strips (MAQS) or Formic Pro. Both of these treatments are Formic Acid. They cannot be put on if the daily temperatures exceed 85 degrees F. during the first three days they are on. But the honey supers can be on when using this type of mite treatment. MAQS is a seven day treatment, Formic Pro is a fourteen day treatment. When you put formic on, you should not have messed with your bees for three days. Meaning, if you looked at your bees, pulled some frames, did a mite check etc. The hive should be closed up and the formic should be put on three days later. If you put the formic on immediately after you dug into the hive, jumbling up the bees, you can have a greater mortality to the bees.