I have received several calls from beekeepers going out to put on their winter covers only to find there are no bees in the hive. The hive was empty, no bees, full of honey.
In every case the bees died of Varroa mites. Classic mite kill is in late fall.
Varroa builds up with time and in September and October the mite populations explode. The mites become so prevalent that they overcome the colony. The bees know they are sick and will all drift away from the colony over about a two week period.
Many of the hives that died were over wintered from last year. Some were new colonies from this year.
Some of the beekeepers in these cases did treat for mites but did not treat properly and there was not much effect on the overall mite population.
After a mite treatment the hive should be checked again for mites to see how effective the treatment was.
One beekeeper treated his colonies with MiteAway Quik Strips. I think he had about 45 colonies. He checked his colonies after the treatment and found some of the colonies still had some very high mite counts. He had to treat those with Hopguard. But this check saved his colonies.
A nation wide study that was completed recently. Several bee labs took 30 colonies in many locations around the country. Bees were installed and left on their own with no treatments of any kind.
The results were interesting. Most of the colonies were dead after two years. All of the colonies were dead after three years. The research showed that in every case, the bees died of Varroa mites.
The conclusion is, to check the mite load of the hive and treat if necessary. There are two food grade treatments available that are very safe for the bees, the hive, and the beekeeper.