OK, bear with me here. I have been keeping bees for three years now and have yet to lose a hive over winter (Chicago Area). I always insulate and run with an upper and lower entrance (reduced). Last year one of my hives went through around 30 lbs of stores from October until March. I thought that was real good until Finski (the Finish beekeper on this forum) basically called my bees pigs. He thought that 30 lbs was a very large amount of stores to go through. Many on this forum and others said that running an upper entrance causes them to burn through much more stores than they normally would.
Now, most of us know that an upper entrance is used to prevent condensation over the bees in the winter. A hive in the winter is a very humid environment. Without an upper entrance, hot humid air created by the bees would rise, hit the top of the hive, condense, and drip on the bees, killing them. However, if you have the most insulation over head then condensation over the bees would be practically impossible, instead condensing on cooler surfaces (the sides or bottom). So, if you throw a bunch of insulation over top of the bees, why have an upper entrance?
I have been thinking of this for some time and was on the fence. I mean, how can you argue with my success for overwintering? Why change? I think I'm going to change though. The tipping point came with this month's beeculture and ABJ. In this month's CAP article, they discussed varroa mite biology. There was a section on humidity in the hive which stated that varroa hate humidity. The higher the humidity in the hive, the less varroa are able to reproduce. The section concluded with the following statement, "[i]f there are ways to artificially increase the [relative himidity] to about 80%, then the varroa mite population will never increase to a damaging level." Whoa!
We all know that hives are incredably humid during the winter, and most of us do everything we can do to reduce this. It now occurs to me that we might actually be helping out the varroa when doing this. So, I'm going to give the "no upper entrance" thing a try. Insulating heavily over the top of the bees is a must.