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Here is an article on bee hive temperature regulation. Just some general info. (I don't endorse any particular vendor) https://www.honeybeesuite.com/physics-for-beekeepers-temperature-in-the-hive/

The thing to watch for is a lot of bees fanning at the entrance, They are trying to increase the air flow to regulate hive temps. If brood gets much above 95 F. it might die. (I think sort of like us getting a high fever?) So increasing the hive opening is what you want to manage. They are fully capable of insulating a brood nest and considering that field bees will increase numbers in the hive during the night when temperatures drop, it is more likely that having larger opening vs one too small is the direction to err toward. So I start the season off with a weak hive with an opening of just an inch wide or so. I then graduate toward a 2-3 inch opening and eventually remove entrance reducers if the hive is really strong. Especially with outside temps above 90 F.

 

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If you have a screened bottom board without the insert, there is no need to open the entrance reducer beyond the 4" opening. Assuming this is a fairly new hive as an established colony does not need an entrance reducer now at all.

Hi JW, I have a couple of somewhat related questions. I have generally used screen bottom boards although I have a board to slide in to close off the screen ventilation. This slides in underneath at a distance that allows me to insert a lunch tray that I put veg oil in another slot. I normally remove the solid bottom when it starts getting hot outside if I remember. So there is more ventilation than a fully open entrance.

I found out today that supposedly some Michigan BK use a screen top as well as screen bottom. The claim is that because we see high humidity during our Summers that bees have trouble controlling the temperature. I had never heard of this before, and am a bit dubious of the claim. All the same we are surrounded by water and I have seen Texan's flee during our hot and humid Summers because they "couldn't stand the heat".

So: is there a case to be made that in some climates a bit more top ventilation might help bees with temp control. From my limited experience the bees I had in screen bottom board hives seem to do just fine. I haven't much data to draw any conclusions. People don't associate overheating issues with Michigan very often. I have had 2 borderline heat stroke experiences in Michigan. Hi humidity is not fun at all. You are so soaked with humidity and perspiration that you don't remember to drink enough water. Unlike many Southern states where the heat is dry and dry skin might tip you off to drink.

 

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So a top screen cover might help on the days when it's in the 90's with 90%+ relative humidity.

It occurs to me that some shade over the hive might help as well. Or maybe just a popsicle stick under the cover. That would be rather easy.

Michigan weather can be a real pain. Once commonly fished for brook trout in the UP. One day in the upper 80's, humid, and mosquitoes were brutal. One pulled off a "death star" on me flying into my lung. Massive hacking caused a bloody nose...you can guess, like blood in the water for sharks. Fishing was good however. Next day 39 degrees and snowing. Caught bigger trout.