Perhaps in areas of cold winters an inner cover will provide a layer of insulation and ventilation that might help prevent water condensation from gathering on the underside of the cover and dripping down on the bees? Sort of like crating an 'attic'?
If you use telescoping covers, you need to use an inner cover of some kind. It not, at some point you will find that they have attached burr comb from the top of each frame to the telescoping cover. In cool weather, it will be very difficult to get the lid off. Very disruptive to the colony. Don't ask how I know this.inch:
Migratory covers work good to and you don't need an inner cover.
Traditionally the inner cover was basically used, as mentioned, to keep the telescoping outer cover from being cemented in place by the bees with propolis. The oblong hole in the inner cover was to be used in conjunction with a "bee escape" device, which is rarely used any more.
I've taken to using covers that I designed myself. I have also eliminated any bottom entrances, so I have even designed my covers with a small hole routed into the rim in order that even with the covers square on the hive top there is a small entrance to allow for ventilation and to make it easier for the bees to defend their hive if robbing were likely. In order to give the bees a larger top entrance all that needs to be done is slide the cover back a little.
It is shown with the rim assembly unattached to the flat cover assembly.
Most of my nuc boxes came with migratory covers, but I bought a case of unassembled nuc boxes from a retired beekeeper one year. They had telescoping covers but not an inner cover.
I built one and was using it when one time I tried to get the cover off and found that the frames had been glued to the lid. Quite a mess. You can't get your hive tool in there to pry the lid from the frames. There is a real chance to damage bees and your queen while trying to get the lid off of all of the frames while pulling all (or some)of the frames out of the box stuck to the lid.
I took changed the rest of the nuc boxes from the case and made the side pieces braces for the top like a migratory lid. Basically leaving the sides off. They work great. I still have the telescoping lid, but it hasn't been used since. I need to knock off the side pieces. Or make an inner cover for the nuc box.
There are times when the inner cover comes in handy as I learned during our fist spring here for putting on feeders and such but overall not much use..Also in some other countries, with more cost sensitivity, I've seen beeks use just an old coffee sacks and the like on top of the frames to prevent the bees from gluing the telescoping cover ..Seems to be a cheap solution which can also help with humidity control somewhat..Overall I'm more and more intrigued with top entrances and dumping the telescoping in favor of a simpler cheaper cover such as on Michael Bush's website...
When it rains here, it usually rains very hard, I'm sure that lots of rain goes into the many upper entrances my hives have during the Summer. So far, most all the rain that goes into the hives drains out the bottom screen, I haven't seen any bees drowning from the rain. It doesn't just come in the little vent/entrance port, but I slide the cover and honey supers back to create entrances on the top of each super --->
But I'm sure it isn't nearly the amount of rain experienced by those few colonies that build their combs in the open air.
1) Since you can't easily pry up a telescoping cover the inner cover is used to prevent propolis sticking it down.
2) It is used a lot for feeding. If you just put an empty chamber on the bees will build comb in all the open areas.
3) It can be used to allow more ventilation by propping up the telescoping cover with a stick.
4) With rim up in the winter it can be used to hold "emergency feed" as a safety against starving.
5) Rim down in winter provides a small air space above the frames. Usually a piece of foam board is placed above it with a tunnel as a top entrance and for moisture to leave the hive.
6) During harvest the inner cover is put under the first honey super (the one right above the chambers) with a bee escape placed in the hole. The bees will exit the supers and be unable to re-enter (drones tend to get stuck though from what I hear).
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