The reduced entrances are for the hive to have a smaller area to defend when necessary. The smallest opening only takes one bee to defend but creates a traffic jam. It is good to use when other hives, either feral or otherwise within a couple of miles are looking for an alternative honey source during the dearth and they turn to robbing. You can run the full open entrance when the flow is on and the other hives have plenty of forage and there is no need to rob. Any other size opening or alternative entrances you can experiment with, keeping in mind security, ventilation and free access.
I had mine on the smallest opening when they were just starting out, and switched to the larger opening when they started expanding rapidly (about the time I added the second deep). I also use the smallest opening (combined with a robbing screen) if robbing occurs. Otherwise I use the larger opening.
I always have my entrance reducer in. The larger opening in the reducer doesn't cause much of a traffic jam, and is much easier to guard and defend than a wide-open hive. Since I can't be around all day to keep an eye on my bees, I leave the reducer in to give them a better chance of keeping robbers and moths out.
I didn't realize how robbing is such a deal. Last year I had all hives open but in September I saw a wasp walk right in my best hive. I was too late, as the hive was empty of stores. Wont happen again. I learner. Traffic jam is ok. Because bees may not have bigger opening in nature. In my area after black locust/ tulup poplar is a general dearth. Till fall that's when you have to be careful. Also bee numbers peak around aught first and brood peaks 2 weeks earlier. That's when reducers needed
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