I leave the entrance block in! You can get different types of mouse guard, some with holes over the whole length and some just with a central section, whatever you only need an inch or two for the bees to get in and out over winter and that space to have mouse guard holes covering it. Some rely only on a reduced entrance in width and height but that is a bit risky. If you ever have mice once you won't want them again! Yuc!
I put it on when the weather gets cold enough for the bees to stop flying and when they are obviously not stood at the entrance looking for intruders.
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