Mice can cause huge problems in hives and can be hard to permanently get rid of. In early fall they will enter a hive and mark it with scent. Once a hive is marked the mouse will return in late fall and hide out until winter. They are bad house guests and make a huge mess inside of the hive. I have been told mice can fit into the large hole with a standard entrance reducer. Small mice can fit into holes the size of a dime. They build nests by eating part of the comb of several frames and filling it with grass. If you check your bottom board during the winter and find a large amount of wings and legs; there is a good chance you have a mouse. If temperatures are extremely low you may not be able to remove the mouse until spring. I was fortunate enough to have a beekeeper friend give me one of these mouse guards when I started beekeeping. It is a simple and effective designed at almost no cost.
To “build” you cut hardware cloth (metal screen) the width of your entrance and about 7″ tall. One of 7″ sides are cut smooth. This will be the bottom. The top cut you with the small metal bits sticking out. They are sharp so watch out. Then you bend it into a U, about 3″ from the smooth side. Then angle the top spikes facing up, about an inch in from the end. When I do this I use leather gloves and a piece of wood to help bending.
Insert the guard into your hive with the exposed sharp points sticking up. It should be a tight fit but not dig into the wood. The sharp points are designed to keep skunks away. When the skunk attempts to scratch the hive, their paws get cut on the spikes. Bees do not seem to mind the guard and I have watched bees carry pollen into the hive with no problem.
I gave this to a beekeeper who was finding a mouse in their hive every weekly inspection for a month. Once the guard was installed the mouse did not return.
UPDATE: The screen size is 1/2 inch and will get a little smaller when bent.