Looking at the long range weather forecast it appears we will be getting repeated nights with temperatures below freezing (32F/0C) in the next two weeks. This will trigger the field mice to begin looking for a warm winter home. If you don't want your bee hive to become this home it is time to put on your mouse guards. Over the course of a winter mice can do a lot of damage to a hive; chewed frames, destroyed foundation, urine smell, feces, etc. A mouse guard can save you a lot of repair work and expense come spring. At the same time you should also position your entrance reducer so its smallest opening is controlling the movement of the bees.
The mouse guard can simply be metal hardware cloth with a 1/4 inch size. Larger sizes may permit a mouse to squeeze through. Mesh sizes down to 5 holes per inch can be used. Anything smaller, such as 6 or 8 holes per inch may prevent movement of the bees through the mesh and should NOT be used. A more expensive alternate is a metal mouse guard procured from your favorite bee equipment supplier.