After finding the propolized remains of a mouse in one of my hives years ago, I started using the Universal Entrance Reducers sold by several suppliers. Here is an example:
http://betterbee.com/products.asp?dept=351
They are held in place with two cork board thumb tacks, so they can be removed in a pinch, and they pretty much last forever. But, I leave them on year round and just slide open the main opening, once the weather warms. Have not had a rodent inside, since I started using them. They are also useful to reduce the opening to help alleviate robbing. I do use top entrances (Imirie shims - the ones from BetterBee with the round hole that comes with a plug) in the winter, since the snow can really pile up some winters (not this one).
Bill
Claremont, NH
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