This is a bit more detail about the mouse guards that I use on the Brookfield Farm beehives. What I use has evolved over time, and will probably continue to evolve. For the moment it keeps out the deer mice, which plague all of my yards.
At the farmyard, a team of mouse-control experts closely inspects my mouse guards on the beehives.
The cats do offer an extra advantage that they are not bothered by the bees, and their hunting helps keeps down the mice. But inanimate mouse guards are still needed.
As seen in the image, the mouse guards run the entire length of the hive’s entrance.
The wooden mouse guards are cut in an elongated “O” shape, from cedar (preferable) or pine. Cedar’s better because of our rains. Pine tends to rot unless painted. Half-inch hardware cloth is then cut and stapled to the back of the wood.
If I run the center of the hardware cloth down the center of the “O” the remaining space is less than ½ inch and greater than ¼ inch. This seems to keep the mice out, or slow them down enough to give the bees time to attack.
They are held in place by what I call plant hanger bolts, but are, I believe, called anchor bolts in the hardware store. The key is that they are basically screws with no heads. These bolts are screwed into each side of the bottom screen. The mouse guards just fit over two holes that are drilled into the mouse guards. In the image of the front of the hive you can see two somewhat rusted bolts sticking though the mouse guard.
I have also used 2 pieces of 1/8-inch hardware cloth, which are folded in half to create a wedge. These are cut to lengths that allow only a one-inch opening, when they are placed at each side of the hive entrance. If you do this, make them secure. Mice are rather powerful for their size.
If I think a hive is weak, especially in the spring, I will often put these 1/8-inch hardware cloth wedges behind (inside the hive) my mouse guards. This helps reduce the amount of area to be guarded by the bees, but still allows ventilation.
I started with the traditional wooden ones, but unless you nail them into the hive, which I don’t want to do, they don’t work. The mice push them away.