It’s March 2019 and I’ve deleted and retitled the 2011 post that used to be here (though the comments are still intact). But here’s the gist of it:
No matter how it’s installed, half-inch (~12mm) mesh will not prevent shrews from getting into a hive. Shrews, or more accurately, the pygmy shrew, can even slip through standard 3/8-inch metal mouse guards. That’s why I use quarter-inch (6mm) mesh to keep both shrews and mice out of hives.
I know beekeepers in Newfoundland who only use half-inch mesh to keep mice out of their hives and have done so for years. I took most of my cues from them when I first started beekeeping. None of them ever told me about shrews. I wish they had.
I didn’t have a problem with shrews when I kept my bees near downtown St. John’s. But shrews destroyed most of my colonies once I got out of the city. I didn’t know what was happening with my bees until it was too late.
Shrews slip into a hive and pick away at the cluster of bees, one bee at a time. They pick the bodies apart and eat away the innards, leaving behind desiccated pieces of bees. Here are the big quotes from Fletcher Colpitts in New Brunswick, Canada:
Shrews target the thorax by removing the head or entering through the top making a large hole hollowing out the thorax. They may also consume some of the exoskeleton leaving what looks like dirt (wings, legs and bit of the abdomen).
They are able to go through a hole less than 1cm. [0.4 inches]
With shrews weighing about 3 grams and requiring 125% of their body weight in food per day, they may consume over 450 grams of bees in 120 days of winter.
Here’s the math on that one: An average honey bees weights 1/10th of a gram, so every gram of bees is 10 bees. A shrew that eats 450 grams of bees over the winter has eaten on average 4,500 bees. And that’s only if ONE shrew is eating away at the bees. Even if all the bees aren’t destroyed, judging from what I’ve seen in my hives, the cluster shrinks and the bees becomes so stressed that they die. Shrews are the worst.
Neither mice nor shrews can get through quarter-inch (6mm) mesh.
I staple or use push pins to attach the mesh over my bottom entrances usually by the first week of October, or whenever I think it’s getting so cold that the bees are beginning to cluster. I could probably wait until December, but having lost 6 out of 8 colonies once, I just can’t risk it. Even if the bees are still bringing in pollen and the mesh knocks some of the pollen off their legs, too bad. They’ll live.
I even add the 6mm mesh to the top entrances once I know the bees are huddled down in their winter clusters for good. I probably don’t need to attach mesh up top, but again, I’m extremely cautious. When I lost my hives to shrews, the hives were buried in snow most of the winter and the shrews were able to get into the hives through the top entrances. So it may be safe to keep the top entrances open, but if the level of snow is anywhere close to the top entrances, the mesh goes on.
I keep the mesh on until the bees start bringing in pollen again, which for me is usually some time in April or May. But if there’s still snow on the ground or crazy Newfoundland freezing rain and snow is happening, the mesh stays on. If the bees are cold and clustering, and therefore unable to defend themselves against mice or shrews, the mesh stays on.