At its simplest, a queen excluder is a mesh divider between the brood box and the honey super in a hive. The mesh is sized so the worker bees can pass through easily but the larger queen can not squeeze through.
Queen excluders may be made of plastic or metal, occasionally with a wooden frame. The material is a matter of personal preference. The plastic excluders are a bit less expensive; the metal ones will be more durable over the long run.
Why Use a Queen Excluder?
It may seem counter productive to keep the queen from some parts of the hive. She is the queen after all. In this picture you can observe a good reason to keep the queen from the honey super. Note the darker cells in an arch pattern near the bottom center of the frame. This is brood. The queen placed eggs in these cells. Since the cells are capped, we know that the developing bee is somewhere between 8 and 21 days old. Workers have stored honey in the rest of the frame which appears as a lighter color. This is a normal pattern and fine in the brood box. But when it comes to harvesting the honey, let's just say no one wants a pupae on their biscuit.
How To Use a Queen Excluder
Before adding a honey super, place a queen excluder on top of the brood box, then set the honey super on top of the excluder. This will act as a preventative before the queen starts laying eggs in the super.
If you have not previously installed an excluder and notice brood in a super, carefully examine every frame in the super. Ensure the queen is not on these frames. If you do find the queen, gently move her down into the brood box. Then install the excluder between the brood box and honey super.
All is not lost if you do find brood in the super. Simply install the excluder, wait until the bees have hatched, let the house bees clean out the cells and hope they will refill with honey. If they do not refill these cells, the rest of the frame can still be harvested but the yield will be slightly less.
Why Not to Use a Queen Excluder?
More experienced beekeepers often do not use queen excluders in their hives. Bees may build comb on the excluder, blocking some of the access to and from the honey super. The larger drones may attempt to pass through and become stuck, again blocking the passage. Either of these situations will decrease the honey production.
When not using an excluder, the beekeeper should keep watch for where the queen is laying new eggs. Boxes can be rotated in the stack if necessary to keep the queen from climbing into the honey super. This technique is typically used in the early spring and keeps empty cells above the queen.
Given the operation at Five Feline Farm as hobby level beekeepers, it is far easier to prevent the problem in the first place. We are now adding queen excluders at the same time as the first honey super. This is the first year any of our hives had brood in the honey super, so we have learned another step we wish to take.
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