The Queen Excluder
Queen Excluders can be a very useful tool in a beekeeper's arsenal.
They are grid screens that have large enough spaces for the worker bees to pass through, but small enough spaces that the queen would be excluded from using parts of your hive that you’ve blocked off. Queen Excluders are usually plastic or metal and have been designed to create a barrier between the brood boxes and the honey supers. The reason you might want to have this barrier is because most beekeepers do not want brood eggs in the middle of their honey frames.
Even if you allow the eggs on a honey frame to hatch, the postnatal cocoons will acquire a dark brown tint from a variety of things. This include a combination of remnants from the egg, debris that has entered the hive (usually dirt tracked in by the bees’ feet), as well as feces from the bees within the hive already.
The tastiest and most pure honey comes from frames that have never had eggs in them. The best way to guarantee that your queen won’t lay eggs in your honey frames is by making these frames inaccessible to her using a Queen Excluder.
So Why Don't All Beekeepers Use Queen Excluders?
Many do, but some not-so-jokingly say that Queen Excluders should rather be named Honey Excluders because they can sometimes slow down the workers from filling the honey frames as well.
Secondary Uses of Queen Excluders!
Some experienced beekeepers have been known to use Queen Excluders below their bottom brood box, but above the bottom board.
Having a barrier here can be twofold:
1. If you just bought a new nuc, it will help prevent your new hive from swarming away. Since the queen is trapped in the hive, she will start laying eggs, once she has started laying eggs, she will consider your new hive her new home and you can remove the Queen Excluder from the bottom of your hive.
2. This position will also help prevent large wasps, large spiders, mice, and even the infamous murder hornet from getting into your hive and making a mess.