Everyone should have a robber screen for every one of their hives! 

When you need it, you need it over night. Not having one on hand could doom your hive(s). 

Some people leave these on year round. It is best to have them on whenever you are feeding and BEFORE robbing starts.

It doubles as a mouse guard too and also keeps out robbing honey bees and native wraps. 

In times of dearth robbing can break out among your hives. Even when internally feeding, it attracts neighboring hives to invade weaker hives, and rob out ALL of their honey. If left unchecked, your hive will fight violently, yet can be overwhelmed and starve, abscond, or even dwindle in population until they are all gone. This can happen with hives right next to each other or over large distances.

Watch your bees; if things seem frantic, bees walking fast on the surface of the hive, bees pushing each other, grabbing other bees by the legs, ... then robbing has started. I catch it quickly with that last one. When I see activity by the entrance that seems panicked, then I get my face a couple feet away and watch. I watch incoming bees, looking for them to land and walk to the entrance. If I see one or two bees try to drag that one away, I know robbing has started. I know it needs to be addressed THAT night.

Since I catch it early, I put on a screen that night when all the bees that belong to the colony have returned and the robbers have retreated. The screen goes on and I monitor it the next few days to see if it needs to be adjusted.

I make my screens so they fit any of our hives. 

It's mounted with screws so the true hive entrance is either centered or at the opposite end from the screened entrance that's shrouded with flashing.

You'll see in these pictures that the Robbers are attracted by scent. They are huddled on the screen right in front of the original entrance. You'll see bees walking all over the hive body, searching for a way in, tricked by the screen and the tiny gaps between the boxes. They are about 10" away from the true entrance that's shrouded by the metal flashing. This stops them from seeing the colony leave and enter the true entrance.

You'll see we mounted these on the side wall of a top bar hive and on a Warré hive. All it takes is a couple screws.