Hubby built an anti robbing screen for one of the hives last night. I think this is going to work. The bees come out of the entrance of the hive and into the screen at the bottom where the notch is cut out. They travel up and across to the notch at the top to leave the hive. The robber bees have not orientated to this situation because they have never had to use it to leave the hive. The bees that have left the hive, know to use the notch at the top to come and go. the robbers hang out at the bottom corner of the hive and try to get in.
On the hive last night.
Robbers trying to get into the hive.
It worked so well, I made one for the nuc today.