First I thought they were all home with nothing to do. Been communal feeding. As I look there is some fighting going. Perhaps this is the way they will pare down the population going into winter? There doesn't seem to be fresh capping below the hive. What think you all? I thought the strongest hive was the least likely to get robbed. How frustrating.
reduce the entrance first of all. then check your hives for food stores. While communal feeding (imo)is easier, less time consuming, less stressful i think you ending up feeding a lot of other possibly unwanted guests. place your sugar water into a plastic bag(2 lb) tie it and drop it into the hive. the bees will pierce the bag and there will be no leaking to mess up the hive and attract ants.
http://www.bushfarms.com/beesrobbing.htm