Ever see a bunch of bees buzzing around the entrance to a small, weak, or sickly colony, seemingly not exactly sure where they are going, or being repulsed by the guard bees at the entrance to the hive? You likely were observing robbing. Several things can cause this. Some breeds of bees are more prone to rob than others. Extremely weak hives kept in the same yard as extremely strong hives will also create a situation where the strong rob the weak, sometimes to the point of the weak colony collapsing because they literally starve to death.
How to prevent robbing
If you are feeding a colony with an inner feeder, this can induce the robbing behavior. Only feed this colony at night. Night feeding will prevent robbing, because bees don’t fly at night, but they will drink syrup.
If you must feed through the day, use an open feeder. Don’t put it in a hive body, or the bees will get used to robbing. It’s like training them to steel.
Reduce the entrance of the weak hive. A small colony doesn’t need their entrances wide open. A small hole, roughly an inch in diameter should be perfect. You can use a square stick or grass covered with duct tape to close unnecessarily large entrances. This way the guard bees have less they have to cover.
Entrance reduction disks are also an option, although I tend to use what I find in the bee yard or easily have on hand. If they are more your style, beekeeping supply catalogs have them fairly inexpensively.
Another option is to take frames of emerging brood from the very strong hive and add them to the very weak hive. This will work to average them out somewhat. Generally bees are loyal to the colony they emerge in. Shake the bees off in the original hive and only move the brood, being especially careful not to take the queen from the large hive with the brood.
It is always an option to separate the two hives, moving one or the other to a separate yard several miles (at least) away from each other.
Whatever route you choose, robbing is a serious situation that can quickly grow out of hand. Any potential diseases or pests are transferred between the two hives, and the weak hive may very easily starve. When you notice robbing it is important to address it immediately so no further harm is done.