New beekeepers are usually told to never feed their bees using an open feeder because it can trigger robbing and fighting instincts in the bees, transforming them into bees that are not fun to be around. The bees act like they’re hepped up on caffeine. They behave frantically. As usual, I don’t believe anything until I try it myself and, indeed, through the wonderful world of experiential learning, I discovered that open feeding can have that effect on the bees. But not always.
In my experience, any sugar syrup close to the hive, whether in an open feeder or spilled on the ground, can trigger the robbing instinct, especially if the syrup is spiked with something like anise extract. However, if the open feeder it placed 30 metres or so from the hive (~65 feet), the bees might crowd in on the syrup, but don’t usually start fighting each other to the death to get at the syrup, and when they fly back to their hives, they don’t try robbing the neighbour hive of the all honey.
I prefer this method of feeding at certain times of the year — long before or long past harvesting any honey — because it’s so easy to do. I don’t have to load up each hive with a feeder. I just refill the open feeder once it gets empty. Open feeding usually doesn’t last more than week.