Bees have a gland on the tips of their butts called the Nasanov gland
. You can see them on the landing boards, or around the entrances with their butts in the air and wings fanning. What they're doing is scenting the air to tell the other hive members the location of the entrance.
Lemon Grass (Cymbopgon Citratus) Oil mimics Nasanov pheromone. It's pretty potent and a drop or two will bring them in to check out whatever it's on. I had a swarm trap out within spitting distance of two of my colonies and they seemed to camp out in it, coming and going without actually doing anything inside. Fooled me into thinking I had caught a swarm several times before I took it down in frustration.