I’ve heard for a long time that lemongrass oil is an excellent swarm lure. A few drops inside a swarm box full of old drone comb and the bees will be all over it.
So I went ahead and got myself some lemongrass oil ($5 at my local Bulk Barn), sprinkled five or six drops of it on some old comb (drone comb, comb with patches of honey, etc.) and set up a few swarm boxes. And within hours the bees were all over them.