Haven't tried it my self, but a neighbor has a spray bottle of water & vanilla flavoring, he sprays both old & new bees, claims they all get along fine.
Any body else tried this ?
This is true when combining bees from 2 different hives. Even combining races of bees can cause a knock down drag out so a masking agent helps. Vanilla is a very good one.
You can mix brood frames from several hives together when making splits without incident but when it is limited to 2 hives and you shake bees from one into the other or togethe into a split the bees will fight unless a masking agent, a strong one, which vanilla is, is used.