Slavs believed that bees were the purest beings and the only ones whose soul devil can't corrupt and that beehives are the only place where devil can't hide.... In South Slavic languages, the word "uginuti" means "to die" but is only used for animals. The word "umreti" also means "to die" but is only used for people. And bees... The reason for this could be that in some parts of Serbia people believed that the soul of the deceased migrated into a bee. So bees and humans had the same soul. Slavs believed that bee only bites people who committed some kind of sin. In Lika (Croatia), at Christmas people would fill a small wooden vessel used for scooping flour from storage with grains, and would stick three beeswax candles in the grain, one for dead, one for bees and one for grain.From the Old European Culture blog.
There are also rituals which strongly suggest that the ancient Slavs knew that bees were closely linked to the fertility of grain, although they probably did not know precisely the way that pollination caused this to occur.