Yggdrasil is the Cosmic Tree of North-European mythology, although the name "Yggdrasil" is specifically Norse
I believe that Yggdrasil was originally a real tree somewhere in Europe, far back in the mists of time, which was venerated as the "axis mundi", the axis of the world. Eventually, as tribes migrated and the memory became mythology, Yggdrasil's basis in reality was forgotten and lost to posterity.
Yggdrasil is described as an evergreen tree that takes its nourishment from the Well of Wyrd. It has a hollow space inside, in which the last man and woman will be preserved at the renewal of the world. Four stags roam its branches, eating the leaves and dripping honeydew from their horns, which falls to the ground and returns to the Well. Bees make honey from the honeydew. The tree is afflicted with some kind of fungus, and the three Wyrdae/Norns must wash it with water from the Well each day to keep it healthy (the well-water is said to make the tree “white”).
Interpretation: The tree is obviously a yew tree. Yews are evergreen and are often hollowed-out inside while new living wood continues to grow around the empty centre; they are the only tree which can live indefinitely. Although it is unlikely the ancient Europeans were aware of the role aphids play in honeydew production, the “stags” may represent aphids, which suck sap from the leaves (sap drawn up from the Well-waters) and excrete honeydew, which coats the leaves of the tree and rains down in sticky-sweet drops on everything beneath. Aphids even have “horns” with which they can do battle with each other. Trees hosting aphids often suffer from “sooty mould”, a greyish-black fungus that grows on the leaves and bark when they are coated in sticky honeydew. The mould itself is not harmful, but if it becomes too abundant it can prevent adequate light from reaching the leaves and inhibit photosynthesis. The fungus can be removed by washing the tree, as the Wyrdae/Norns bathe Yggdrasil, to “whiten” it and maintain its health. Bees are attracted to honeydew and make honey from it just as they do from flower nectar.
In this context, drinking mead made of fermented honeydew becomes an indirect imbibing of the waters of Wyrd's Well. Honeydew mead is thus literally “the nectar of the Gods”, the “mead of inspiration or wisdom”, given to human beings and ingested by us as a way of participating in the cosmic, primal cycle of nourishment that begins and ends in the Well of Wyrd. Bees, and perhaps aphids, must have been seen as sacred, almost mystical creatures in the eyes of our ancestors.