I think it is important for beekeepers, even beginners, to know the life cycle of Varroa destructor, otherwise it is difficult to understand what we are trying to achieve with various treatments.

It is difficult for the ordinary beekeeper, including me, to work out for themselves what the life cycle is from our own observations, so I set out below what is the most likely, based on the most authoritative information I can find. The information available varies considerably in timing, but I think it is more important to know what happens, rather than exactly when. The observations may have been made under different conditions, hence the variation.

Varroa mites can only reproduce under capped brood cells. They emerge with the bees to take up a phoretic stage, which gives mites a chance to transmit to other colonies. It is often stated that adult mites only enter worker and drone cells, with a preference for drone, but on several occasions I have seen them in queen cells, even in very lightly infested colonies.

In a simplified form, the following is what happens when there is brood in the colony. For more detailed information please consult more authoritative sources, but please remember it does vary somewhat, so even the experts disagree. For that reason, I only give the basics, which I think is all the ordinary beekeeper needs to know. When there is no brood, adult varroa mites are apparently able to live for 5-6 months on adult bees.

A female mite enters a brood cell before capping, sinking into the brood food below the larva.
The cell is sealed and the female starts to lay eggs, the first is always male, all subsequent eggs are female.
It takes 6-7 days for both sexes to develop into adult mites within the cell from the laying of the egg.
The male mates with females in the cell.
The mother and female offspring emerge from the cell with the bee, the male and immature females dying in the cell.
They transfer to an adult bee, although they move between bees, where they stay for 4-11 days, before entering another cell.
I hope this gives a brief account of the life cycle of varroa, or enough to understand what treatments are trying to achieve.