Varroa mites enter a hive through a bee, they attach themselves to a bee’s abdomen, and it only takes one bee with a varroa mite to enter your hive through drift or robbing for an infestation to begin to grow. As soon as that bee is in the hive, it will jump ship off the bee and search for the brood cells with larvae in them.
These varroa are female. Before the cell is capped, it will climb in underneath the larvae and hide. As the worker bees don’t know it is in there, they cap the cell. While the larvae are pupating and growing the varroa is feeding off it. The varroa mite lays an egg (which is always male to start with) followed by female eggs. Once they have hatched mating begins within the cell.
When the bee hatches out, the varroa load will normally only increase by one mite (the original varroa and one fertilised mature female varroa). There are normally quite a few other varroa, but they haven’t matured enough to leave and lay eggs.
As drone brood takes a longer time to pupae, this allows more time for varroa to hatch and mature. So, when drones leave their cells, they will bring at least 2 new varroa mites out with it.
Due to this, varroa mite choose drone comb to allow them to multiply more. As varroa feed on the pupae (sucking bodily fluids) some of the drones won’t hatch or will hatch malformed due to the varroa feeding off them. Brood that isn’t hatching or a high load of malformed bees is often a sign of varroa infestation.