Varroa mites are a parasitic mite affecting honeybees. Mites poke a hole in the bees to drink bee blood. They move from bee to bee, drinking blood and transmitting disease. Mites feed on both adult bees and brood. An adult mite enters the cell of a larvae and then is sealed inside the wax cell. The mite lays eggs in the cell and eggs hatch, mature and mate in the sealed cell. Mites prefer to enter drone cells because they are larger. When mite loads in a colony are large, it can kill the colony. Some colonies can handle low mite infestations.
Varroa mites are large and reddish brown. You can see them attached to honeybees. They are about 1/6 the size of the bee. You can sample a colony to estimate the mite load using a powdered sugar roll.
Use resistant lines of bees. Bees with hygienic bee behavior clean out diseased brood. VSH bees are specifically bred to resist varroa mites. Buy queens from these lines of bees.
Replace old comb. Some beekeepers replace all their comb every 3-5 years or 1/3 of their comb every year. With this second method, cull the darkest comb.
Maintain strong colonies with a young, prolific queen.