Varroa mites are external honeybee parasites that attack both the adults and the brood, with a distinct preference for drone brood. They suck the blood from both the adults and the developing brood, weakening and shortening the life span of the ones on which they feed. Emerging brood may be deformed with missing legs or wings. Untreated infestations of varroa mites that are allowed to increase will kill honeybee colonies.
The adult female mites are reddish-brown in color, flattened, oval, and measure about 1 to 1.5 mm across. They have eight legs. They are large enough to be seen with the unaided eye on the thorax, most commonly, and on the bee’s abdomen. Their flattened shape allows them to hide between the bee’s abdominal segments.
Mites develop on the bee brood. A female mite will enter the brood cell about one day before capping and be sealed in with the larva. Eggs are laid and mite feed and develop on the maturing bee larva. By the time the adult bee emerges from the cell, several of the mites will have reached adulthood, mated, and are ready to begin searching for other bees or larvae to parasitize. There is a preference for drone brood. Inspection of the drone brood in their capped cells will often indicate whether or not a colony is infested.
Mites spread from colony to colony by drifting workers and drones within an apiary. Honey bees can also acquire these mites when robbing smaller colonies.