The mite Varroa jacobsoni is a brood parasite of the Asian hive bee, Apis cerana. The recent switch in host from A. cerana to the western honeybee, Apis mellifera, offers an exceptional opportunity for studying preadaptation and host-parasite relations. The fact that this host shift appears to have happened on at least two separate occasions, with differing outcomes, must be unique. At another level, the rapacious spread of this mite throughout the world is testimony to the ineffectiveness of international quarantine laws.