A population of a species living on and showing a preference for a host which is different from the host or hosts of other populations of the same species. Host races represent a continuum between forms that freely interbreed to those that rarely exchange genes. The latter may approach the status of a species, generally regarded as an interbreeding population reproductively isolated from all other such populations. Bush 1969, P237.
A more practical definition was published by Drès and Mallet 2002.