migrant labour
(US migrant labor)
noun
1A workforce which travels from place to place in search of (temporary) employment, or is recruited for such employment, usually to an urban from a rural area; the existence or exploitation of such a workforce.
2 South African specifically the black labour force working under the migrant labour system; the system under which such workers are employed.
Origin
Early 20th century; earliest use found in American Journal of Sociology.