Eurostat: Farm labour Annual agricultural labour figures by country with a specific column (G)  for temporary labour,  by subnational region

Farmlabour*.xls = Annual agricultural labour figures by country with a specific column (G)  for temporary labour,  by subnational region:  this doesn’t specify migrant labour specifically,  but may be a proxy for it. Separate sheet per year (2013, 2016),  with figures for All people,  males and females

National (NUTS1) and subnational (NUTS2) level data downloaded in excel format and zipped from the Eurostat data explorer.

Metadata link: https://ec.europa.eu/eurostat/cache/metadata/en/ef_esms.htm