The European Union reformed the Common Agricultural Policy (CAP 2013) to include greening measures with the aim to decrease negative impacts of farming on the environment and biodiversity. The degree to which greening measures such as permanent grassland or fallows of different ages enhance biodiversity is still debated. We investigate the effect of fallows in two different age classes and permanent grassland in the surrounding landscape on the taxonomic and functional diversity of two numerically dominant groups of natural enemies in cereal fields: soil-emerging carabid beetles (Family Carabidae, Order Coleoptera) and ground-active linyphiid spiders (Family Linyphiidae, Order Araneae). The species richness, abundance and Hill–Shannon diversity of carabids and linyphiids did not differ significantly between fallows and cereal fields and was not significantly related to the proportion of permanent grassland in the surrounding landscape. The species composition of both communities differed significantly between cereal fields and fallows. The functional distinctness, as an index reflecting the similarity among species in terms of functional traits, of linyphiids was significantly higher in fallows than in cereal fields. The trait composition of carabids was significantly related to the proportion of permanent grassland depending on field type (cereal or fallow). Our results document considerable species turnover in natural enemy communities of adjacent cereal fields and fallows, and support the assumption that older fallows (>8 years) produce functionally more diverse natural enemy communities. Maintaining fallows for a long period is an important measure to promote the functional diversity in predaceous arthropod communities.