Carabid beetles could potentially provide a number of ecosystem services in arable fields, including the regulation of weeds by eating seeds. Spatio-temporal variability in the incidence of predation combined with a limited understanding of the interactions between carabids and seed predation, currently limits our ability to assess the effectiveness of this service. We carried out a long-term field study of carabid communities and the incidence of predation of the seeds of 5 species of weeds and laboratory cafeteria choice-tests to quantify the preferences of the most abundant species of seed-eating carabid for the seeds of 10 species of weeds. The field study revealed important temporal variation both in carabid activity-density and seed predation, with the seed of Viola arvensis and Capsella bursa-pastoris being particularly preferred by the carabids. The laboratory cafeteria tests confirmed the predation preferences measured in the field, in showing strong preferences by both Pterostichus melanarius and Pseudophonus rufipes for the seed of V. arvensis. The percentage predation of the seed of V. arvensis was associated with the activity-density of carabids, while that of the seed of C. bursa-pastoris was not. Integrating laboratory-measured daily consumption rates, per carabid taxa, into models improved the ability to predict the intensity of predation of the seed of V. arvensis based on the composition of carabid communities.