none Conspecific and heterospecific associations of aphid parasitoids (Hymenoptera, Braconidae, Aphidiinae) were used to determine the interactions between crops forming the main arable crop ecosystem (wheat, alfalfa, isogenic and [Bt Bacillus thuringiensis] transgenic maize) in an irrigated area in Catalonia, Spain. Species composition and instars of parasitized aphids (mummies) were used to detect the parasitoid population movements between the crops and proved interactions (alternation) between the crops, including transgenic maize. Parasitoid interactions were species-dependent, appearing in the movements between alfalfa-wheat-maize in the individual species. Aphidius ervi Hal., A. rhopalosiphi DeSt.-Perez, A. uzbekistanicus Luzh., Lysiphlebus testaceipes (Cress.) and Praon volucre (Hal.) were classified as key-stone species in the target agroecosystems in the area. No differences were found in aphid-parasitoid associations on Bt transgenic maize and isogenic maize for aphid species composition and associated parasitoid guild. Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Habitat; Man made habitat; Land zones; Palaearctic region; Eurasia; Europe Aphidiinae (Aphidiidae); Hemipteran hosts; Aphididae; Host parasite associations & interactions; Spain; Catalonia; Almenar; Associations & interactions with hemipteran hosts, arable crop system Aphididae (Aphidoidea); Hymenopteran parasites; Aphidiinae; Host parasite associations & interactions; Plant hosts; Arable crops; Cultivated land habitat; Spain; Catalonia; Almenar; Associations & interactions with hymenopteran parasites, arable crop system