none no species Field experiments with manipulations of natural enemies of plant-feeding insects may show how a diverse enemy group ensures an important ecosystem function such as naturally occurring biological pest control. We studied cereal aphid populations in winter wheat under experimentally reduced densities of: (i) ground-dwelling generalist predators (mostly spiders, carabid and staphylinid beetles); (ii) flying predators (coccinellid beetles, syrphid flies, gall midges, etc.) and parasitoids (aphidiid wasps), and a combination of (i) and (ii), compared with open controls. Aphid populations were 18% higher at reduced densities of ground-dwelling predators, 70% higher when flying predators and parasitoids were removed, and 172% higher on the removal of both enemy groups. Parasitoid wasps probably had the strongest effect, as flying predators occurred only in negligible densities. The great importance of parasitism is a new finding for aphid control in cereal fields. In conclusion, a more detailed knowledge of the mechanisms of natural pest control would help to develop environmentally sound crop management with reduced pesticide applications. Nutrition; Diet; Prey; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Population dynamics; Habitat; Man made habitat; Land zones; Palaearctic region; Eurasia; Europe Arthropoda; Insect prey; Aphidoidea (Hemiptera); Predatory role in regulating pest populations; Cultivated land habitat; Cereal crop fields; Germany; Lower Saxony; Reinshof research farm; Predatory role in regulating hemipteran pest populations, biocontrol implications, cereal fields Aphidiidae; Hemipteran hosts; Aphidoidea; Parasitoid role in regulating pest populations; Cultivated land habitat; Cereal crop fields; Germany; Lower Saxony; Reinshof research farm; Parasitoid role in regulating hemipteran pest populations, biocontrol implications, cereal fields Metopolophium dirhodum; Rhopalosiphum padi; Sitobion avenae (Aphidoidea ); Hymenopteran parasites; Aphidiidae; Population dynamics; Population development; Effects of different natural enemy types, biocontrol implications; Population density; Predators; Arthropoda; Cultivated land habitat; Cereal crop fields; Germany; Lower Saxony; Reinshof research farm; Different natural enemy role in population regulation, cereal fields