not in Young Effect of Bt maize MON 810 expressing Cry 1 Ab toxin on braconid Aphidius colemani in tritrophic plant - herbivore - parasitoid system was studied under greenhouse conditions by rearing the bird cherry-oat aphid (Rhopalosiphum padi) on genetically modified maize cultivar DKc 307 (MON 810) and its isogenic parent cultivar - Monumental of Monsanto Ltd. R. padi aphids reached higher population level when maintained on Bt-maize cultivar DKc 307 (MON 810) than on its isogenic parent cultivar - Monumental both in the winter and summer experiments. The braconid A. colemani developed higher population on R. padi aphid population feeding on Bt maize plants in the summer experiment and on non-Bt plants in the winter experiment. The observed effect of season on parasitation level by A. colemani on R. padi host feeding on Bt and non-Bt maize plants indicates that results obtained in a single greenhouse experiment may lead to questionable conclusions and should be confirmed by other e! xperiments. Rhopalosiphum padi (Aphidoidea): Host, Parasite; Aphidius colemani (Aphidiidae): Parasite. Aphidius colemani: Biological control, Food plants, Tritrophic plant-herbivore-parasite system, Hemipteran hosts, Rhopalosiphum padi, Cultivated land habitat, Genetically modified crops, Fertilizer and pesticide pollution, Bacterial insecticide effects on non-target organism, Fertilizers and pesticides; Rhopalosiphum padi: Biological control, Hymenopteran parasites, Aphidius colemani, Plant hosts, Zea mays, Bacterial diseases, Bacillus thuringiensis, Food webs, Tritrophic plant-herbivore-parasite system, Cultivated land habitat, Genetically modified crops, Fertilizers and pesticides, Bacterial insecticide effects on non-target organism. Arthropods; Hymenopterans; Insects; Invertebrates; True Bugs. none