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With the cultivation of Bt cotton,the produced insecticidal Cry proteins are ingested by herbivores and potentially transferred along the food chain to natural enemies,such as predators.In laboratory experiments with Bollgard Ⅱ cotton,concentrations of Cry1Ac and Cry2Ab were measured in Lepidoptera larvae (Spodoptera littoralis,Heliothis virescens),plant bugs (Euschistus heros),aphids (Aphis gossypii),whiteflies (Bemisia tabaci),thrips (Thrips tabaci,Frankliniella occidentalis),and spider mites (Tetranychus urticae).Tritrophic experiments were conducted with caterpillars of S.littoralis as prey and larvae of ladybird beetles (Harmonia axyridis,Adalia bipunctata) and lacewings (Chrysoperla caea) as predators.Immunological measurements (ELISA) indicated that herbivores feeding on Bt cotton contained 5％-50％ of the Bt protein concentrations in leaves except whiteflies and aphids,which contained no or only traces of Bt protein,and spider mites,which contained 7 times more Cry1Ac than leaves.Similarly,predators contained 1％-30％ of the Cry protein concentration in prey.For the nontarget risk assessment,this indicates that Bt protein concentrations decrease considerably from one trophic level to the next in the food web,except for spider mites that contain Bt protein concentrations higher than those measured in the leaves.Exposure of phloem sucking hemipterans is negligible.