The cultivation of CrylAb-expressing genetically modified MON810 (Bt maize) has led to public conce in Europe,regarding its impact on nontarget arthropods (NTAs).We have assessed the potential effects of DKC 6451 YG (MON810) maize on canopy NTAs in a farm-scale study performed in Central Spain during 3 years.The study focused on hemipteran herbivores (leafhoppers and planthoppers) and hymenopteran parasitic wasps (mymarids) collected by yellow sticky traps,which accounted for 72％ of the total number of insects studied.The dynamics and abundance of these groups varied among years,but no significant differences were found between Bt and non-Bt maize,indicating that Bt maize had no negative effect on these taxa.Nonetheless,the CrylAb toxin was detected in 2 different arthropods collected from Bt maize foliage,the cicadellids Zyginidia scutellaris and Empoasca spp.A retrospective power analysis on the arthropod abundance data for our field trials has determined that Z.scutellaris and the family Mymaridae have high capacity to detect differences between the Bt maize and its isogenic counterpart.The use of these canopy NTAs as surrogates for assessing environmental impacts of Bt maize is discussed.