Abstract: One aspect of the Environmental Risk Assessment (ERA) of genetically modifiedplants is the potential ecological impact on the receiving environment. Plants with genes fromBacillus thuringiensis (Bt) that produce proteins with entomotoxic properties need to be assessedfor their potential effects on non-target organisms (NTO), especially beneficials.One important group of NTO are the ladybird beetles (Coleoptera: Coccinellidae), as they serveimportant biological control functions. Their exposure to the Cry proteins from Bt-plants dependson their consumption of exposed prey and plant materials such as pollen. Bt-plants expressingCry3 proteins directed against the Western corn rootworm Diabrotica virgifera virgifera(Coleoptera: Chrysomelidae) especially pose a potential hazard to beneficial Coleoptera.To assess the suitability of higher tier test systems (i.e. semi-field and field experiments) withinthe ERA, we analysed data from two 3-year, field-scale experiments with Bt-maize varietiescontaining the transformation events MON810 and MON88017. We present data on the densitiesof Coccinellidae in maize and calculated confidence intervals (CI) suitable for a test ofequivalence between the Bt-maize varieties and their respective near-isogenic lines. We alsoreport the results of a Monte Carlo simulation study assessing the probability that 90% CI areincluded in pre-specified margins of irrelevant change of mean abundances. The results show thelimits of addressing questions regarding the non-target impact of Bt-maize on Coccinellidae infield experiments, given their low densities and the large natural variability.