The present study investigated whether case-specific monitoring (CSM) would be an appropriate option to detect possible effects on butterflies during commercial cultivation of Bt-maize (Cry1Ab). The analysis of an existing dataset on butterfly communities in Switzerland allowed quantifying the variability in species richness and abundance of nine ecological groups of butterflies resulting from habitat features, agricultural field management, landscape features, regional farming characteristics, spatial variability and temporal fluctuations. The data set also enabled to estimate the sample size needed to detect potential effects of Bt-maize in a CSM programme within the determined variability. Fifteen of the 24 tested descriptors of environmental and farming context induced significant variability in butterfly species richness and abundance of generalist butterflies. In case regulatory authorities would request a CSM, a stratified sampling design considering habitat, landscape, and regional farming types would be essential to account for existing sources of variability. However, recording only the presence and abundance of butterfly species will not be sufficient to explain occurring changes and sampling of additional explaining variables is crucial. The analysis showed that CSM will at best detect large effects on ubiquitous butterflies, whilst the detection of small effects would need a considerable sampling effort. A sampling effort of 100 pairs of fields or field margins will only allow detecting changes exceeding 30% in species richness or abundance of the most abundant species. Rare butterfly species can hardly be monitored in CSM and causalities between changes in butterfly communities and the cultivation of Bt-maize will be difficult to determine due to the high variability of communities and the multitude of influencing environmental factors. CSM is thus unlikely to reduce remaining uncertainties on potential effects on butterflies during commercial cultivation of Bt-maize. Ultimately, potential effects might be evaluated more rigorously in pre-market risk assessment by studying hazard and exposure of sensitive butterfly stages to Bt-maize through experimental toxicity studies in the laboratory or in the greenhouse.