Post-market monitoring (PMM) consistent with Swiss and European Union legislation should ensure the detection and prevention of adverse effects on the environment possibly deriving from commercial cultivation of genetically modified (GM) crops. Insect-resistant GM crops (such as Bt-maize) raise particular questions regarding disturbances of biological control functions provided by beneficial insects such as predators and parasitoids (so-called natural enemies). Consensus among regulators, scientists and the agricultural biotech industry on appropriate PMM plans allowing the detection and possibly prevention of such adverse effects is still lacking. The aims of this study were to identify the necessity for PMM of Bt-maize expressing Cry1Ab on natural enemies and to develop an appropriate PMM plan. The approach chosen consisted in determining what type of monitoring is most appropriate to address potential effects of Bt-maize on natural enemies during commercial cultivation. This included identifying whether there remain substantial scientific uncertainties that would support case-specific monitoring. Existing pre-market risk assessment data indicate that Bt-maize (Cry1Ab) comprises a negligible risk for disturbances in biological control functions of natural enemies. As a consequence, a faunistic monitoring of specific groups of natural enemies is not considered an appropriate approach to detect failures in biological control functions. Alternatively, an approach is proposed that consists in indirectly analysing biological control functions by surveying outbreaks of maize herbivores. Unusual herbivore outbreaks could indicate failures in biological control functions of natural enemies. Data could be collected via questionnaires addressed to farmers growing Bt-maize. Significant correlations between unusual occurrences of specific maize herbivores and the cultivation of Bt-maize would subsequently need specific studies to determine possible causalities in more detail. The here proposed approach has the advantage of covering different natural enemy groups. It represents a cost-effective strategy to obtain scientifically sound data as a basis for regulatory decision-making.