Since 1998 we are studying the evolution of resistance to Bt maize of two key maize pests, Sesamia nonagrioides and Ostrinia nubilalis, and a secondary one, Mythimna unipuncta. We have modeled the resistance evolution of S. nonagrioides and we are analyzing different biological parameters relevant in the development of resistance to optimize the model. In addition, we have detected for the first time in the EU an allele of resistance to Bt maize in field populations of this species.