M. De la POZA, G. P. FARINÓS, P. HERNÁNDEZ-CRESPO, F. ORTEGO and P. CASTAÑERA
CSIC, CIB, Dpto Biología de Plantas, Velázquez 144, 28006, Madrid, Spain

Maize is the main summer cereal crop in Spain with a cultivated area of about 450,000 ha. The Mediterranean corn borer (MCB), Sesamia nonagrioides Léfèbvre, and the European corn borer (ECB), Ostrinia nubilalis (Hübner), are key pests in most of the Spanish maize growing regions.
The commercialisation of transgenic maize expressing the Cry1Ab toxin from Bacillus thuringiensis (Bt-maize) provides a new tool for an effective control of these two major maize pests. However, the development of resistance in target pests to Bt plants has been considered the main risk for the success of this control strategy. So far, field resistance to Bt-maize has not been documented, but it is expected that large-scale planting could result in rapid selection for resistance in field populations of corn borers.
A surface ranging from 20,000 to 25,000 ha of Bt-maize (event 176, cv. Compa CB, Syngenta) has been grown annually in Spain since 1998. Accordingly, a monitoring research project was established (funded by the Spanish Ministry of the Environment) to detect changes in susceptibility of corn borers through regular monitoring on Bt-maize fields.
Baseline susceptibility to the Cry1Ab toxin was determined for Spanish populations of MCB and ECB from larvae collected on non-transgenic maize in the most important growing areas (Galicia, Ebro, Madrid, Andalucía, Badajoz and Albacete). Annual monitoring of field populations of both species collected on Bt-maize in the same geographical areas has not revealed changes in susceptibility after three years of Bt-maize cultivation in Spain.
The expression of the toxin in Compa CB tissues is not maintained all season and, therefore, laboratory selection to induce resistance might be more relevant than in other cultivars where high expression of Bt toxin is maintained throughout the maize cycle. Laboratory selection for eight generations yielded ECB and MCB strains 5.0- and 2.2-fold less susceptible to Cry1Ab than unselected populations, respectively.