Characterization of resistance to the Cry1Ab toxin from Bacillus thuringiensis Berliner in a field-derived strain of European corn borer, Ostrinia nubilalis (Hubner) (Lepidoptera: Crambidae)

The Cry1Ab toxin from Bacillus thuringiensis is expressed by 80% of the commercially available transgenic maize, Zea mays L., that targets the European corn borer, Ostrinia nubilalis (Hubner). The resistance management plan for O. nubilalis is based on the high dose refuge strategy and monitoring of field populations to detect resistance. We compared methods to quantify Cry1Ab toxin for standardization of insect bioassays. Standardization of Cry1Ab production and quantification should improve consistency in bioassay data to quantify resistance and to identify changes in susceptibility of O. nubilalis to Cry1Ab toxin. A field collection of O. nubilalis from Kandiyohi County, Minnesota exhibited significantly lower mortality at the diagnostic concentration developed for O. nubilalis. We used standardized bioassays, on-plant assays, life table analysis and biochemical studies to characterize resistance in insects from this field collection. The resistant strain developed high levels of resistance to Cry1Ab, variable levels of cross-resistance to other Cry toxins, and the ability to survive on transgenic corn by feeding on tissues of low Cry1Ab expression. Studies to determine the inheritance of resistance and its fitness costs revealed that resistance is most likely autosomal, incompletely recessive, and polygenic. The inheritance of fitness costs of resistance to Cry1Ab in O. nubilalis varied from incompletely recessive to recessive among the parameters evaluated using life table analysis. Cry1Ab binding to brush-border membrane vesicles (BBMV) of susceptible and resistant larvae was compared using surface plasmon resonance and ligand-toxin immunoblot analysis to test the hypothesis that resistance involves reduction of toxin binding to receptors. We also examined proteolytic degradation of Cry1Ab as a candidate mechanism of resistance. Although we found reduced Cry1Ab and Cry1Aa binding in BBMV of resistant insects that were consistent with the cross-resistance pattern observed in bioassays, the exact nature of the resistance remains uncertain.