Development and characterization of resistance to the Cry1F toxin from Bacillus thuringiensis Berliner in the European corn borer, Ostrinia nubilalis (Huebner) Lepidoptera: Crambidae

A strain of European corn borer, Ostrinia nubilalis (Hubner) (Lepidoptera: Crambidae), obtained from field collections throughout the U.S. Corn Belt in 1996 was selected in the laboratory for resistance to Cry1F by exposure to the toxin incorporated into artificial diet. The selected strain developed more than 3000-fold resistance to Cry1F, yet it was as susceptible to Cry1Ab and Cry9C as the unselected control strain. Only a low level of cross resistance (7-fold) to Cry1Ac was observed. Dose-response of reciprocal parental crosses indicated that the resistance is autosomal and recessive. Backcross of the F1 generation with the selected strain revealed that a single locus or a set of tightly linked loci is responsible for the resistance. Greenhouse experiments with Cry1F-expressing corn plants indicated that some resistant larvae survived the high dose of toxin delivered by Cry1F-expressing plants although F1 progeny of susceptible by resistant crosses had fitness close to zero. Comparison of life-history traits and population growth rates of genotypes homozygous and heterozygous for resistance relative to susceptible genotypes indicated existence of weak fitness costs associated with resistance. Analyses of toxin binding using ligand-toxin immunoblotting and Surface Plasmon Resonance (SPR) to measure Cry1F binding to brush border membrane vesicles of midgut epithelia from susceptible and resistant larvae suggested that reduced binding was not associated with resistance. Additionally, no differences in activity of luminal gut proteases or altered proteolytic processing of the toxin were observed in the resistant strain. Although the resistance mechanism remains uncertain, there is no direct evidence that altered binding and proteolytic processing of toxin are involved. Considering the relatively narrow spectrum of cross resistance and the evidence of monogenic resistance, the resistance mechanism in this Cry1F-selected strain of O. nubilalis appears to be specific and maybe distinct from previously identified resistance mechanisms reported in other Lepidoptera. The results of this research have important practical implications for resistance management in that they suggest that Cry1Ab and Cry1F are compatible to resistance management, and more importantly, provide the first direct evidence that the high dose/refuge strategy currently in place to manage resistance in Cry1F-expressing maize is appropriate.