BACKGROUND: The high dose plus refuge is one of the major components of the resistance management plan mandated for transgenic corn expressing Cry toxins from Bacillus thuringiensis Berliner (Bt) that targets the European corn borer, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae). This strategy was based on assumptions such as functional recessive inheritance, which has not been previously tested for O. nubilalis. The authors used a field-derived resistant strain of O. nubilalis to define the nature of resistance to Cry1Ab toxin by examining the inheritance and on-plant survival of susceptible and resistant insects and their F1 progeny.
RESULTS: The resistant strain exhibited >800-fold resistance to Cry1Ab. Resistance was primarily autosomal and controlled by more than one locus or multiple alleles at one locus. The degree of dominance D calculated on the basis of LC50 values was −0.45 (h′ = 0.27), indicating that resistance was incompletely recessive. No survivors were found on vegetative-stage Bt corn, although both resistant larvae and their F1 progeny were able to survive on reproductive corn 15 days after infestation.
CONCLUSIONS: A field derived O. nubilalis strain exhibited high levels of resistance to Cry1Ab and survived on transgenic corn by feeding on tissues with low Cry1Ab expression. The Cry1Ab resistance was primarily autosomal, incompletely recessive and polygenic. Tissue and on-plant survival data indicated that dominance varies depending on plant stage.
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