European corn borer, Ostrinia nubilalis Hübner, is a major pest of maize (Zea mays L.) in North America that has been managed using transgenic maize producing insecticidal toxins from Bacillus thuringiensis Berliner (Bt) for over 25 years. The first case of field-evolved resistance of O. nubilalis to a Bt toxin, Cry1Fa, was discovered in Nova Scotia, Canada, in 2018. The objectives of this study were to determine the inheritance and fitness of the first field-evolved Cry1Fa-resistant strain of O. nubilalis. Fitness characteristics such as lifespan, fecundity, and developmental time were compared among homozygous Cry1Fa-resistant, homozygous Cry1Fa-susceptible, and heterozygous reciprocal lines of O. nubilalis. Cry1Fa-resistance was found to be recessive based on diet-overlay and tissue bioassays. The homozygous Cry1Fa-resistant strain had 2.4 times greater fecundity, 2 day longer adult lifespan, and 6.5 day longer developmental time than the homozygous Cry1Fa-susceptible strain. Cry1Fa-resistant populations are expected to have 1.6 times greater net reproductive rate than Cry1Fa-susceptible populations. These results suggest that limited fitness costs are associated with field-evolved Cry1Fa-resistance, therefore the spread of resistant populations should be urgently managed.
Keywords: Ostrinia nubilalis; Bt maize; resistance.
© The Author(s) 2026. Published by Oxford University Press on behalf of Entomological Society of America.