The fitness of Ostrinia nubilalis Hübner (Lepidoptera: Crambidae) in Canada with multiple Bt resistance

European corn borer (Ostrinia nubilalis Hübner), is a major pest of maize (Zea mays L.) that has been controlled by transgenic Bt corn since 1996. However, concern regarding the persistence of Bt corn durability began after the first case of field-evolved resistance to Cry1Fa was found in Nova Scotia, Canada in 2018. Laboratory studies with this population suggested differences in fecundity, developmental timing, and lifespan between Cry1Fa-resistant and susceptible strains that would benefit the persistence of Cry1Fa-resistant individuals in the field. Field-evolved Cry1Ab resistant O. nubilalis were collected near Truro, Nova Scotia in 2022. We observed a loss of Bt resistance in this strain from laboratory rearing for multiple generations in the absence of selection. The potential fitness costs of multiple Bt-resistant O. nubilalis will be discussed and its implications for management.