Nosema pyrausta and Cry1Ab‐incorporated diet led to decreased survival and developmental delays in European corn borer

The high dose/refuge strategy for delaying evolution of resistance to Bt maize [Zea mays L. (Poaceae)] relies on random mating between resistant European corn borers, Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae), and susceptible O. nubilalis from the refuge. However, differences in developmental rate caused by feeding on Bt maize, or infection with the microsporidium Nosema pyrausta Paillot (Microsporida: Nosematidae) may result in assortative mating. Developmental delays and mortality caused by infection with N. pyrausta and feeding on Bt maize were quantified alone and in combination in Cry1Ab‐resistant and susceptible O. nubilalis. Feeding on Cry1Ab‐incorporated diet significantly increased number of days from hatch to pupation and decreased survival in the resistant population. Infection with N. pyrausta increased mortality and lengthened development in both the resistant and susceptible populations. The combination of Cry1Ab‐incorporated diet and infection with N. pyrausta in resistant O. nubilalis lengthened development and increased mortality to a greater extent than either factor alone. Greater larval delays of resistant O. nubilalis feeding on Bt maize could lead to temporal isolation from adults emerging from refuge maize. The resulting assortative mating would hasten the evolution of resistance. Developmental delays caused by infection with N. pyrausta may increase the likelihood of mating between resistant and infected susceptible adults emerging from refuge maize, producing infected offspring that are also more susceptible to Bt maize.