Microsporidia infection rate in Ostrinia nubilalis larvae collected in Russia in 2011-2016 ranged from 0 to 16 %. Totally, among 262 examined insects, there were as many as 13 infected specimens, resulting in average prevalence of 5 % over the period indicated. In all positive samples but one diplokaryotic spores 4 um long were observed corresponding to diagnosis of Nosema pyrausta. Nevertheless, in one case (i.e. 0.4 %) the infected larva contained monokaryotic spores about 2 um long. After experimental infection of a substitute host Ostrinia furnacalis with monokaryotic spores only Nosema-like spores were observed in laboratory assays. Ribosomal RNA and RPB1 gene portions were 100 % identical in samples of both mono- and diplokaryotic spores. This observation shows that Nosema pyrausta can form uninucleate spores under yet to be described conditions in nature and that molecular genetic analysis is essential for correct species identification.
