Impact of two microsporidia, Nosema pyrausta and Vairimorpha necatrix, in Nosema pyrausta infected European corn borer (Ostrinia nubilalis) larvae

Larvae of the European corn borer, Ostrinia nubilalis (Hübner), infected with the microsporidium Nosema pyrausta (Paillot) or free of N. pyrausta infection, were fed either additional N. pyrausta, or Vairimorpha necatrix (Kramer), or a combination of N. pyrausta plus V. necatrix spores. Nosema pyrausta and (or) V. necatrix spores increased the number of tissues infected and the intensity of infection. In laboratory studies, V. necatrix, alone or in combination with N. pyrausta, killed more larvae than did N. pyrausta alone. Significantly fewer insects pupated and emerged in the V. necatrix plus N. pyrausta treatment than in the V. necatrix or N. pyrausta treatments alone. There was a correspondingly greater percentage of dead pupae in the V. necatrix plus N. pyrausta treatment. Nosema pyrausta was more detrimental under field conditions than under laboratory conditions. Of larvae surviving the winter, those that had been fed V. necatrix had significantly fewer spores per milligram of infected tissue than did larvae that had been fed N. pyrausta. These data suggest that the larvae with a higher intensity of V. necatrix spores did not survive the winter.