The relatively recent finding that certain species of Nosema may form two kinds of spores reveals that our understanding of sporogony and microsporidia development is incomplete. This is especially true for those that are transovarially transmitted. The limited information that is available indicates that microsporidia may undergo changes when they pass through the egg and developing embryo. Causing rather benign diseases after oral infection, N. pyrausta can cause high mortality in the progeny of infected European corn borer females. Continued research in this area may reveal important determinants of pathogenicity.
Before any attempts are made to identify infectivity or virulence factors, it is necessary to establish the degree of variation in these parameters that actually exists. Furthermore, host specificity and transmission patterns of the microsporidia that infect European corn borer should be investigated in several insect species. Conclusive evidence for the existence of strains in the three species discussed here is lacking. However, strains have been identified in other microsporidia, and there are no a priori reasons to expect that N. pyrausta, N. furnacalis, or V. necatrix should be an exception. Geographical isolates of the same species, most notably N. pyrausta from Europe, should be obtained for comparison. The variation that exists within isolates from a given geographical region should be examined in more detail to determine the degree of heterogeneity within a population. This information should provide a basis on which to decide whether native parasite populations should be screened for strains with better biological control potential or whether exotic isolates should be sought. Because of the apparent clonal nature of microsporidia that have been examined to date, it may be necessary to mutagenize microsporidia, as has been done with nuclear polyhedrosts viruses ( Wood et al. 1981). This approach could yield variants with greater virulence, wider host range, and better transmission potential or persistence. As there is every reason to believe that sexual recombination does not occur in Vairimorpha and Nosema, it will be difficult, if not impossible, to obtain crosses by conventional means that display improved biological control characteristics. Alternative methods based on the introduction of foreign genes into microsporidia may be needed.
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