none Epizootiological studies of Nosema pyrausta in natural European corn borer populations show that while vertical transmission is the primary way in which N. pyrausta is transferred from one host generation to the next, it is horizontal transmission that is responsible for the annual build-up of infection in each overlapping generation. During the first generation, larval corn borer migration to adjacent corn stalks is minimal and increases in the prevalence of N. pyrausta within the population result from horizontal transmission of infection among borers that inhabit the same stalk. During the second generation, corn borer larvae actively disperse to other corn plants and this results in an increased level of infection. Factors which facilitate pathogen dispersal in this generation include (1) higher host densities, (2) longer periods of larval development, (3) lower mortality among young larvae, and (4) possible mechanical transmission by the braconid parasitoid, Macrocentrus grandii.