The Epizootiology of Nosema Pyrausta (Paillot) in The European Corn Borer, Ostrinia Nubilalis, in Central Illinois

Abstract

Nosema pyrausta (Paillot), a microsporidium, is a chronic pathogen of the European corn borer, Ostrinia nubilalis (Hubner). A four year study was conducted in Woodford County, Illinois, from 1980 through 1983 in order to elaborate the relationship between corn borer density and the incidence of N. pyrausta. Factors contributing to the mortality of both generations of the corn borer are presented as partial life tables. First generation infection levels were determined primarily by the transovarial input from the first moth flight, while second generation infection levels were determined by the interaction between horizontal transmission and the first generation input of N. pyrausta. An empirical threshold for horizontal transmission was calculated for the second generation corn borers. Horizontal transmission increased when the average weekly density exceeded 50 borers per 100 corn stalks, but the initial inoculum of the pathogen also plays an important role in determining the level of N. pyrausta.

Laboratory studies were conducted on both field collected and laboratory reared corn borers in order to evaluate lethal and sublethal effects of the pathogen. N. pyrausta infections reduced adult lifespan and increased larval mortality. Environmental stress also increased larval mortality and infection combined with environmental stress resulted in the highest mortality. The sequence of infected egg masses laid influenced larval mortality, with the egg masses laid on the first two days having the lowest mortality.

Over the four year period, the braconid Macrocentrus grandii (Goidanich) became increasingly important as a first generation mortality factor of the corn borer. Laboratory studies indicated that infected M. grandii females did not die during the preoviposition period as had been previously reported for the Connecticut strain of the parasitoid. Infected female parasitoids oviposited successfully and transmitted N. pyrausta to their progeny; they served as vectors for the pathogen as well. A limited tolerance to N. pyrausta may be developing in the Illinois strain of M. grandii and the parasitoid may play a role in the transmission of the pathogen in first generation corn borers.