No. 3/April 11, 1997
Infection of European Borers by Disease
The potential for infestations of first-generation European corn borers is always difficult to predict. We know that if growers plant early and the weather is favorable for survival and development during the early part of June, first-generation corn borer larvae establish a foothold and cause problems. In addition, our annual fall survey for larvae gives us some indication of the size of the overwintering population in different areas of the state. However, the size of the overwintering population doesn't always correlate well with the level of infestation the next year. One reason for this lack of correlation is the impact of disease on corn borer populations. Thanks to the work of a couple of graduate students last fall and this spring, we have some indication this year about the potential impact that one disease might have on corn borers this spring.
Two diseases affect populations of European corn borers in Illinois. A disease caused by the fungus Beauveria bassiana kills overwintering borers outright. This fungus attacks many species of insects and is transmitted from host to host by asexual spores (conidia). Corn borers killed by Beauveria usually are coated with a white or pink-tinged layer of conidia. However, the disease that probably has the greatest impact on populations of European corn borers over time is a microsporidian, Nosema pyrausta. This disease pathogen overwinters with the borer but does not kill it outright. When the diseased larva breaks diapause in the spring and resumes development, the pathogen endures into the pupal stage and then the adult stage. When the infected female lays eggs, she transmits the disease organism transovarially (inside the eggs) so that all larvae that hatch are infected. These young larvae often succumb to the disease. Consequently, the impact of Nosema pyrausta is multiplied from one infected female to many infected larvae.
Chris Pierson, Rick Weinzierl's graduate student, and Maria Venditti, my graduate student, sampled some populations of overwintering corn borers, returned the larvae to the lab, and examined them for infection by Nosema pyrausta. Chris examined corn borers collected from LaSalle, Marshall, and McLean counties last fall and detected that 62%, 38%, and 47%, respectively, were infected with Nosema pyrausta. Maria examined corn borers collected from Sangamon County this spring and determined that 61% of them were infected. Although the numbers of borers examined were relatively low, the infection rates were impressive. These results suggest that the level of mortality of first-generation corn borer larvae this spring could be rather high. As more samples are examined and the season progresses, we'll keep you informed about factors that discourage or encourage infestations by corn borers in June.
Kevin Steffey, Extension Entomology, (217)333-6652