Biased sex ratios, mating frequency and Nosema prevalence in European corn borer, at low population densities

The widespread adoption of transgenic Bt maize in the Midwestern United States has led to historically low populations of the European corn borer, Ostrinia nubilalis (Hübner). Reduced population densities might influence mating and/or disease dynamics in O. nubilalis, potentially amplifying or diminishing the benefits of areawide suppression. We hypothesized that O. nubilalis mating success and infection by the microsporidium Nosema pyrausta Paillot would be decreased in areas of low pest density. Over 2years (2009, 2010), we collected moths from black-light traps, evaluating sex ratio, mating status and infection by Nosema. We found several locations that exhibited female-biased sex ratios: to our knowledge, this is the first report of female-biased sex ratios in O. nubilalis in the Midwestern United States. Despite this bias and subsequent rareness of males, proportion mating was still high (>90%) in most locations, and there were no statistically significant relationships among population density (as measured by black-light trap catch), sex ratio and proportion of female moths that were mated. Nosema prevalence was highly variable among locations, ranging from 0% to 87% of female moths infected. Nosema infection was not significantly related to population density in either year, but was positively associated with sex ratio in 2009. It is possible that Nosema or some other microbe may be manipulating reproduction in O. nubilalis. Regardless of mechanism, our results indicate that Nosema is being maintained in O. nubilalis, which is an encouraging indication that this important natural enemy may continue to provide biological control to complement transgenic technology. Additional surveys are warranted to better understand the extent of female-biased sex ratios in low-density populations of O. nubilalis. Population models developed for studying the risk of Bt resistance in this pest should consider the possible impact of female-biased sex ratios (vs. the conventional 1:1 sex ratio).