Biased sex ratios, mating frequency andNosemaprevalence in European corn borer, at low population densities

Abstract

AbstractThe widespread adoption of transgenicBtmaize 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 inO. nubilalis, potentially amplifying or diminishing the benefits of areawide suppression. We hypothesized thatO. nubilalismating success and infection by the microsporidiumNosema pyraustaPaillot would be decreased in areas of low pest density. Over 2 years (2009, 2010), we collected moths from black‐light traps, evaluating sex ratio, mating status and infection byNosema. We found several locations that exhibited female‐biased sex ratios: to our knowledge, this is the first report of female‐biased sex ratios inO. nubilalisin 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.Nosemaprevalence was highly variable among locations, ranging from 0% to 87% of female moths infected.Nosemainfection was not significantly related to population density in either year, but was positively associated with sex ratio in 2009. It is possible thatNosemaor some other microbe may be manipulating reproduction inO. nubilalis. Regardless of mechanism, our results indicate thatNosemais being maintained inO. 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 ofO. nubilalis. Population models developed for studying the risk ofBtresistance in this pest should consider the possible impact of female‐biased sex ratios (vs. the conventional 1 : 1 sex ratio).