Field collected, over-wintered European corn borer (Ostrinia nubilalis) larvae were observed for parasitoid infestation for multiple years (2003, 2004 and 2005) and locations (4 locations in Pennsylvania). Collected larva were reared under non-diapausing conditions of 25^oC, 18:6 L:D, and more then 60% humidity. The ratio of univoltine and multivoltine individual in a population can varied significantly between years at the same location. While their race ratio is thought to be due to climatic variation, other factors may involve. For instance, the larval parasitoid’s (Macrocentrus cingulum) development is synchronized to the multivoltine population only and shifted the race ratio balance towards the univoltine population in the area where both races co-occurred. In addition, M. cingulum influence the sex ratio of the post diapause European corn borer population by infesting more female larvae.