The leek moth, Acrolepiopsis assectella (Zeller), was first discovered in Ottawa, Canada, during the 1993 growing season, representing the first known occurrence of this species in North America. Since then, it has become a significant concern in Allium vegetable production including garlic, leeks, and onions. Acrolepiopsis assectella was first detected in the contiguous United States during the 2009 growing season in northern New York. In this study, we evaluated the development of the US A. assectella population in the laboratory and commercial onion fields. Our results showed that this population required 443.9 degree-days to complete its life cycle on onions in the laboratory. The development of A. assectella on onion did not significantly differ from populations reared on garlic or leeks. Field studies revealed three distinct flight periods for overwintered, first- and second-generation adult males in northern New York. Life cycle duration in the field ranged from 4 to 8 wk. The degree-day prediction model evaluated in this study provided accurate estimates of the occurrence of the following generation. We conclude that this model can help growers to implement appropriate management strategies for different life stages in a timely manner and lessen damage by this new invasive pest.