Early in the investigations of the European corn borer, Pyrausta nubilalis (Hbn.), in the Lake States, according to Caffrey & Worthley (1927), it was evident that the insect in this part of the infested territory was producing only a single generation annually, although later a very small partial second generation was noticeable in certain seasons. In more recent years, however, and particularly in 1936 and 1937, it became apparent that midsummer pupation of the corn borer in sections of Ohio, Michigan and Indiana had increased in frequency and, in some localities, had reached measurable proportions. Interest was immediately aroused as to the possible extent of this phenomenon, its underlying causes and the character of the second generation material in an environment hitherto considered adapted to the development of only a single generation of the pest annually.