Abstract Base temperatures for use in the growing degree day prediction method of European corn borer instars were determined based on a 25-year larval-stage observation record from a south-central Minnesota station. The results indicate a linear or curvilinear trend with the base temperatures increasing from about 2.8°C (37°F) to 5.6°C (44°F) for instars one to five. A base temperature of 10°C (50°F) seems to be acceptable only for the mature larval to pupal stages. These results provide useful field operation base temperatures. In addition they substantiate observations of veteran field observers that the corn borer larvae are active during periods too cool for active corn growth, for which 10°C (50°F) is the generally accepted base temperature.