Hypera postica(Gyllenhal) larval development time was investigated for both eastern and western strains. The minimum effective developmental temperature was 8.9°C for the eastern strain and 10.6°C for the western strain. Development times were calculated for each larval instar, prepupa, and pupa of both strains. There were some differences between instars of the 2 strains. The eastern strain required 212C degree days to complete larval development while the western strain required 207C degree days.Developmental stages were subjected to temperatures below their minimum effective developmental temperatures to determine negative effects on overall development time. No significant negative effects were found where a single stage was held at 0.0° or 4.4°C for 12 h or 1 week. Some differences were found when each stage was held at the lower temperatures for 1 week.Field samples of larvae were taken at 3 different latitudes in Illinois to determine differences in larval population developmental rates.