Temperature thresholds and thermal requirements for egg hatching in an Illinois population of western corn rootworms, Diabrotica virgifera virgifera LeConte, were determined under controlled conditions. Developmental rate for first hatch increased linearly with increasing temperature. The threshold temperature ( t ) and thermal constant ( K ) for first hatch were t = 12.7°C and K = 209.7 degree-days (DD), respectively. The respective values for 50% hatch were y = 0.235 x − 2.616, R2 = 0.97, t = 11.2°C, and K = 426.2 DD. Developmental times were longer than those reported for Minnesota and South Dakota populations at similar temperatures, suggesting significant physiological differences among populations. Based on 4-yr mean monthly temperatures that simulated those at the 10-cm depth at Champaign, 1L, first hatch occurred on 7 June (176.5DD, t = 12.7°C), 50% hatch occurred by 18June (353.8 DD, t = 11.2°C), and last hatch took place on 4 July, 27 d after first hatch. In separate studies in Illinois and Ontario, Canada, a small percentage of eggs from both locations underwent prolonged diapause. In Illinois, 0.14% of 4,202 D. virgifera virgifera eggs hatched only after passing through two simulated winters. Similarly, in studies in Ontario, 0.21%of 1,446 D. virgifera virgifera eggs hatched only after the passage of a second winter. Ours is the first report of prolonged diapause in the western corn rootwoml.