The threshold temperature for hatching of eggs of Diabrotica longicornis (Say) was determined as 52°F. The thermal constant for tile first hatching was about 400 degree-days above 52°F. First hatching in the field can be expected when the average soil temperature condition at the 5—10-cm level reaches 368–396 cumulated degree-days. First hatching also coincided with the air temperature cumulation of about 500 degree-days. This index must be scaled up or down depending on the condition of the ground cover.