The univoltine life history of Western and Mexican subspecies of Diabrotica virgifera (LeConte) are closely synchronized with the phenology of corn (Zea mays L.). Data from ten years of field observations at three locations in the Texas High Plains were used to develop and validate a degree-day phenology model of cumulative proportional D. virgifera adult emergence. Climatic data and records of cumulative proportional adult emergence from the Texas High Plains were used to determine a functional lower development (base) temperature of 50̊F and optimum date at the VE (emergence) stage of corn for the start of degree-day accumulation for a model using a modified 4-parameter Gompertz equation (y=96.5*exp(−(exp(6.0−0.00404*(x-4.0 )))) to predict cumulative proportional adult emergence expressed as a percentage. The model base temperature corresponds closely to known physiological lower limits for D. virgifera and Zea mays L. Validation showed the dates of predicted and observed adult emergence were very close and correspond closely to corn phenological stages from tasseling to the end of silking. This was accurate enough to give corn producers a well defined period and ample lead time to anticipate and implement adult control practices.