First-instar larvae of Sitona lineatus L. (Coleoptera: Curculionidae) penetrate and feed on the nodules of Pisum sativum, which causes nitrogen fixation to stop. A model, based on experiments conducted at constant temperatures and the Sharpe and DeMichele formula, was derived to predict mean egg hatch times as a function of temperature. Variability was introduced through the use of the Weibull distribution around the mean. The model was validated in artificial conditions under naturally varying air temperatures. The fit between actual and predicted times was good. The data generated herein was compared with that of Andersen’s, which was collected 70 yr ago in the area of Munich, Germany. No biologically significant differences between the parameters derived from the two sets of data were identified. The model can be used to accurately predict egg hatch under natural conditions.