The weekly aphid population on timely sown mustard crop and daily weather parameters recorded at ICAR-Directorate of Rapeseed–Mustard Research, Bharatpur and Hisar stations, during rabi seasons of 2003-04 to 2013-14 were used to develop forewarning model for aphid. In most of the cases, the appearance of population and its peak activity occurred during 51 standard meteorological week (SMW) and 8 SMW. The study revealed that the thermal time or growing degree days (GDD) consistently explained the aphid multiplication significantly in all seasons. The impact of other weather parameters on pest population was observed to be significant only in some seasons. The aphid population increased exponentially with increase in GDD. Further, the exponential coefficient varied among the varieties. The population build up was compared with the model predicted values in three common species (Brassica juncea, B. rapa ssp. sarson and B. napus) grown at two stations. The results revealed that the predicted values derived using the model, were in close agreement with those of the observed.