Knowledge of the larval-age distribution in the field is important for prediction purpose and timing of insecticide applications for insect pest management. This studies acts by calculating the average of thermal units in degree-days (dd’s). The average of thermal units required for completion of generation is 544.98, 640.63 and 599.66 degrees-days (°C) as calculated from air temperatures derived from thermograph and satellite images, and soil temperatures from satellite images, respectively, considering 9.89 °C as a developmental threshold. These were higher than the estimated value of dd’s based on laboratory data (524.27 degrees-days (°C)). There was a difference between degree days obtained from air temperatures derived from satellite images and thermograph by 59.2 dd’s, this value represented only about 2.85 days. In order to improve the predictability, a factor was estimated between them which is 0.81, 0.96 and 0.87 in case of thermograph, soil and air temperature that derived from satellite images so the predicted stages was highly improved. Egg hatching was estimated to be 80% complete by ≈80.45 dd’s. At 174.85 DD, mostly all larvae in the field experiment were from the first to third instars. The presence of more mature larvae (fourth to sixth instars) was not noticed until 197.59 dd’s. These data indicate that, the best timing for control application against Spodoptera littoralis would be at 174.85–197.59 dd’s. The results are important for quick prediction purposes, control timing and also as valuable tools used in an integrated control program for managing S. littoralis in Egypt.