Threshold of development (t) and accumulated heat units (k) for total preimaginal development of Cassida vittata Vill. (Coleoptera : Chrysornelidae) were calculated to be 12 ^°C and 288.6 degree — days (dd's) . Development of egg stage required 79 dd's above 10.5 °C, while larval stage demanded 103 dd's above 13.8 °C and this value increased to 105 dd's above 11 °C for adult stage . Accumulation of 528 dd's were sufficient for the completion of one generation. Field observations and population dynamics of C vittata showed two annual generations in 1995 & 1997 and only one generation in 1996 . Variations in insect phenology from year to year was explained and discussed. The accumulated thermal heat units (T. U.) or effective degree - days estimated through insect activity time in 1995, 1996 and 1997 were sufficient to develop two generation in 1995, 1997 and only one generation in 1996. Assessment of thermal heat units for each development stages was successfully used for insect forecasting in sugar beet fields.