Abstract: Obviously the most important factor to determine the differentiation of the Pine Caterpillar in the second generation is photoperiod experienced in its early larval period and the differentiation is associated with its population dynamics in nature along the Yangtze River area. The ratio of the third generation produced in field is closely related with the hatching date of eggs in the second generation each year. Most of the larvae hatched in early Augst can complete the second generation and grow up into the third generation, but the larvae hatched in mid-late August can not do so. The annual differentiation varied with the temperature and other factors could greatly effect the population density in hibernation. The critical photoperiod required by the Pine Caterpillar is around 13.5 hrs and the critical date fell on early August in Chongqing region, Sichuan Province, so these larvae hatched before the critical date can produce the third generation or vice versa. The present paper provide some useful date and hints to forecast and manage the pest in the Yangtze River area.