Abstract: The seasonal susceptibility of masson pine caterpillar to insecticides were studied.It was found that the larvae before hibernation was the most sensirive to deltamethrin and dimilin Ⅲ in Nangjin, Jiangsu Province and Guangzhou,Guangdong Province. The total haemocytes counts,esterases activity was found to be lowest and it was also the suitable time to control this pest by chemical insecticides. The mechanism of the seasonal adaptation of the larvae was discussed.It was suggested that the chemical control strategies should be focused on the selection of suitable chemical and the timing of application to coincide with the susceptible life stages.