Abstract: The present investigation was made in the course of three years (1961—1963). Thirty-two plots,each of 2—3 mou,in Pinus massoniana stands were selected for esti- mating the fluctuation of population of this serious pest.The estimations comprised of several items,such as sex ratio,fecundity,mortality and others. Based on the results of observations,it was found that in the hilly areas of Kiangsf Province,the outbreak of this pest seemed to be of intermittent epidemic type.The out- break intensity,as measured by the number of generations between two successive out- breaks,varied from 2 to 3 or more generations. The population clensity varied from generation to generation. As a rule, the first generation had the highest population density, and the overwintering generation the low- est. The population density of each generation was mainly decided by the mortality of the first three instar larvae. No close relationship between populations of two successive generations was found. In those areas where there were 2—3 generations a year,the larvae of the second generation when developing to fourth instar,differentiated into two groups.The larvae of the one group went into diapause right away and those of the other continued to de- velop and overwintered as 3rd instar larva in the third generation.The number of larvae in each of the two groups varied with environmental conditions.The ecological significance of this phenomenon should be studied further.But based on the results of examinations,it was found that this phenomenon exerted no influence on the population change of the overwintering generation. Besides the human effect,there were three factors affecting the population changes, namely the climatic,food and natural enemy factors.The effect of climate on popula- tion fluctuation was found to have been greater than that of the food or of the enemies. In Kweichi,Kiangsi,the precipitation in May and in August exerted a decisive effect on the occurrence of outbreak in a year.The food factor affected the population change only in certain localities where the pine needles were completely consummed by this pest. The natural enemies exercised a great effect on the population immediately after the collapse of the outbreak. The number of generations completed in one year varied from the south to the north.There were 3—4 generations in Canton,Kwantung Province,while in Hsinyang, Honan Province only 2—3 generations a year.The highest fecundity was 881 eggs per female with an average of 681.5 eggs.But in general,a female laid only 200—300 eggs.