Abstract:According to the observations in Xishan Area of Beijing in 1985—1991, the population of Dendrolimus tabulae formis was in a state of low density successively with an average of 0.4—11.7 larvae/plant:4. 2 larvae/plant in 1985; 11.7 in 1988; 0.5 in 1991. During the past 7 years, the average annual parasitism of Casinaria nigripes was 25. 22± 11.14%. The parasitism rate was negatively proportional to the host density, y= 34.406 1.909x, r= -0.925. If the parasitism rate was high than 25% in this year, the population density of D. tabuiaeformis would lower down in the following year. There were two parasitism peaks, which appeared in 1986 and 1990 respectively with an interval of 3 years. There was an interspecific competition existing between C. nigripes and Carcelia matsukarehae, In the year when the parasitism rate of C. nigripes was high, that of C, matsukarehae would be rather low or even zero. The number of overwintering larvae of D. tabuiaeformis parasitized by C. nigripes, which began to be active earlier after hibernation, was more than that of the parasitized D. tabuiaeformis, which began to be active later after hibernation. The alternate hosts of C. nigripes in the field are Malacosoma neustria testacea, Lymantria dispar and Ivela ochropoda. It has 4—5 generations in a year in different species of hosts. Its overwintering host is D. tabuiaeformis. The population of C. nigripes was influenced by these factors such as alternate hosts, hyperparasitism and high temperature in summer. The seasonal population dynamics is like this:its population is more in spring, and it decreased significantly in summer, and it rises again in autumn. According to the observations, it has exerted an obvious suppressive function on the number of D. tabuiaeformis at low density.