Periodical samplings were made in the fields, located Hwasong-Gun, Banwal-Myon, Doondai-Ri, for two years to study the major factors responsible to the population of the overwintered pine needle gall midge, Thecodiplosis japonensis Uchida et Inouye. The population densities of the overwintered larvae reduced greatly at the time of pupation and the changing patterns seemed to be influenced by environmental conditions. The estimated 50% pupation day was about two weeks earlier for the 1980 generation compared with the 1979 generation, and much higher average and the daily maximum temperatures in 1981 affected on the development of the larvae. The relative emergence rates were 14.1% for 1979 and 14.9% for 1980 generation. The relative emergence rates(Y) were affected by the moisture contents of soil ($X_1$) and its variance $(X_2),;Y=-68.41+4.3206X_1-0.6887X_2$. The relative emergence rates seemed to be decreased with the increased variance of the moisture contents of soil. Percents of needle gall were 49.63% for 1980 and 86.87% for 1981 generation.