Abstract: The effect of the 3rd generation differentiation ratio and drought on the temporal dynamics of the mason pine caterpillar, Dendrolimus punctatus, is revealed by correlation analysis of 27 years' data on the abundance of this species. The 3rd generation differentiation ratios of mason pine caterpillars fluctuate periodically and usually can be divided into three levels: 1%-10%，11%-40% and 41%-100%. The peaks occur about every 5 to 6 years with low differentiation ratios before and after the peaks. Moderate and low differentiation ratios alternate between peaks. Correlation analysis indicates that the 3rd generation differentiation ratio has a distinct relationship only with the occurrence rate of the present generation and not following generations. There is no correlation between 3rd generation differentiation ratios and the occurance rates of previous generations. The key climatic factor affecting differentiation ratios is the temperature in July. Under higher temperatures more larvae hatch before the critical photoperiod that causes high differentiation ratios. Though drought affects the differentiation ratio slightly, it has a delayed, but distinct, affection on occurrence rate. The most affected generation is the first generation of the following year. The results reveal that drought not only affects the dynamics of current generations of mason pine caterpillars but also, more importantly, affects the pines, which in turn affects the next generation of mason pine caterpillars.

Key words: Dendrolimus punctatus, generation differentiation, drought, population dynamics, climate