Abstract: The effect of Dendrolimus punctatus Walker caused defoliation on the growth of Pinus massoniana Lamb and Pinus thunbergi Parl is studied. A empirical formula is derived;management index=larvae density when 50% crown totally defoliated/6.83 It is not nesessary to make a control decision until the larvae density per tree exceed the management index. A atable based on a significant coefficient between breastheight diameter and foliage biomass is shown.From this table,one can find out the management index conveniently if only he measures the breastheight diameter of tree.