Abstract: The experiments to study the effect of defoliation of Pinus tabulaformis (intensity, season and needle age) on photosynthesis, transpiration, chloro-phyll content, starch and soluble sugar content, tree growth and its resistanceto Dendrolimus spectabilis were carried out. The results show that the 30%defoliation rate is able to enhence the resistance of trees to the pest insect.After larvae of D. spectabilis being fed by the needles, the growth rate ofweight decreases obviously. It has been proved that the principal stored nutri-tional substance is fat, and the storage organ is needle. The nutritionalproduction and storage in current needles surpass that in old ones. The shootgrowth is smaller and the tree vigor, photosynthesis and transpiration inpines defoliated the current needles are weaker than that defoliated the oldones. By using the current needles to feed on larvae of Dendrolimus specta-bilis, the larvae grow more rapidly. For the purpose of increasing the treevigor and resistance, it is reuired that current needles must be well preserved. The young larvae of D. spectabilis attack mainly the current needles.The larvae fed on current needles have lower mortality and gain greater weight than those fed on old needles. In order to protect current needlesand tree vigor, it is urgent to control the young larvae of D. spectabilis ofthe new generation. Based on the experiments the strategy to control the Dendrolimus in North China has been put forwared.

Key words: Pinus tabulaeformis, Defoliation resistance, Dendrolimus spectabilis