Shoot damage by Tomicus sp. (Coleoptera: Scolytidae) and effect on Pinus yunnanensis resistance to subsequent reproductive attacks in the stem

摘要

In South-western China, Yunnan pines Pinus yunnanensis, suffer considerable damage from an undescribed Tomicus sp. previously thought to be T. piniperda. To assess the effect of shoot maturation feeding (during which an aggregation process appears to occur) on host resistance to attacks on the bole, the relation ships between shoot damage, bole attack density and tree survival were studied. Attack distribution in the crown and in the stem did not vary between killed and surviving trees, indicatingthat mortality is determined by the quantity of attacks. The level of shoot damage and bole attack density were positively and linearly correlated. This can be explained by the fact that bole attacks are caused by beetles coming from the crown of the same tree. A critical threshold of bole attack density (around 80 attacks/m~2) above which trees die was observed. However, because attacks continue after this threshold is reached, the density of failed attacks on the killed trees was used as an estimatorof the threshold density. It decreased when shoot damage increased. The existence of a critical threshold of shoot damage (60 percent damaged shoots) was also demonstrated. Above this threshold, stem attack density was always sufficiently high to kill trees. The results emphasize that concentration of shoot attacks is the main reason for the extensive tree damage observed in China. A model of relationships between shoot and stem attacks is proposed, suggesting that management to reduce shoot attacks would protect trees from dying by both decreasing the number of bole attacks and raising the threshold for successful attack density on the bole to levels that could not be attained.