Abstract:Trunk attacks by the pine shoot beetle \[Tomicus yunnanensis Kirkendall and Faccoli (Coleoptera: Curculionidae: Scolytinae)\] on Yunnan pine (Pinus yunnanensis Franchet) are regarded as the principal cause of tree death. The current studies were conducted under field conditions by controlling the beetle densities that were allowed to attack the trunk. The results showed a positive relationship between attack density and mortality rate of Yunnan pine trees. The field studies demonstrated that an attack density of 115 tunnels/m2 of bark surface area was the threshold for mortality of Yunnan pine trees. All Yunnan pine trees survived when attacked by beetle densities below 26.4 tunnels/m2, but only a portion survived at the attack densities between 26.4-115 tunnels/m2. All trees were killed at the attack densities higher than 115 tunnels/m2. These attack densities were calculated using both galleries where the parent laid eggs and where no eggs were laid. The maternal galleries without eggs appeared related to host tree resistance. Trunk attacks where no eggs are laid can apparently still weaken pine trees, while attacks with eggs can cause more severe damage to the phloem tissue, and thus was regarded as the critical factor in causing mortality of Yunnan pine trees.