242 PROTECTION AGAINST INSECTS. 



may be imported from time to time in the bark of unseasoned 

 spruce-timber. 



Bark-beetles are generally slow and lazy insects, which only 

 in very warm weather will fly to the top of trees. A flight of 

 them to remote places is therefore a rare occurrence, due to 

 over rapid multiplication and want of food. 



c. Relations to the Forest. 



The beetle, both as a larva and as a perfect insect, does 

 physiological damage to conifers. 



It chiefly attacks old spruce trees, generally those between 

 80 to 100 years old, and very seldom when under 50 years. 

 It is said to have been found quite exceptionally on larch and 

 Scots pine and on the Cembran pine. Even if, in these 

 cases, it has not been confused with the extremely similar 

 species T. amitimts, Eichh., on the larch, and T. ceuibrae, 

 Heer, it must be admitted that T. typographies only appears in 

 swarms in spruce woods, and only attacks trees with thick bark. 



The injuries are confined to the bast-layer, and are fatal to 

 the trees which are attacked. 



The resulting disease is called spruce-canker. Symptoms 

 of the attack are yellow or red discoloration of the needles, 

 greyness, loosening or falling off of the bark, numerous bore- 

 holes through its substance, and the presence on the trunk of 

 boring-powder ejected from the burrows. 



Trees infested in the spring appear differently affected to 

 those injured in the summer. The needles change colour 

 rapidly in the former case ; in the latter the needles remain 

 green, even whilst the bark has alr.eady partially fallen 

 off. This depends on the difference in the movement and 

 composition of the sap at the different seasons of the year. 

 In the spring, the ascent of water from the ground is cut off 

 from the crowns of the trees by the destruction of the bast, 

 and the foliage at once begins to change colour. In the 

 summer, the descent of the supply of nutritive material 

 prepared by the leaves is cut off, while the crowns still get 

 the nutriment, hence the needles remain green while the 

 bast is killed. Nevertheless trees attacked in summer 

 eventually die. 



