132 TIMBEES AND THEIE USES 



bark and sapwood of tlie younger parts of the 

 trees. 



There are two families of beetles, whose 

 members are all injurious to forest trees or to 

 timber. The Bark Beetles or Scolytidce are 

 common in this country, whilst the other family, 

 the BostrycJiidcB, are almost strangers to Britain 

 and will be mentioned later. Fortunately, many 

 of the bark beetles only attack dead or sickly 

 trees, but there are important exceptions. From 

 a practical point of view these insects fall 

 naturally into three groups, according to the 

 damage they cause to trees. Some of them bore 

 directly into the wood, others damage the sur- 

 face of the wood just below the bark, others 

 again only penetrate into the bark. As so often 

 happens in the insect world, certain species 

 confine their attentions to definite species of 

 trees ; thus Cryphalus abietis only attacks Spruce; 

 C. ficecB is partial to Silver Fir ; C. fagi is a pest 

 of Beech and C. tilice frequents Lime. The 

 various parts of the tree itself also harbour 

 different species, and therefore several species 

 may be found on one tree. Take, for example, 

 a comparatively young Northern Pine. In the 

 top twigs we may have Pityogenes bidentatus, 

 and in the smaller branches Hylastes palliatus. 

 The stem may harbour Hylurgus piniperda and 

 the root surface Hylastes ater. 



