DAMAGE DONE BY INSECTS. 



163 



resists insects. On poor sandy soil, and in unfavourable 

 situations, frost-holes, etc., the damage done is greater than 

 to plants in good localities, as the repairing force of Nature is 

 then greatest. 



c. Character of the Attack. 



The attack may be either primary or secondary in character. 

 In the former case, 

 healthy trees are 

 injured by insects 

 attacking the leaves, 

 buds, or seeds ; by 

 many shoot-borers, 

 root-gnawers, and 

 insects which attack 

 young plants or sap- 

 lings. The attack 

 of other insects is 

 only secondary, that 

 is, it is made ex- 

 clusively, or by pre- 

 ference, on plants 

 already weakened by 

 other causes (wild 

 animals, fungi, 

 drought, frost, etc.). 

 This is specially 

 true of bark and 

 wood insects, which 

 abound in old coni- 

 ferous trees. The 

 rich flow of tur- 

 pentine from sound 

 trees would kill the 

 young larvae. Cer- 

 tain species, such as 



the bark-beetles of broadleaved trees, according to circum- 

 stances, may make at one time primary and at another 

 secondary attacks. 



This distinction is of practical importance, as nothing can 



1/1 



Fig. 61. Galleries made by Tenthredo cingulata, Fabr. 

 a Common bore-hole. b Larval passages. 



