OAMAOE DONE BY INSECTS. 161 



Ichneumon-wasps and jiai-asitic flies increase in numbers in 

 proportion to the abundance of their hosts, which bring about 

 an insect-calamity ; they thus assist in suppressing it, whilst 

 other animals, incapable of rapid multiplication, can only keep 

 down the numbers of injurious insects in ordinary times. 



Section VI.— Injurious Forest Insects. 

 • 1. Damage done. 



The grouping of insects that are injurious to forests may 

 follow either the degree of damage done, or the kind of damage, 

 or nature of the attnek. 



(I. Degree of Damage. 



In accordance with the amount of damage they do, we may 

 distinguish forest insects as highly injurious, decidedly, or 

 slightly injurious. The degree of resistance of the species of 

 tree, the part of the tree attacked, and the severity of the 

 attack, as well as the abundance and voracity of the insect in 

 question, decide the degree of injuriousness for any case. It 

 is, however, impossible to assign any strict limits to the several 

 groups. 



An insect is considered highly injurious when by the nature 

 and duration of its attacks, masses *of plants or whole woods, 

 otherwise healthy, may be killed over more or less extensive 

 areas. 



To this category belong, e.g., Melolontha vulgaris, Fabr., 

 Hylohius ahietis, Fabr., Tomicus typograplius, L., Gasteropacha 

 pint, L. The two former insects frequently destroy extensive 

 areas of young plants, and the two latter large areas of forest 

 trees. 



Decidedly injurious insects destroy certain organs only of trees, 

 such as the leaves, inflorescence or fruits, shoots, or stems, or 

 they weaken and eventuajly kill plants here and there in the 

 woods. Most injurious insects belong to this group. 



Slightly injurious insects hardly deserve notice from a forest 

 point of view, as they only cause trifling damage ; they either 

 attack dead stems or tree-parts without impairing their com- 

 mercial value, or the damage done by them to leaves, shoots, 



F.P. M 



