THE CAUSES OF INSECT ATTACKS 265 



more serious consequence in the case of coniferous than in 

 hardwood forests. Pure stands of conifers are particularly- 

 susceptible. There appear to be two reasons for the greater 

 damage to conifers. First more species and more destructive 

 insects attack the conifers; and secondly conifers do not 

 recover so easily from insect injuries as do hardwoods, be- 

 cause the latter replace injured parts more readily.^ As an 

 illustration, complete defoliation, by the gipsy moth ordi- 

 narily causes death of a conifer, whereas a hardwood tree may 

 survive several defoliations. 



The killing of trees by insects increases the amount of in- 

 flammable material in the forest and hence the fire danger. 

 Insect epidemics are often followed by disastrous fires made 

 possible by the fuel in the insect killed trees. On the other 

 hand fires injuring the standing timber may lead to the de- 

 velopment of a big insect outbreak, altho this is not always 

 the caseJ Insects may be responsible for injuries by fungi, 

 which find access to the tree through the galleries and holes in 

 the bark. 



Insects Responsible for the Damage. — The wood boring 

 insects, particularly the bark beetles and grubs, constitute one 

 of the two most destructive classes of insects; while defoHating 

 insects form the other."* For detailed lists and descriptions 

 of injurious (and beneficial) insects the literature treating 

 with forest insects should be consulted. 



Forest insects may be classed as injurious, beneficial or 

 neutral. The beneficial insects function either as parasites 

 living within injurious insects or as predaceous insects feed- 

 ing externally upon the injurious insects. Neutral insects 

 are those present in the forest which have no distinctly 

 beneficial or injurious influence. 



The Causes of Insect Attacks. — Insects attack trees for 

 one of two purposes; either to obtain food of which defoli- 



