24 FOREST PROTECTION 



cidentally from one country to another, is much more likely 

 than a native species to cause serious losses, because of the 

 absence of native parasites and other enemies which serve 

 to keep it in check in its original habitat. The extensive 

 ravages of the Gipsy Moth in Massachusetts, which have lasted 

 over a long period of years, is without precedent in European 

 countries, although the species has been abundant over a 

 large part of the continent of Europe, probably for several 

 centuries. 



It may be that insect plagues play a role in the natural change 

 of species of plants coinciding with geological periods, but 

 the question is one of speculation, not demonstration. 



VI. Species of Trees Affected. There are no species which 

 are not liable to insect attack, but some are much less sus- 

 ceptible than others. Conifers have, on the whole, less re- 

 cuperative powers than broad-leaved species, and consequently 

 succumb much more readily to insect attacks. In this coun- 

 try, the spruces and pines, wherever occurring in pure and 

 even-aged forests, are the species which suffer most. 



VII. Condition of Trees Affected. We may divide injurious 

 insects into three classes according to the condition of the 

 trees attacked. 



a. Certain species, notable those that feed on leaves 

 and pith, usually prefer healthy to diseased plants. 

 They may either kill the tree outright or weaken it 

 to such an extent that conditions are made favor- 

 able for the attacks of — 



5. species which generally prefer unhealthy trees. Or- 

 dinarily these species never attack healthy plants, 

 but in years of plagues they may be forced to do so. 

 Thus in years of extreme abundance, millions of 

 bark-beetles may be drowned in the resin of healthy 

 pines before the trees are weakened to an extent 

 sufficient to allow subsequent millions to propagate 

 the species. 



c. Certain other species only attack the trees after they 

 have been killed. Dead timber, either standing or 

 on the ground, should be marketed as soon as pos- 

 sible as a precaution against damage. Decaying logs 

 and stumps are always found infested with numerous 

 species of insects which cannot be classed as injur- 

 ious since they merely hasten the process of decay. 

 Those insects of this class which are injurious are 



