CONSERVATION AND MANAGEMENT OF FORESTS 339 



is estimated at $100,000,000 annually. Insects attack and cause 

 malformation or destruction of buds, flowers and seeds, leaves, 

 bark, wood, and roots. Such forest products as furniture, dwell- 

 ings, posts and poles are destroyed or weakened. Many of these 

 insect pests are native to our American forests, and under 

 normal conditions of nature, are held fairly well in check by their 

 natural enemies — parasites, predators, and climatic factors. 

 Occasionally, however, a series of years favorable to the rapid 

 propagation of a particular pest results in the building up of an 

 abnormally large population. This condition is known to ento- 

 mologists as an "outbreak." It is only by careful study of life 

 histories and requirements of the many and various destructive 

 forest insects that such outbreaks may be forecast or recognized 

 before they have reached peak proportions. Then necessary 

 control measures can be applied to check the damage; these 

 include removal and burning of insect-infested trees and general 

 cleaning up of logging debris which would favor conditions for 

 insect multiplication. Special methods of growing and maintain- 

 ing forest stands also help in preventing undue insect damage. 

 An example of such a silvicultural method of control is seen in 

 the planting of white pine beneath a light cover of hardwoods 

 in order to protect it from damage by the white pine weevil. 

 Especially destructive are introduced insect pests, since they are 

 not suppressed by the natural checks which existed in their 

 native environment. Sometimes insectivorous birds and rodents 

 do not readily accept these newcomers as items in their diet, so 

 that the insects breed and multiply unmolested in their new 

 home. The gypsy and browntail moths, Japanese beetle, and 

 European pine tip moth are familiar examples of such intro- 

 duced pests. 



Fungus diseases of forest trees are caused by heterotrophic 

 plants (see Chap. 25), which, being colorless, must secure their 

 food at the expense of green plants and in so doing cause disease 

 and deterioration of the host. All parts of forest trees, of all ages, 

 are subject to attack by wood-rotting or disease producing organ- 

 isms — leaves, stems, and roots, as well as fruit, being invaded. 

 In the case of root and stem diseases, entrance is usually gained 

 through breaks in the bark of the stems or roots, which may be 



