INSECT ENEMIES OF WESTERN FORESTS 13 



to control all insect outbreaks. Much of the insect damage to forest 

 trees of low value will have to be allowed to run its course, for if a 

 policy of combating all threatening insect outbreaks were adopted 

 the cost would be enormous and in many cases would exceed the 

 damage probable if Nature were allowed to control the epidemic 

 in her own way. The older forests, as they stand today, are ripe 

 and an easy prey to bark-beetle attack, and if we are not prepared 

 to utilize such timber and are willing to wait for Nature to replace 

 any losses by the slow process of growing a new crop of trees, no 

 further consideration need be given to control. But where timber 

 is in demand and satisfactory control measures are available, fail- 

 ure to take the necessary protective measures should be viewed 

 in the same light as failure to control forest fires. 



DETERMINING CAUSES OF 

 FOREST-TREE DAMAGE 



Before observed damage is charged to insects, other possible 

 causes should be investigated. Often several agents, such as fire, 

 insects, fungi, and physiological injuries, are so closely associated 

 or interrelated that it is difficult to determine the primary cause 

 of the damage. 



Injury by fire is usually easy to identify. Destruction of the 

 ground cover, scorching of the bark, and reddening of the needles 

 constitute ample evidence of fire damage. Usually bark beetles, 

 either primary or secondary species, attack fire-weakened trees 

 and complete their destruction. In some areas fire scars serve as 

 important entrance points for fungi. Witches' brooms and damage 

 by mistletoe are frequently conspicuous in either killing small 

 trees or so distorting them that they can never grow into timber 

 trees. Injuries by fungi, bacteria, and higher parasitic plants are 

 not so easily determined by the layman, and can rarely be identi- 

 fied without the assistance of a trained forest pathologist. The 

 diseases, decay, and wood rots caused by these various organisms 

 are not discussed in this publication. 



Mechanical and physiological injuries are frequently the pri- 

 mary cause of sickliness, weakness, or death of forest trees. The 

 insects that invade the wood after such injuries have occurred are 

 usually only secondary enemies, and cannot be charged with pri- 

 mary responsibility. 



In some years a combination of weather conditions causes what 

 is known as winter injury, red belt, or parch blight; that is, 

 all trees of certain species on exposed hillsides within definite alti- 

 tudinal limits turn a bright wine-red color. The injury is thought 

 to be due to excessive transpiration during warm periods in win- 

 ter when the ground, roots, and tree trunks are frozen and water 

 cannot rise to supply the deficiency in the leaves. Twigs are some- 

 times killed, but the trees usually recover unless subsequently 

 attacked by bark beetles or fungi. 



Sometimes the tender bark on the south and southwest sides of 

 trees and on the tops of branches is killed by the sun's heat. This 

 is referred to as "bark scorch," or "sun scald." The bark breaks 



