INSECT ENEMIES OF WESTERN FORESTS H 



DETERMINING CAUSES OF FOREST-TREE DAMAGE 



Many agencies may cause injury or death to forest trees, so before 

 observed damage is charged to insects, other possible causes should 

 be investigated. Often several agencies, 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 impor- 

 tant 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. In- 

 juries by fungi, bacteria, and higher parasitic plants are not so 

 easily determined by the layman, and can rarely be identified without 

 the assistance of a trained forest pathologist. The discussion of 

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

 not within the field covered by this publication. 



Mechanical and physiological injuries are frequently the primary 

 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 primary respon- 

 sibility under such circumstances. 



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 winter 

 when the ground, roots, and tree trunks are frozen and water cannot 

 rise to supply the deficiency in the leaves. Twigs are sometimes 

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

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

 from the wood and sloughs off. Such damage is rare under forest 

 conditions but occurs more frequently in young trees grown in open 

 plantations. 



Excessive quantities of dust in the air, as along dirt roads, causes 

 a clogging of the stomata or breathing pores of leaves and results in 

 l^artial suffocation of trees. In the Western States such injury is 

 frequently followed by an attack of scale insects, which add to the 

 injury and in some cases have caused the death of many young trees. 



Smelter smoke, and chemicals or oils deposited on the ground in 

 some instances cause injury to trees which leads to attack by many 

 species of insects. 



Mechanical injury to trees may result from a number of causes, 

 such as logging operations, lightning, road building, and packing of 

 soil or exposure of roots (as in camp grounds), or from the work of 

 animals such as bear, beaver, and porcupines and that of sap-sucking 



