INSECT ENEMIES OF WESTERN FORESTS 11 
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 1mpor- 
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 1s 
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 
partial 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 
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