INSECT ENEMIES OF WESTERN FORESTS 169 
FIRE 
Trees scorched or killed by forest fires are particularly attractive 
to a large number of forest insects, which may be drawn to them 
from a radius of several miles (64). Subsequent insect damage 
augments the fire losses, as bark beetles often kill many trees which 
otherwise might have survived. Wood-boring species then enter the 
wood and so riddle the interior that within a few years it becomes 
valueless for lumber purposes. 
Forest fires are not of any benefit in destroying injurious bark 
beetles, as is sometimes supposed. Sometimes hght burning has 
been advocated as a means of controlling bark beetles, but’ studies 
have shown that such fires are more apt to have the opposite effect. 
Destructive, tree-killing bark beetles never breed in and seldom 
inhabit the forest litter and duff and hence are seldom killed by 
light ground fires, and can only be killed in the trees by a fire severe 
enough to kill the bark on the trunks. Such a fire obviously would 
do more harm than good. 
NATURAL ENEMIES 
Insects, like other living things, have natural enemies that prey 
upon them and tend to hold them in check. Three of the most im- 
portant of these are birds, disease, and other insects. 
BIRDS 
Many species of birds are insectivorous. Nuthatches, chickadees, 
creepers, warblers, kinglets, and many other species search for insects 
on tree trunks and folhage, while woodpeckers dig through the bark 
and feed on larvae of bark beetles and wood borers. Counts have 
shown that fully 75 percent of the western pine beetle population 
in patches of pine bark worked over by woodpeckers have been 
destroyed by these industrious workers. But not all birds are bene- 
ficial in this respect. Some are as destructive to beneficial insects 
as to the harmful species, and their feeding has ultimately little effect 
in reducing the injurious forms. 
DISEASE 
insects are subject to many fatal diseases, that sometimes are a 
potent factor in suppressing an outbreak of some harmful pest. 
These diseases are represented by many different micro-organisms, 
including bacteria, fungi, and the causes of polyhedral bodies. Few 
of these have been adequately studied. One of the most common 
examples is a wilt disease that spreads rapidly through outbreaks 
of various caterpillars when these are excessively numerous. The 
caterpillars suddenly sicken and die and are seen hanging from leaves 
and twigs in a blackened, shriveled condition. 
BENEFICIAL INSECTS 
Many species of insects belonging to different orders and families 
are distinctly beneficial in that they devote their lives to preying 
upon certain harmful species (57). These beneficial forms may be 
