INSECT ENEMIES OF WESTERN FORESTS 159 



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 (^^). 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 light 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 oh 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 foliage, 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 (51). These beneficial forms may be 



