INSECT ENEMIES OF WESTERN FORESTS 215 



this would mean that the burning would have to be done in 

 midsummer or early in the fall, and this would present a fire 

 hazard. Spreading- the slash so that it will receive the direct 

 rays of the sun will kill a high percentage of the insects in the 

 more southern latitudes, where high temperatures can be attained 

 in and under the bark in this way. 



FIRE 



Trees scorched or killed by forest fires are particularly attrac- 

 tive to many forest insects, which may be attracted from a radius 

 of several miles (122). Subsequent insect damage augments the 

 fire losses, as bark beetles often kill many trees that 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 (96, 138). 



Forest fires do not help in destroying injurious bark beetles, as 

 is sometimes supposed. Sometimes light burning has been advo- 

 cated 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, hence are seldom killed by light 

 ground fires and can be killed in the trees only 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 

 important of these are birds, disease, and other insects. Small 

 mammals, such as chipmunks, mice, and shrews, may be im- 

 portant predators of insects under certain conditions (136) . 



BIRDS 



Many species of birds are insectivorous. Nuthatches, chicka- 

 dees, 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 beneficial 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, which sometimes 

 help to suppress an outbreak of some harmful pest. These 

 diseases are produced by many different micro-organisms, in- 

 cluding bacteria, fungi, and the causes of polyhedral bodies. Few 

 have been adequately studied. One of the most common examples 

 is a wilt disease that spreads rapidly through outbreaks of various 



