16 MISC. PUBLICATION 657, U. S. DEPT. OF AGRICULTURE 
to the belief that it is best to regard defoliation as the primary factor 
in the death of the trees and consider the subsequent attack by bark 
beetles or boring insects as secondary and not justifying contr ol meas- 
ures. In other words, it is of little avail to treat a symptom when the 
seat of the trouble cannot be attacked. 
NATURAL ENEMIES 
By R. C. Brown 
Insects, like other living things, have natural enemies which prey 
upon them and tend to hold them in check. These include other in- 
sects, birds, small mammals, bacterial, virus, protozoan, and fungus 
diseases, and parasitic nematodes. 
Birds 
Many species of birds are insectivorous. Nuthatches, chickadees, 
creepers, warblers, kinglets, and many other species search for insects 
on tree trunks and foliage, and the woodpeckers dig through the bark 
and feed on the larvae of bark beetles and wood borers. Over 75 per- 
cent of the broods of the eastern spruce bark beetle and southern pine 
beetle have been destroyed in patches of bark worked over by indus- 
trious woodpeckers, and frequently local outbreaks are completely 
checked. Hall (208) reported woodpeckers as one of the most impor- 
tant single factors in the control of the locust borer. 
Small Mammals 
The meadow mouse, the white-footed mouse, moles, shrews, voles, 
chipmunks, and squirrels play important roles in the destruction of 
forest insects hibernating in the duff or soil. These small mammals 
often consume a large percentage of the cocoons formed in the leaf 
litter and have been “reported as practically controlling outbreaks of 
some of the sawflies. These mammals are also impor tant in destr oying 
broods of bark beetles exposed during the peeling operation of control 
projects. Rust (366) and Graham (297) have discussed predatory 
mammals as control factors. 
Nematodes 
Certain species of parasitic nematodes attack bark beetles, wood- 
boring insects, soil-inhabiting grubs, ana lepidopterous larvae, causing 
sterility or death of the host. “Little is known of the real importance 
of such parasitic animals. 
Insects 
Many species of insects and related forms, such as mites, belonging 
to several orders and families, are distinctly beneficial in that they prey 
on the harmful species. These beneficial forms may be divided into 
two groups—parasites and predators. The line of demarcation be- 
tween a parasite and a predator is not a rigid one, as both live at the 
expense of their host. A parasite is usually considered as one capable 
of completing its life history in or on the body of one host, whereas 
a predator feeds upon a succession of individuals. 
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