170 MISC. PUBLICATION 273, U. S. DEPT. OF AGRICULTURE 
divided into two large groups, (1) the parasites, which live in, on, 
or with some particular host and gradually consume it, and (2) 
the predators, which feed externally and devour their prey. The 
line of demarcation between a parasite and a predator is not a rigid 
one, since both live at the expense of their host. A parasite is usually 
considered as one capable of completing its life history in or upon 
the body of one host, whereas a predator feeds on a succession of 
individuals. Often both the immature and adult forms of predacious 
insects feed directly on all stages of their insect hosts. 
Most of the parasites belong to two or three families of wasps 
(Hymenoptera) and one family of flies (Diptera). The wasps 
frequently have long ovipositors with which they deposit their eggs, 
often within the body of their insect hosts. Here the egg hatches 
and the young parasite grows to maturity, feeding on and finally 
killing its victim. Fbhes lay their eggs on the surface of the host, and 
the maggots burrow within. Native forest insects are plentifully 
supplied with parasitic enemies. Unfortunately, even the beneficial 
insects are not immune from attacks of other parasites, called hyper- 
parasites. -Hyperparasitism is occasionally carried to the third and 
fourth degrees, making parasitism an extremely complex relationship. 
The most active predacious insects are beetles belonging to the 
families Cleridae, Ostomatidae, Carabidae, and Coccinellidae; lace- 
wing flies of the family Chrysopidae; and several families of true 
bugs belonging to the order Hemiptera. Other important predators 
include such small mammals as chipmunks, mice, and shrews (7Ja). 
The larvae of some of the roundheaded borers are very voracious 
bark feeders and'‘are often unwittingly beneficial in that they devour 
the inner bark so rapidly as to rob the bark beetles of their food. 
This is a case of competition between two species of insects, one 
of which is capable of killing trees and the other harmless in this 
respect, with the harmless species depriving the destructive one of 
necessary food material. 
Under normal conditions, the operation of these physical, nutri- 
tional, and biological forces counteracts the enormous reproductive 
capacity of the insects and tends tc keep the destructive and beneficial 
insects more or less in balance. The few harmful individuals which 
escape their enemies live and feed on their hosts without doing 
conspicuous injury. The defoliating insects feed on a few leaves 
or needles, but the damage is so small as to escape notice. The bark 
beetles kill an occasional tree or breed in down logs and broken tops. 
The aggregate damage is negligible, and the annual growth which 
the trees acquire exceeds the drain, so that there is a net accretion 
of volume in the stand. Insect infestations which continue under 
these conditions are termed “endemic.” This is the normal cond1i- 
tion in nature and it is a hopeless and unwise undertaking to try 
to exterminate native insects under such conditions. The result of 
an effort in this direction would be more apt to disrupt the delicate 
balance than to accomplish the objective. 
Under certain conditions, the natural balance may be broken by any 
one of a number of factors. For example, the beneficial insects or 
other enemies of harmful species may become reduced in numbers; 
the resistance of the trees may be lowered through drought, fire, or 
stagnation; large quantities of slash or other breeding material may 
