X70 ^IISC. PUBLICATION 273, U. S. DEPT. OF AGPvICULTURE 



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. Flies 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 {71a). 



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 tAvo 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 to 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 condi- 

 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 



