216 MISC. PUBLICATION 273, U.S. DEPT. OF AGRICULTURE 



caterpillars. The caterpillars suddenly sicken and die and are 

 seen hanging from leaves and twigs blackened and shriveled. 



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 (81). These beneficial forms 

 may be 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 predaceous 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. Parasitic flies lay their eggs on the 

 surface of the host, and the maggots burrow within. Native 

 forest insects have many parasitic enemies. Unfortunately, even 

 the beneficial insects are not immune from attacks of other 

 parasites, called hyperparasites. Hyperparasitism is occasionally 

 carried to the third and fourth degrees, making parasitism an 

 extremely complex relationship. 



The most active predaceous insects are beetles belonging to the 

 families Cleridae, Ostomatidae, Carabidae, and Coccinellidae; 

 several families of flies (Diptera) ; lacewing flies of the family 

 Chrysopidae; and several families of true bugs belonging to the 

 order Hemiptera. 



The larvae of some of the roundheaded borers are very vora- 

 cious 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 reproduc- 

 tive 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 accx^etion of volume in the stand. Insect 



