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



will be so poor and the trees so weak that insects will have a 

 fertile field for their activities. Here, too, there is the opportunity 

 to select varieties of trees not only adapted to the particular site 

 but also insect-resistant or capable of making such rapid growth 

 as to overcome any set-backs from insect attack. Drainage, the 

 mixture of species, and the spacing of the trees also must be 

 given special consideration. 



BIOLOGICAL CONTROL 



As has been previously mentioned, parasitic and predaceous 

 insect enemies and insectivorous birds and mammals are often 

 of great value in holding destructive forest insects in check. To 

 increase the effectiveness of these natural control agencies 

 through artificial means offers hopeful possibilities, but is beset 

 with many difficulties (62). 



If the native insects already have a full complement of natural 

 enemies, an effort can be made to create favorable conditions for 

 the multiplication of these beneficial agencies. Direct control 

 methods, such as burning or sun-curing, can be modified so that 

 the beneficial insects will not be destroyed in as large numbers 

 as the harmful species. Sometimes only a slight change in this 

 direction will give the beneficial insects the upper hand, and they 

 will quickly bring an epidemic back to normal balance. To in- 

 crease these native insect enemies through artificial propagation 

 presents seemingly insurmountable difficulties with little hope of 

 any lasting benefit. 



Another possibility is the introduction of a new parasitic or 

 predaceous enemy. To do this, it is first necessary to find an insect 

 not already present within the infested area that will prey upon 

 the harmful species. Even though such an enemy may be found, 

 there are many complex factors that will influence the success 

 of the introduction and its ultimate effectiveness. The life history 

 of the new enemy must synchronize with that of its host if it is 

 to be on hand at the proper time for attack. If a parasite has 

 more generations annually than the host, other insects must be 

 present for it to attack at other periods during the season. The 

 parasite must be capable of wide distribution and have a greater 

 reproductive capacity than that of its host. Moreover, its ability 

 to adapt itself to the change in climatic conditions in its new 

 environment may be an important factor in determining its 

 ability to succeed. So far, no introduction of a foreign parasite 

 or predator has been outstandingly successful in the control of 

 any of our native forest insects. 



The introduction of parasites and predators has been confined 

 largely to cases where injurious insects have been accidentally 

 imported into new regions while their natural enemies have been 

 left behind. Under such conditions the introduction of -parasites 

 from the original home of the pest have, in several instances, 

 been entirely successful, although the continuation of artificial 

 control has frequently been necessary. In only a few instances 



