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



these native insect enemies through artificial propagation presents 

 seemingly insurmountable difficulties with little hope of any lasting 

 benefit. 



Another possibilit}' is the introduction of a new parasitic or pre- 

 dacious enemy. To do this, it is first necessar}^ 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 which 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 parasitic enemy 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 if it is to succeed. 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 im- 

 ported into new regions while their natural enemies have been left 

 behind. Under such conditioTis 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 fre- 

 quently been necessary. In only a few instances have introduced 

 beneficial insects been able to control destructive epidemics completely 

 without other assistance. 



In view of the fact that the application of biological control meas- 

 ures often presents almost insuperable difficulties, a thorough knowl- 

 edge of all associated insects is of vital importance, in order that the 

 more direct control methods may be applied so as to take full 

 advantage of any aid from parasites and predators. 



DIRECT ARTIFICL\L OR REMEDIAL CONTROL 



"When nature fails to keep an injurious insect in check, and valu- 

 able forest crops are threatened with destruction or serious injury, 

 direct remedial or artificial control measures are called for. These 

 consist in such mechanical methods as the application of insecticides, 

 as in spraying, dusting, fumigating, or baiting : felling or burning in- 

 fested trees; or the use of trap trees or solar heat. In brief, these 

 artificial control measures are employed to destroy the harmful 

 insects directly in one way or another. 



The control measures applicable to the control of cone and seed 

 insects, root-feeding nursery pests, terminal or twig insects, and in- 

 sects injurious to forest products have already been considered in 

 connection with the discussion of these special groups. The control 

 of defoliating insects and bark beetles affecting mature forest trees 

 has been given special attention in western forests, and. since spe- 

 cialized methods have been developed, these will be given detailed 

 consideration. 



