174 MISC. PUBLICATION 273, U. 8S. DEPT. OF AGRICULTURE 
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 pre- 
dacious 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 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 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 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 ARTIFICIAL 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, 
