86 SOME INSECTS INJURIOUS TO FORESTS. 



ciples of insect control in the management of forests and manufac- 

 turing enterprises. 



Beneficial Insects. 



The beneficial insects comprise those which are internal or external 

 parasites of the immature or mature stages of the injurious insects, 

 and predators which feed on the young or adults of insects either 

 before or after they make their attack on the trees or products. These 

 two beneficial factors are doubtless far more effective in the long run 

 than any other agencies of natural control. Yet they, in combination 

 with all other factors, can not be always relied upon to render con- 

 tinued and efficient control. They can, however, be relied upon to 

 respond to artificial assistance in reducing the numbers of the depre- 

 dators. 



Beneficial Diseases of Insects. 



It is very evident that the parasitic fungi and bacteria which some- 

 times cause epidemics among injurious insects often exert a powerful 

 influence toward the control of extensive outbreaks or invasions of 

 insect enemies of forests. Indeed, it appears that the greatest serv- 

 ice rendered by this class of natural enemies is in the frequent sud- 

 den appearance of an epidemic which kills off a destructive species of 

 insect after the latter has increased to such numbers and extended 

 its depredations over such vast areas as to be far beyond the control 

 of man or his insect and bird allies. Numerous examples of this kind 

 of natural control are found in the sudden ending of widespread 

 depredations by various species of caterpillars and sawfly larvae which 

 defoliate deciduous and coniferous trees. As a rule, however, the 

 beneficial effects of the diseases of insects prevail only after the 

 injurious insects have increased to excessive numbers. Therefore this 

 factor of insect control can not be depended upon to hold the insects 

 in check or prevent outbreaks. The fact, however, that it operates on 

 a class of insect enemies of the forest (defoliators) which at present 

 can not be controlled by any known artificial methods renders the serv-' 

 ices of the diseases all the more valuable. 



It is believed that with further knowledge of nature's method of 

 propagating, perpetuating, and disseminating the diseases which 

 cause epidemics among insects, they may be utilized more or less 

 successfully through artificial propagation and dissemination to 

 prevent threatened invasions of defoliating insects. 



Bkneficial. Bikds. 



It is very evident that certain kinds of birds, such as woodpeckers, 

 render valuable service toward the natural control of destructive 

 bark and wood boring insects. They appear to render the greatest 



