CHAPTER XIV 

 BIOLOGICAL CONTROL 



Biological Control of Insect Pests, p. 869. I. Parasite Intro- 

 ductions. The Introduction of Vedalia into Ccdifornia, p. 370 ; The 

 Hawaiian Islands, p. 371 ; Later Work in California, p. 376 ; The Gipsy 

 and Brown-tail Moths in New England, p. 379 ; Other Biological Control 

 Problems in the United States, p. 381 ; Neiv Zealand, p. 385 ; Canada, 

 p. 386 ; Fiji, p. 387 ; Experiments in England, p. 389. 



In its natural or original habitat a species of animal or plant is 

 ordinarily maintained in a condition of equilibrium ^ by the 

 interaction upon it of biological and other agencies. If, on the 

 other hand, any factor or factors disturb this state of equilibrium, 

 an animal or plant may be able to maintain a supernormal 

 ascendancy for a variable period. Similarly, one or the other 

 may find a home in a foreign country, affording it a favourable 

 environment where those agencies which control it in its original 

 habitat are ineffective or wanting. Under either of these conditions 

 the probability is that the animal or plant, as the case may be, will 

 multiply and spread so as to assume the status of a pest. 



Among the natural agencies exercising a controlling influence 

 upon insect life the complex of meteorological factors constituting 

 climate is ever present. Simultaneously another complex of a 

 biological nature is exerting its influence, and represents the sum 

 total of the activities of bacterial, fungal and other diseases, of 

 insectivorous birds and mammals, and of parasites and predators. 

 The restraint exercised by these several biological agencies is 

 known as biological control, and in so far as insect life is 

 concerned, the influence of parasites and predators alone is of 

 immense significance. Plant life is likewise affected by a complex 

 of biological agencies including fungal, bacterial, and other 



^ Equilibrium, in the strict meaning of the word, is not implied, but a 

 condition involving relatively small fluctuation within narrow limits. 



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