392 BIOLOGICAL CONTROL 



of pest repression ; in the most successful cases it may result in 

 artificial methods being no longer necessary, while in others 

 biological control may serve to supplement insecticidal or other 

 treatment. It is only in rare instances and in the most favourable 

 environment that almost complete suppression of a pest results, 

 and in most cases a successful outcome is regarded as having been 

 achieved once an appreciable degree of permanent control over a 

 given pest has been accomplished. Thus if an injurious insect A, 

 feeding within the stems of a cereal crop and exempt from the 

 effects of any practicable artificial method of control, is affected 

 by biological measures to an extent of reducing the damage which 

 it occasions by an average of 20 per cent., taken over a period of 

 years, few would assert that the experiment was not justified. Or 

 again, if a scale insect B, infesting fruit trees to an extent which 

 demands spray treatment several times annually, is reduced by 

 biological control so that spraying becomes necessary, on average, 

 only once every three years, immense benefit will have resulted 

 to growers in the saving of expenditure incurred. 



It is desirable at this point to discuss the more important 

 factors and principles which have a definite bearing upon the 

 success or failure of parasite introductions. 



Geographical Location. Up to the present time some of the 

 most spectacular and complete examples of the application of 

 biological control have resulted from parasite introductions 

 made under insular conditions. The successes achieved in the 

 Hawaiian Islands, New Zealand and Fiji are examples in question. 

 In continental areas the most encouraging results have been 

 achieved where parasite introductions have been made, (a) with 

 respect to certain tracts which are, ecologically, separate islands, 

 e.g., California ; and (h) with respect to the control of pests 

 affecting a localised crop, e.g., citrus fruits, mulberry, etc. In 

 the case of widespread pests affecting crops or trees whose range 

 of distribution extends over a vast continental area, involving 

 varied climatic and other conditions, no indubitable success has 

 so far been achieved. 



(a) Insular Areas. The most favourable type of physical and 

 biological environment afforded by insular conditions is 



