CHAPTER XV 

 BIOLOGICAL CONTROL— continued 



II. Factors Governing Parasite Introductions, p. 391. Geo- 

 graphical Location, p. 392 ; Multiparasitism, p. 396 ; Hyperparasitism, 

 p. 398 ; The Sequence Theory of Parasitic Control, p. 401 ; Other Factors, 

 p. 402. III. The Utilisation of Indigenous Parasites, p. 405. 

 — Conservation of Parasites, p. 406 ; Direct Increase of a Parasite 

 Population, p. 407 ; Transference of Parasites to New Areas, p. 409. 

 Biological Control of Noxious Weeds. Principles Involved, 

 p. 410 ; Control of Lantana, p. 412 ; Control of Prickly Pear, p. 413 ; 

 Control of Prickly Pear in Other Lands, p. 416 ; Control of Clidemia, 

 p. 418 ; Problems in New Zealand, p. 418. Literature, p. 419. 



II. Factors Governing Parasite Introductions 



In the foregoing section a number of the more important 

 parasite introductions are discussed. They are by no means 

 exhaustive, and many other experiments have been undertaken 

 in various parts of the world. Certain of these have yielded 

 beneficial results, but in a much larger number of cases no 

 successful outcome has been recorded ; in other instances the 

 introduced species failed to establish themselves or the experiments 

 were doomed to failure from the outset owing to the haphazard 

 nature, lack of requisite knowledge and other causes. 



In judging success or failure it is, of course, important to bear 

 in mind that the primary objective is to bring about an economic 

 reduction in the abundance of the pest concerned. If, for example, 

 introductions are attempted with ten different species of parasites 

 and only two of these succeed in estabhshing themselves, and these 

 are effective in restraining the pest in question, the experiment 

 has been successful. It would be wide of the mark to assert 

 there had been eight failures, since the objective was pest control, 

 and once this is achieved it matters little as to what becomes 

 of the ineffective parasites. Also it needs to be pointed out that 

 biological control aims at bringing about a permanent measure 



391 



