410 BIOLOGICAL CONTROL 



resulted in complete control of the woolly aphis (Eriosoma lanigera) 

 in that region. Owing to some unexplained factor, or factors, the 

 natural spread of the host in the area mentioned had been accom- 

 plished without the usual accompaniment of the Aphelinus. In 

 the third example the experience of Hazelhoff (1929) may be 

 quoted. He states that the transfer of the Chalcid Encarsia 

 scutellum, from the older or established cane-fields in Java into 

 newly planted sugar cane areas, has proved definitely successful 

 in maintaining the aphid Oregina lanigera under control. The 

 powers of dispersal of this parasite, it appears, are very restricted, 

 and consequently its incidence in new cane -fields is greatly 

 delayed if the insect be left to disseminate itself by its unaided 

 efforts. 



Biological Control of Noxious Weeds 



The application of biological control to weeds presents itself 

 as a possibility where alien plants have invaded and colonised 

 land to the extent of rendering it useless. It is well known that 

 in certain parts of the world introduced weeds have resisted all 

 attempts at cultural, chemical or other methods of control, or 

 such methods have proved to be economically impracticable. 

 It is in circumstances of this character that insect control of the 

 noxious plants concerned has been attempted in several different 

 countries. 



Principles Involved. In any scheme to control noxious weeds 

 by the introduction of insects living at their expense certain 

 guiding " principles " require emphasis (vide also Imms, 1929). 



1. It has already been pointed out that there is abundant 

 evidence that alien insects which have entered countries 

 unaccompanied by their natural parasites can become firmly 

 established and cause great depredations to cultivated crops. 

 On the other hand, there appears to be no valid reason why 

 suitable types of insects deliberately introduced, free from their 

 natural enemies, should not likewise exercise a destructive effect 

 upon noxious alien weeds, given an environment favourable to the 

 colonisation of such insects. It is this principle that is fundamental 

 to the problem concerned. 



