NOXIOUS WEED CONTROL 411 



2. The second principle involved is that any insects utihsed 

 must be species known to exercise a controlHng influence over 

 the weeds concerned in their own habitat. At the same time 

 they must be kinds which are known to be specific to the host 

 concerned or confined to a very restricted range of host-plants 

 and, unlikely in a new environment, to infest important economic 

 plants. 



With certain plant-hosts, such as Opuntia, the possibihty of 

 insects which feed upon them resorting to other kinds of plants is 

 remote. Insects feeding upon plants of such unusual characteristics 

 and endowed with acrid juices, resistant cuticle, along with other 

 specialised morphological and physiological properties, exhibit so 

 peculiar and rigid an adaptation to that particular mode of life 

 that they are unlikely to infest plants with entirely different 

 physico-chemical attributes. With plants of the natural order 

 Rosacese, for example, on the other hand, there is always the 

 looming possibihty that in a new environment insects which 

 normally feed upon a single genus or species of that order will 

 infest one or other of its numerous cultivated members. 



3. In contingencies where a pest-plant has close allies of 

 economic importance, insect control requires exceptionally 

 cautious procedure. In such instances the method is beset with 

 manifold difficulties, and lack of foresight may only too easily 

 result in the remedy proving worse than the disease. 



If insect control of such types of pest-plants commends itself as 

 a feasible economic measure, recourse should be rigidly confined, 

 in the writer's opinion, to the introduction of species of specialised 

 habits and behaviour. Root-borers, stem-borers and internal 

 seed- or fruit-feeders are preferable to leaf-feeders, since their 

 economy is usually more delicately adjusted to their specific 

 hosts, and for this reason they are probably less liable to seek 

 other hosts in a new environment. ^ 



4. Any insects deemed likely to prove valuable in the biological 

 control of weeds require exhaustive testing with reference to the 

 possibility of their feeding upon plants other than the particular 

 host concerned. Such experiments require to be carried out 

 firstly in the native country of the insects, and, if the tests prove 



