276 THE PRACTICAL APPLICATION OF ECOLOGY 



The foregoing examples will serve to illustrate the influence 

 of plant nutrition, and soil conditions, in relation to the 

 susceptibility of crops to insect infestation. Recent research 

 emphasises the complexity of the subject, but, at the same time, it 

 indicates the importance of studying the ecological relations 

 between host and pests as an interconnected problem. Progress 

 of knowledge on this field is necessarily slow, and consequently 

 its application to cultural practice can only result very gradually. 

 We are only just beginning to realise that persistent heavy 

 attacks by insects may be the result of the particular cultural 

 conditions under which a given crop is grown. The possibility 

 is, therefore, opened out of so regulating the growth of the crop 

 that it will be rendered less prone to such attacks, and still yield 

 an adequate economic return. 



Resistant Varieties 



In the wild state many plants appear to suffer relatively little 

 injury from insect attacks as compared with their cultivated 

 representatives. They appear to have acquired, through many 

 thousands of generations, a degree of tolerance to such interference, 

 whereas cultivated varieties of these same plants suffer severely 

 from repeated infestation. This difference in plant response 

 appears attributable to two main causes. (1) The conditions 

 under which the cultivated varieties are grown which favour 

 infestation. (2) It would seem that during the process of selection 

 and breeding, which has led to the production of cultivated 

 varieties, certain properties, inherent in the wild state, have been 

 out-bred ; that the plants are rendered in this way, not only more 

 attractive to their respective insect enemies, but also less able 

 to sustain the injuries incurred. 



The problem which presents itself is likewise two-fold. Firstly, 

 an ecological study of the conditions in which a given crop is 

 grown, in order to trace those conducive to infestation, which has 

 already been discussed. Secondly, the discovery of varieties or 

 strains of plants which betray high resistance to insect attack 

 and, at the same time, exhibit desirable properties from the stand- 

 point of the grower. In practice, strains which exhibit one or 



