FOOD SUPPLY 57 



been started, and that its limitations are in a broad way de- 

 termined primarily by the insect species against which it is 

 directed. Aside from the repeated application of poisons and 

 other insect-destroying materials, it is the only feasible means 

 except those which entail changes in agricultural procedure. 



Of the latter methods, there are several which can be em- 

 ployed without seriously disturbing agriculture, and at least 

 one of them is, in fact, only a slight modification of the prin- 

 ciple of the rotation of crops which had its origin not in rela- 

 tion to insects, but as a practice to delay or prevent the loss 

 of soil fertility. 



We have already seen how the continued presence of the 

 same crop plant in the same place from year to year serves 

 to augment insect depredations. Since many different crops 

 have insect enemies in common, it follows that when such 

 crops follow one another on the same area, the opportunities 

 for insect multiplication are not conspicuously less than if 

 there were no rotation. Similarly the planting of newly 

 plowed grass-land to corn, wheat or other grasses courts dis- 

 aster in the same way. Rotation practised with insect injury 

 in mind, however, where the crop plants are without impor- 

 tant enemies in common, serves to reduce insect injury to at 

 least some extent, dependent mainly upon the ability of the 

 insects concerned to migrate, and it need in no way impair 

 the other advantages derived from the process. Agricultural 

 entomology has now reached such a condition that reliable 

 advice with reference to many insects can be given in this 

 matter. 



A concrete example of the utilization of this method in the 

 control of certain insects that are difficult to combat other- 

 wise has recently been demonstrated. In this case the insects 

 are the large grubs of May -beetles which feed on the roots of 

 many plants. They are particularly injurious to corn in the 

 northern middle states, but do not injure clover nor do they 

 deposit their eggs in soil that is covered by a stand of clover. 

 In this region corn is commonly planted in rotation with 

 timothy, oats or barley, but if clover is grown the year before 



