FOOD SUPPLY 



45 



usually related, species of plants for their sustenance. Still 

 others appear to be monophagous, or restricted to a single 

 food plant, like the Mexican cotton-boll weevil and many 

 others of our important agricultural insect pests. 



From the agricultural standpoint, monophagous insects 

 feeding upon the crop plants are usually the most destructive. 

 If present in the locality where cultivation takes place, they 



\ 



FIG. 21. Map showing the spread of the European white cabbage butterfly (Potitia rapce), 

 following its first introduction, indicated by white spot, near Quebec in 1860. This insect which 

 spreads with great rapidity feeds in the larval state upon cabbage and other closely related plants. 

 (In part after Scudder.) 



seek out their host plant with unerring accuracy to find enor- 

 mous opportunities for development and multiplication, a 

 condition which, like the vacuum, would be abhorred by na- 

 ture, but which is a corollary of all agricultural development. 

 Further than this, the same crop is planted and carefully 

 cultivated during succeeding years, to give never ending op- 

 portunities for increasing hordes of insects. The economic 

 entomologist has no criticism to make of such a method of 

 agricultural procedure, as he cannot, except perhaps within 

 very narrow limits, propose a better one. He cannot, how- 

 ever, hold himself guilty when it is pointed out to him that 

 insect ravages are constantly increasing in spite of his feeble 

 efforts to reduce them. 



