VII.] PESTS CHANGE THEIR HABITS. 183 



eats this insect is beneficial; that is, he is beneficial to 

 man, not to the other insect. Thus everything in nature 

 is a benefit to something and an injury to something: 

 and every time that conditions of life are modified, the 

 relationships readjust themselves. 



Cultivation is a powerfully disturbing factor in 

 nature. It affects the relationships of depredaeeous 

 insects and parasitic fungi to man chiefly in the fol- 

 lowing ways : 



1. Cultivation induces change of habit in the insects 

 and fungi. Animals do not live and plants do not 

 grow where they most desire to live and grow, but 

 where they are allowed to live ; and all organisms are 

 susceptible to new enticements or new advantages. A 

 cultivated plant may be a more attractive source of food 

 to an insect than the wild plants upon which it has 

 been forced to live, or it may be a more congenial 

 host to a fungus. In such cases, the organism is 

 likely to abandon its old habits and to spread into cul- 

 tivated grounds. Or, man may destroy the natural 

 food-plants or host-plants, and the organism is obliged 

 to seek other food, and it is likely to take that which is 

 most nearly like its habitual food, and which is most 

 abundant. We are always liable, therefore, to have 

 insects and diseases transferred to the orchard from 

 our wild crabs and plums. The apple maggot, which 

 lived once upon wild thorn apples, began to attack cul- 

 tivated apples in New England some thirty years ago, 

 and has now spread westward to the Mississippi Valley. 

 But this same species of insect also occurs in a wild 

 state in this same western region, but it did not so 

 early attack apples here, if, in fact, it has done so to 

 the present day. It is not possible to discover why it 



