structure, growth and economics of insects 45 



Inter-relations Between Insects and Plants 



The idea of inter-relations in Nature was first emphasized by Spren- 

 gel, Darwin and Miiller, and later ecological studies reveal still more 

 clearly how all Nature is linked together into a system, one part de- 

 pendent upon another in an intricate web of life. Disturbances in one 

 portion of the system are followed by disturbances in another. 

 In this chapter reference is made to some of the relations be- 

 tween insects and plants, between insects and birds, and between 

 insects and their parasites. Numerous other relations might be 

 mentioned but these are sufficient to show that a knowledge of these 

 relations is an important part of the equipment of the economic 

 entomologist who would deal successfully with the problems confronting 

 him. 



In a region undisturbed by man the various parts of the system of 

 Nature have practically reached a state of balance through the ceaseless 

 action for long ages of the "struggle for existence." Plant struggles 

 with plant, animal with animal, and both with the environment. With 

 the advent of man, however, the balance has been disturbed by the 

 clearing of the forests, the cultivation and drainage of the land, the 

 growing of crops, and the introduction of foreign plants and animals, 

 since the new set of conditions will be favorable to the increase in num- 

 bers of certain plants and animals, including insects, and unfavorable 

 to others. This disturbance is often widespread. Favored insects 

 will multiply rapidly on account of the abundant supply of food fur- 

 nished by the cultivated crops, faster at first than their parasitic enemies; 

 and insectivorous animals such as snakes, toads, birds and predaceous 

 insects will be deprived of the necessary shelter and hiding places by 

 the clearing of the land, and become less abundant. 



On the other hand insects not favored, by the destruction of their 

 food plants under the new conditions, will diminish in numbers, 

 as will also their parasites, both sometimes no doubt to the verge of 

 extinction. If, however, as is sometimes the case, conditions again 

 favor the insect it will multiply very rapidly because the development 

 of the parasite lags behind its host. Moreover, there is always a 

 limit to the increase of the parasite, otherwise it would exterminate 

 its host, and eventually itself. 



Many examples of inter-relationships among insects in addition 



