CHAPTER V. 



ORIGIN OF INSECT PESTS. 



IT is well known that the natural increase of any insect would be 

 very large if the restraining checks were removed. It is also true that 

 a species rarely becomes so abundant as to cause serious damage to crops. 

 We may here shortly discuss the causes which lead to the undue increase 

 of an insect, so that it becomes a pest. 



In this book we propose to record about one hundred species of 

 herbivorous insects wliich have become injuriously abundant in recent 

 years ; tliis is a small fraction of the herbivorous insects common in the 

 plains, which can and do feed upon crops. Why do not the larger part of 

 these insects also become abundantly destructive ? There is no answer to 

 this question, nor has it ever been answered in any country ; we can but 

 dimly see the laws which govern the increase of animal life, and in this 

 limited space we can but discuss some of the most obvious laws and 

 phenomena which a close study of nature presents to us. 



In the first place, let us examine the rate of increase of an insect. 

 A single female of one of our commonest moths lays over 500 

 eggs. Of these actually 200 have been reared to moths again under 

 artificial conditions ; these produce 50,000 eggs, since roughly half 

 are females. Of these again we rear 30,000 moths which produce 

 5,000,000 eggs. This is the rate of increase based on the actual ratio 

 that live in captivity. We see that the increase of one moth in three 

 generations is vast, equivalent approximately to ten large and 

 devastating swarms of caterpillars. This is not a peculiar case, just one 

 taken at random and well within the limits of any herbivorous insect 

 investigated in India. We can see then that every moth feeding upon a 

 crop plant should, if there were no checks, produce a vast progeny. 

 Clearly also the checks are very efiicient since this does not occur. The 

 checks upon the increase of insects may be roughly said to be climate^ 

 lack of food J enemies. 



Climatic checks are of great importance, and we cannot pretend 

 yet thoroughly to understand them. Most insects become torpid with 

 cold and hibernate (lie dormant) for some three months. Not only does 

 this stop increase at this time but the hibernating insect is exposed to 

 many dangers during this period — dangers it cannot guard against. 

 Probably a very large percentage do not survive the winter. There is 



