64 ORIGIN OF INSECT PESTS. 



then the period o£ dry lieat, which is to many equally a period of rest 

 during wliich their enemies are especially active. Finally^ there are four 

 months of suitable weather, during- which they can increase ; even this is 

 probably not entirely favourable as it is likely that excessively heavy rain 

 and wind storms do much to kill moths and flying insects. 



The lach of food is another powerful cause ; as a rule, plants grow 

 vigorously during the rains, many grow during the cold weather, after 

 which a great proportion of the vegetation dies down. An insect feeding 

 on a plant that grows only in the rains has but a few months in which 

 to multiply; if it can also feed upon cold weather plants it has some 

 months longer, provided it is not numbed by cold, and finally it has no 

 food for some months of dry heat. 



The third great check is due to enemies j these include the parasites, 

 the predatory insects, the birds, bats, etc. As soon as an insect becomes 

 abundant, these attack it and reduce its numbers (see page 268). 

 Fungoid and bacterial diseases are also operative in insects as they are in 

 man and domestic animals. 



In the jungle or forest, we find that, on the whole, these causes 

 acting against the naturally large ratio of increase, tend to preserve an 

 even level for all insects ; what we may call the balance of life is main- 

 tained, and, neglecting small variations, in nature the numbers "bf each 

 species are more or less constant over long periods. Where man has not 

 interfered, insects do not become destructively abundant ; there are excep- 

 tions, of course, but then the balance adjusts itself very swiftly. We 

 are, however, not dealing with the jungle where nature reigns, but with 

 artificially maintained areas of cultivation. There man has upset the 

 original conditions in very definite ways for which he pays the penalty. 



Firstly, the balance of life is commonly upset by the new forms of 

 life which are introduced by man himself or which come with him. The 

 introduction of the Gipsy Moth to the United States let loose an insect 

 whose increase was so enormous that as much as four lakhs were spent 

 yearly in one State in checking it. The disastrous results that followed 

 the introduction of the Indian mongoose to the West Indies, of the rabbit 

 to Australia, of ferrets, stoats and weasels to New Zealand, of the sparrow 

 to the United States, and of the Indian myna to the Hawaiian Islands are 

 examples of the manner in which the balance of life is upset by the^ intro- 

 duction of new forms. 



Secondly, we may refer to the interference with the climate caused 

 by the changes made upon the earth's surface by man ; such a change as 

 the destruction of the forests is the most obvious case, leading to profound 

 modifications in the climatic conditions of large areas. 



