66 ORIGIN OF INSECT PESTS. 



in our cultivated areas, the increase of insects is automatically and natur- 

 ally checked ; at times it is not checked in our artificial surroundings, 

 whereby we suffer. 



These facts require to be borne in mind in considering- our pests and 

 the manner in which they appear. An outbreak of an insect pest is not 

 due to supernatural phenomena, nor is it wholly due to au east wind, last 

 yearns flood, late rains or other causes. Cultivators commonly believe 

 that the sudden appearance of a pest is more or less supernatural, and for 

 that reason not capable of treatment; it is no more supernatural than 

 when a bullock gets into a young crop and eats it, only we can trace the 

 bullock and cannot always trace the insect pest. Above all we must 

 remember that our insect pests are always with us, but not always 

 abundant. Insect j)ests do not appear suddenly from nothing ; they are 

 the insects common throughout the plains, which under a certain 

 combination of circumstances increase sufficiently to do damage. No 

 conditions of manuring, irrigation or the like can produce them, but 

 suitable conditions can enable them to increase beyond their natural 

 limits and turn them into a pest. 



As can readily be seen, the conditions which govern this very 

 delicate balance of life are extremely complex, and we cannot always see 

 what causes have led to a particular result. The preventive measures 

 discussed in the next section, as well as one simple method of encouraging 

 parasites (see page 271), ai'e based upon our knowledge of these causes. A 

 little consideration further helps us in some cases to anticipate an outbreak 

 of an injurious insect, on perfectly simple reasoning. An entomologist 

 who sees hundreds of the moths figured on page 188 flying in March, as 

 is commonly the ease, will know that, if sufficient wild plants are not 

 available, the crops will be attacked ; he will also know that there must 

 have been abundance of the caterpillars of this moth before the cold 

 weather, and that probably they might then have been destroyed either 

 in the crops or by more careful attention to clearing away weeds on waste 

 strips and headlands. 



Again, if we see a swarm of caterpillars in the fields or in uncultivated 

 land, it is worth while ascertaining whether they are extensively attacked 

 by parasites or not ; if we see many flies (fig. 325) or ichneumons (fig. 323) 

 among these caterpillars or laying eggs on them (a matter requiring but 

 little observation), it is probably advisable not to destroy these caterpillars 

 unless they are doing very great injury ; if, however, we find no parasites 

 or only small numbers, we must use every endeavour to destroy them or 

 prevent them becoming pupae, as the next outbreak will be a very large 

 one, An agriculturist who understands something about these moths 



