INSECTICIDES, PREVENTIVES, AND MACH/NERY. 419 



kinds for them. These enemies in turn must be checked, that 

 they may not exterminate the species they themselves feed upon. 

 Thus there has been estabhshed, gradually and naturally, an 

 elaborate system of checks and counterchecks, by means of 

 which a certain proportion is maintained year in and year out 

 between predaceous, parasitic, and plant feeding insects. Under 

 perfectly natural conditions this ratio does not change much from 

 year to year, and there is no preference on the part of nature for 

 one class as against any other. Any interference with this estab- 

 lished course is almost certain to be disadvantageous in some 

 directions, and man, in his dealings with natural conditions, has 

 persistently created for himself a series of troubles arising from 

 his own acts. By planting large areas of one crop, he has fa- 

 vored the increase of the insects feeding upon that crop. By cul- 

 tivating the land, keeping it cleared of rubbish, stones, sticks, 

 and the like, he has destroyed the shelter needed by predaceous 

 insects, and the result is that he has in two ways given those in- 

 sects, which he now calls injurious, an advantage. The conse- 

 quences, of course, are against him ; and so long as these condi- 

 tions continue, favoring one class at the expense of another, man 

 must suffer, unless he himself fulfils the functions previously as- 

 signed to the predaceous forms. True parasites have not been 

 so much influenced by his actions, and the species, perhaps, 

 suffer as much from them as they ever did ; but the natural 

 checks in operation against the parasites also continue, and 

 while, perhaps, they have increased somewhat in number, they 

 have not been able to make up for the loss of the predaceous in- 

 sects. Furthermore, the feeders upon vegetation suffered also 

 from the attacks of insectivorous animals, like toads, frogs, and 

 snakes among the reptiles, and many small animals and birds, 

 which now also find life insupportable among the artificial condi- 

 tions. The result is that there has been an actual increase in the 

 number of specimens of injurious insects, and their parasites bear 

 somewhat less than their original ratio to them, — that is, each 

 year a certain proportion or percentage of a brood is destroyed 

 by its parasites, and this percentage it is beyond our present 

 power to change to our advantage. In other words, the farmer 

 cannot, except in isolated instances, count upon parasites or 

 natural enemies of any kind to keep down injurious insects. 



