20 INSECTS AFFECTING VEGETATION 



But why is it at all necessary to classify insects for any practical 

 purpose? In reply it must be stated that we can not fight against 

 injurious insects with any hope of success if we do not know their 

 structure. For instance, an insect that has no mouth to bite or chew 

 can not be poisoned, and the application of any arsenical insecticides 

 would in most cases be perfectly useless. Nor is it enough to know 

 the structure of the insects; we must also know their habits and trans- 

 formations, because this knowledge alone will enable us to apply the 

 remedies at the proper time. In fact, notwithstanding the great 

 progress made in economic entomology during the last ten years, we 

 are only able to combat successfully a limited number of injurious 

 insects by means of insecticides. A large number of others, and the 

 most injurious ones at that, can not be reached in that manner. 

 Time, labor and material to do so successfully would cost much more 

 than the whole crop would be worth. The chinch-bug, locusts, and 

 others, if very abundant, can not be fought successfully by means of 

 insecticides. Yet this is no reason why we should not be able to re- 

 duce their ravages to a minimum. But without being perfectly fa- 

 miliar with the habits of these insects, with their life-history in all 

 and every detail, including their insect and plant foes, we can not 

 hope to succeed. But by knowing all this we may be able to dis- 

 cover a weak spot into which a wedge can be driven to break up their 

 ranks. Only by attacking the weak spot of a well fortified castle is 

 victory possible. 



When we consider the immense numbers of insects, and the fact 

 that they devour every and all kinds of organized matter, it seems 

 almost vain even to try to fight against them. All insects are not, 

 however, enemies to man; on the contrary, the great majority are 

 either indifferent to him, or are either directly or indirectly benefi- 

 cial. The indifferent ones eat substances we can not or do not use ; 

 the beneficial ones eat noxious plants, decaying substances, or live in 

 the bodies of other insects, thus purifying the air and making space 

 for other living organisms. Without them the soil would be covered 

 with dead vegetable matter, the now existing plants, i. e., those that 

 are fertilized by the wind, would become smaller and smaller, because 

 their seeds, not eaten by insects, would all have an opportunity to 

 grow, thus crowding, dwarfing and killing each other. Without in- 

 sects the great majority of our brightly colored flowers would not pro- 

 duce seeds, as most of them are dependent upon the work of these 

 animals to cause cross-fertilization. 



The question is frequently asked : Why is it that farmers, horti- 

 culturists, gardeners, etc., are more troubled in the United States 

 with noxious insects than they are elsewhere? or Why is it that more 

 injurious insects and of different kinds are found now than for- 

 merly? The reasons for this increase of insects, both in numbers and 

 kinds, are not so very difficult to give. When settlements were few 

 and widely scattered, the whole country was covered with its virginal 

 vegetation. Plants and animals were adapted to each other, and as 

 soon as one of them became for any reasons exceedingly numerous, 

 natural checks in the form of enemies to such plants or animals soon 



