INSECTICIDES, rREVENTIlES, AM) MACHINERY. 421 



her effort is always to give all her creatures an equal chance to 

 exist. 



Man has further disturbed the natural conditions Ijy introducing 

 into new countries insects that have become adapted to the sur- 

 roundings in others. Sometimes an insect so introduced does 

 not find the new circumstances to its liking ; but, on the other 

 hand, it may find them very much better than those under which 

 it was originally developed. In such cases we get a multiplica- 

 tion out of all proportion to the normal habit of the species, and 

 there may be such a thing as the destruction of the plants that 

 they feed upon. Let us take, for instance, the elm-leaf beetle, 

 introduced into this country many years ago, and apparently 

 without natural enemies suitable for its control. The circum- 

 stances which it finds here are so much to its liking that, if 

 allowed to increase unchecked by the efforts of man, it may in 

 the course of time destroy all the European elms in the regions 

 infested by it ! 



The whole object is to show that, since man has introduced 

 artificial conditions, he must bear the burden of the change 

 caused by them ; and if this means that there are certain injurious 

 insects feeding upon the plants he wishes to grow for his own 

 benefit, he must replace by his own efforts those natural checks 

 which he has removed by the artificial conditions introduced. 

 Those efforts on his part consist in the adoption of measures to 

 prevent increase, or to destroy the injurious insects by means 

 of poisons or otherwise, and this subject will be considered in 

 another chapter. 



A fascinating theory in connection with this subject is the pos- 

 sibility of imj^orting parasites or predaceous forms from other 

 countries for the destruction of pests here. This leaves out of 

 consideration the fact that it would require a readjustment of 

 matters to induce foreign parasites to feed upon American insects, 

 and it has never been proved that such a thing is possible within 

 a reasonable period of time. The case of the Australian Vedalia, 

 imported to prey upon the fluted scale, Icerya purchasi, is fre- 

 quently cited as an example of what can be done, but always 

 without considering the fact that we had an imported insect to 

 deal with in the first place, which increased abnormally in our 

 country because of the fact that its enemies were not brought 



