18 INSECT PESTS OF FARM, GARDEN AND ORCHARD 



common as formerly, and little reflection will show that these 

 scourges are entirely due to natural causes. In fact they are very 

 largely brought about by man himself. Some of these pests are due 

 to the fact that in trying to subdue nature by clearing and cultivat- 

 ing the land, man has deprived the insects of their natural food 

 plants. They must, therefore, needs feed upon that which is sub- 

 stituted by him, and as it is less abundant than the former wild 

 vegetation, the number of insects and the injury they inflict are 

 more apparent. 



By far the larger number of our worst pests, however, are those 

 which come to us from foreign shores. Foreign insects are con- 

 stantly being imported in one way or another, sometimes being 

 already established pests in other lands and sometimes only be- 

 coming so under their new surroundings. These are even more 

 injurious than those native, for whereas many of our native birds, 

 insects, and diseases constantly prey upon native insects and thus 

 keep their numbers in check, the enemies of imported pests rarely 

 accompany them, and they thus increase at an alarming rate and 

 do enormous damage before they are attacked by the natural 

 enemies of similar native pests. It is in the case of these imported 

 pests that the value of parasitic and predaceous insects is most 

 apparent. In an effort to make use of them to fight the gypsy 

 and brown-tail moths in New England, the U. S. Bureau of Ento- 

 mology has for several years been importing large numbers of the 

 parasites and predaceous enemies of these pests and liberating 

 them in affected regions, thus carrying on a practical experiment 

 on a large scale which may show the importance of these parasites 

 in combating imported pests. 



Even with our native pests, however, we have frequent exam- 

 ples of the value of parasitic and predaceous enemies. Thus the 

 southern grain louse, or " green bug," was soon brought under 

 control by the myriads of little parasites which preyed upon it 

 (see page 140), and these were artificially transported for some 

 distance and liberated in large numbers. Though these efforts at 

 the distribution of this parasite may be open to some question 

 as to their effectiveness, other parasites have been successfully 



