46 INSECTS AND HUMAN WELFARE 



The situation is still further aggravated by the constantly 

 increasing number of foreign insects which find their way to 

 our shores and become established to spread gradually 

 throughout those parts of our agricultural regions that enjoy 

 a suitable climate. Such insects, of which a very consider- 

 able number could be enumerated, are our most destructive 

 pests. They are, of all species of animals, the most fortu- 

 nate. They are suddenly dropped in the midst of plenty; 

 they are unhampered by the enemies and parasites of their 

 native land. That they im^nediately make the best of their 

 opportunities and proceed to exploit the country is seen by 

 the phenomenal injury done by the Mexican cotton-boll 

 weevil or Picudo (Fig. 19) to the cotton crop, the gipsy moth 

 or Gro-sse Schwcunmsphiner to deciduous trees, the Hessian 

 fly to the wheat crop, the cabbage butterfly (Figs. 20, 21) 

 to various cruciferous crops, and many other naturalized 

 insects to various other cultivated plants. 



The explanation for the abnormally rapid multiplication of 

 imported insects is not far to seek, and depends upon prin- 

 ciples well recognized by all biologists. As was previously 

 pointed out, approximately one-sixth of our insects are ento- 

 mophagous parasites (Fig. 22), passing their developmental 

 stages within the bodies of other living insects. Some of these 

 parasites prey upon other parasites, but a goodly pro])ortion 

 affect phytophagous insects which they destroy at the time 

 they complete their growth. Like the plant-eating forms 

 they are restricted to definite hosts, generally few in number, 

 with which they show a very definite association. In fact, so 

 fixed are their instincts to restrict themselves to the same 

 host that they ordinarily avoid any insects which they do not 

 normally parasitize. With this in view, if we consider for a 

 moment the conditions surrounding an insect pest introduced 

 from another part of the world, we see that it is no longer at 

 the mercy of the entomophagous parasites that would affect 

 it in its native land, and that the parasites with which it is 

 now associated will leave it unmolested. Since many of our 

 common native insects have a dozen or more well-known 



