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less resistant through removal from their proper homes. Without the 

 assistance of commerce the pests of one region would always be lack- 

 ing-, to a greater or less extent, in another; commerce, however, dis- 

 tributes them freely. In supplies of grain or other food it has carried 

 everywhere the insects which injure stored grain, until nearly all of 

 these insects have become practically cosmopolitan. In the more 

 recently developed commerce in nursery stock, and the still more 

 recently developed long-distance trade in fruit, brought about by rapid 

 transit and cold storage, we have carried, and are carrying, potential 

 destruction in almost every carload or ship cargo. To such an extent 

 has this distribution of injurious insects been brought about, that it is 

 difficult in many cases to ascertain the original home of many species. 



Horticulture is perhaps the greatest sufferer from this commercial 

 distribution of insect enemies of plants, and the injurious insects most 

 readily distributed are the scale insects, since these creatures remain 

 attached to the plant throughout their entire life-round. In the 

 United States we have, in round numbers, more than one hundred 

 species of scale insects, and of these, probably forty have been intro- 

 duced from other countries. These forty, moreover, include nearly all 

 the species of great economic importance. It seems to be a rule that 

 introduced species in this country become far more injurious than in 

 their native home, and far more injurious than the species which 

 already exist here. It is unnecessary to discuss at length any of the 

 reasons for this state of affairs, and in fact, they are not well under- 

 stood. In many cases it is due, ])artly at least, to the fact that the 

 imported species did not bring their parasites with them, while in others 

 we can only attribute it to the fact that through long association our 

 native ciops have become more or less immune to the attacks of native 

 species, but are less resistant to new enemies. 



A few familiar instances may be mentioned. The oyster-shell bark- 

 louse of the apple is a European species introduced into this country 

 toward the close of the last century. It speedily became more destruc- 

 tive than the native scurfy bark-louse, and during the first half century 

 of its existence upon American soil was the principal insect enemy of 

 the apple crop; of late it has become less important. The red scale of 

 the orange in Florida is an introduction from the West Indies or South 

 America; the red scale of the orange in California is an introduction 

 from the Pacific islands. The fluted scale or cottony cushion scale 

 of the Pacific Coast was originally an importation from Australia. The 

 common flat scale and the hemisf)herical scale of our northern green- 

 houses and our southern orchards are European species. The San 

 Jose, or pernicious scale, which for twenty years has been seriously 

 damaging the orchards of the far west, and which, during the last few 

 years, has made a most destructive onslaught on many of our eastern 

 orchards, is also probably an Australian species. 



But the scales are by no means the only insects injurious to horti- 



