64 ECONOMIC ENTOMOLOGY 



and South America by way of Mexico, passing up the Mississippi 

 valley or up the western valleys. Examples of such migration are the 

 Diabroticas, the cotton boll weevil, the Argentine ant, the potato 

 beetle, Halisidota, harlequin cabbage bug and the chinch bug. The 

 fourth was southward from Asia by way of Alaska. Examples of such 

 a diffusion are certain lady-birds, Lina spp. and Silpha spp. 



Regarding the fauna of British Columbia E. M. Walker believes it 

 represents a mingling of Asiatic, Californian and Mexican types. 



With regard to insects that have come in from foreign countries 

 it is generally true that they confine themselves to zones similar to those 

 from which they have come. Of these mention may be made of Crio- 

 ceris asparagi, Pieris rapce, Hylastinus obscurus, the warble" and bot 

 flies, the San Jose scale, and the gypsy and brown-tail moths. Some 

 forms, such as the codling moth and other insects, tend to become cos- 

 mopolitan, but they seldom become epidemic outside of their special 

 zones. 



Similarity of climatic conditions in corresponding zones of two 

 continents renders it easy for species introduced from one to the other 

 to gain a footing, as the absence of native parasites affords an oppor- 

 tunity for excessive multiplication. 



The influence of elevation on the distribution of insects is seen in 

 the southward extension of the zones in hilly or mountainous regions. 

 In an understanding of the distribution of many northern forms often 

 isolated on summits south of their normal zone geological history comes 

 to our aid. The great Tertiary extension of land areas in the north- 

 east and the northwest, making land connections with Europe and Asia, 

 permitted northern forms from these continents to pass to America, 

 and the advance of the Great Glacier southward during the Glacial 

 Period forced many northern forms southward, some of whom were 

 left stranded on mountain summits on the retreat of the ice. 



Methods of Studying Economic Insects 



It has been already pointed out that one of the characteristics of 

 modern-day investigations in economic entomology is the introduction 

 of the "field station" method, whereby each pest is studied under 

 natural conditions both in the open field and in the laboratory. By 

 this method it is possible to study all or most of the factors that play 

 a part in the life of both host and pest. No phase is too trivial for 



