STRUCTURE, GROWTH AND ECONOMICS OF INSECTS 63 



As a rule, however, insects diffuse most readily in the zone in which 

 they originated, and in the direction of least resistance. The Transi- 

 tion and Austral zones are differentiated into an eastern humid, a 

 western arid and a Pacific humid division and certain insects occur in 

 one division and not in the others. 



As examples of the economic importance of a study of the factors 

 of distribution of insects the cases of the imported elm-leaf beetle 

 and San Jose scale may be given. Fernald says: 



"The Elm-leaf Beetle . . . thrives in the Upper Austral Zone, but is 

 noticeably absent in the highlands of the Pennsylvania mountain regions, 

 though it is present again west of them. To the north it has caused serious 

 loss to the elms of New England, resulting in the appropriation of large 

 sums for spraying of the trees to protect them from its ravages. Careful 

 studies of this pest in Massachusetts show that while a serious menace to the 

 life of the elms in the southern part of the state and in the river valleys, it 

 becomes of little importance in the higher and northern parts, and many 

 towns which formerly appropriated money for the protection of their elms 

 from this insect have now learned that this was unnecessary, as the trees 

 would suffer but Uttle at most, in any case. 



"With the San Jose Scale similar facts are now coming to light. This 

 pest finds the best conditions for its hfe in the Lower and Upper Austral 

 Zones, where it has caused the loss of many millions of dollars. Even in the 

 Upper Austral territory of Massachusetts, it is one of the most destructive 

 enemies of the fruit-grower. As we pass into the Transition Zone, however, 

 its ravages become less severe, and by the time the centre of this zone has 

 been reached it is of only medium importance. In this case it has seemed to 

 those studying this problem that this insect was originally limited by the 

 Upper Austral, but has gradually acquired some degree of resistance to 

 lower temperatures and has thus been able to extend into the Transition 

 Zone." 



As Webster has shown, the migration and diffusion of insects have 

 occurred along four main lines. The first was from the east by way of 

 New York or one of the North Atlantic states and the open pathway 

 into the interior past lakes Ontario and Erie. Examples of such 

 migration are the imported cabbage butterfly, the two species of 

 asparagus beetles, the clover-leaf weevil, the clover-root borer, the 

 Hessian fly, the horn fly and the willow curculio. The second line was 

 northward from the West Indies by way of Florida up the Atlantic 

 coast, bringing in many beetles, scale insects, the harlequin cabbage 

 bug and other Hemiptera. The third Hne was northward from Central 



