42 DISTRIBUTION AND DISPERSION OF LEPTINOTARSA. 



southern Ontario, on the northern shore of the lakes, covering great stretches 

 of the country each year (plate 8). In this rapid and direct advance the 

 advantages of the natural highway were not entirely responsible for the 

 accelerated rate of movement. The strong southwesterly winds which sweep 

 down this basin aided very materially in this dissemination, especially in 

 the early years. On the southern side of the lakes the St. Lawrence Valle}- 

 advance was less rapid, the difference in the rate of movement being due to 

 the fact that the southern side is broad, and there was a large area to cover, 

 whereas upon the northern side the habitable area was narrow, and, as a 

 result, there was greater concentration of the animals along the front, and 

 therefore a greater chance for marked advance each year. 



The southern column, working eastward and southward through Indiana, 

 Ohio, and adjacent States, soon encountered the Appalachian highlands. 

 There resulted a turning of the column northeastward, with a concentration 

 in the region of the lakes, and the formation of several independent columns 

 which moved eastward along natural highways across the Appalachian 

 Mountains. That portion of the southern column which had concentrated 

 near the lakes, moving rapidly eastward along their southern shores and 

 down the St. Lawrence Valley, finally turned south along the Lake Cham- 

 plain and Hudson River valleys, penetrated the Appalachian barrier in the 

 north, and gained entrance to eastern New York and New England. The 

 most prominent of the minor columns which were crossing this barrier moved 

 along the valleys of the Mohawk, Delaware, Susquehanna, and Potomac 

 rivers, while still smaller lines of advance were developed in the valleys of 

 the Cheat, Kanawha, Licking, Kentucky, Cumberland, and Tennessee rivers. 

 To the north the barrier was penetrated, first by the Delaware, Susquehanna, 

 and Potomac columns, and later by the Mohawk (plate 9). 



The establishment of the colony at Lancaster, Pennsylvania, in 187 1 or 

 1872, and the arrival of the beetles from the west over the Delaware, Susque- 

 hanna, and Potomac highways produced a strong center of dispersal from 

 which movement along the Atlantic coast began. The northward dissemi- 

 nation along the Atlantic slope during the years 1872 to 1876 was rapid, it 

 being accelerated by climatic factors, principally winds, which retarded the 

 movement to the south. There is evidence also of a strong local control of 

 the direction and rapidity of migration by small features of the topography, 

 as, for example, the movement up the Connecticut River Valley and in the 

 river valleys in Maine. 



In the south the available habitat of the beetle was limited, and the culti- 

 vation of its food plant by no means as general as to the north and west ; 

 consequently, advance was generally quite slow. Over the Piedmont belt 

 the retardation of the southward movement between the years 1872 and 

 1900 was less marked, and here the beetle became fairly common and gener- 



