﻿38 NINTH ANNUAL REPORT 



swarms from one part of a country to another; and the migrating 

 tendency has at times been quite marked in our Doryphora during its 

 eastward march. This tendency is particularly noticeable in the last 

 or Fall brood, and I have seen the beetles in autumn, swarming in the 

 air or traveling in immense armies on foot — all instinctively taking 

 the same direction, which is indeed a peculiarity of all animal migra- 

 tions. There can be little doubt, therefore, that the larger areas 

 have been traversed by this insect in the latter part of the growing 

 season. 



AREA INVADED BY IT. 



From the foregoing account it is manifest that this pernicious 

 beetle has spread over an area of nearly 1,500,000 square miles, or 

 considerably more than one-third the area of the United States. It 

 hafj traveled over two-thirds of the continent in a direct eastern line, 

 and at least 1,500 miles of this distance since 1859. It occupies at the 

 present time, more or less completely, the States of Colorado, Ne- 

 braska, Kansas, Minnesota, Iowa, Missouri, Wisconsin, Illinois, Michi- 

 gan, Indiana, Kentucky, Ohio, New York, Pennsylvania, District of 

 Columbia, Virginia and West Virginia, Maryland, Delaware, New 

 Jersey, Connecticut, Rhode Island, Massachusetts, Vermont, New 

 Hampshire and Maine, in none of which it was autochthonous, except 

 the first mentioned. If we wish to outline the whole territory now 

 occupied by it, we must add to the above, parts of Wyoming and 

 Dakota, where it was native, a large portion of Canada and limited 

 portions of N. Carolina, Tennessee, Arkansas, Indian Territory, Texas 

 and New Mexico. The map given on page 36 (Fig. 12) tells the story 

 better than any words I can employ. 



CAUSES WHICH LIMIT ITS SPREAD. 



There are reasons why the Colorado Potato-beetle did not spread 

 as rapidly along the line of its southern as along that of its northern 

 march. The first is, that the potato is not in such general cultivation 

 along the latter as along the former parallel, and potato fields arc 

 therefore, more scattered; the second, that the insect was northern 

 rather than southern in its native habitat ; the third, that it suffers 

 and does not thrive where the thermometer ranges near 100° F. The 

 larvae frequently perish under such a broiling sun as we sometimes 

 have at St. Louis, and during very hot, dry weather, it frequently fails, 

 as it did in 1868, to successfully go through its transtbrmations in the 

 ground, which becomes so hot and baked that the pupa dries out, and 

 the beetle, if it succeeds in throwing off the pupal skin, fails to make 

 its way to the surface. For these reasons it may never extend its 

 range very far south of the territory now occupied. Its northern 

 spread is not limited by any such cause, and the intensity or length 



