DISP^RSAI, O^ h. DEC^MIvIN^ATA. 2$ 



area it began to spread — at first slowly and in small numbers and unnoticed; 

 then in increasing numbers and more rapidly, making itself felt as an 

 economic factor in agriculture. Onward it advanced yearly, in increasing 

 numbers and speed, until in twenty years from the time of its start it had 

 reached the Atlantic Ocean, a barrier beyond which it has not been able to 

 successfully pass. Fortunately the history of this advance is known from 

 extensive records, so that it can be traced in detail from year to year. 



CHRONOLOGICAL HISTORY OF THE DISPERSAL OF LEPTINOTARSA DECEMLINEATA, 



t859 TO t904. 



Change or extension of habitat is apparently a common phenomenon. I 

 say apparently common, because we suppose that this has happened and is 

 going on continuously in the life of all species of animals ; but those that we 

 can trace and study from year to year are few. It is true that cases of 

 which we have records enabling us to trace the dispersal may not add exten- 

 sively to our ideas or speculations as to what the results of dispersal and 

 change of habitat have been ; they are, however, of prime importance in 

 judging the results derived from less certain cases and in keeping speculation 

 within bounds. Hence is presented in this place the history of the dissemi- 

 nation of decemlineata as it has been worked out from records and data of 

 diverse kinds. 



It has already been shown from whence this beetle came and how it 

 probably attained its center of dispersal along the eastern slope of the Rocky 

 Mountains, where, for a time at least, it was in a state of stabilit3^ It has 

 also been shown how a very slight change in the flora of the habitat was 

 sufficient to upset the stability of the species and to start it upon a prolonged 

 and extensive dispersal. I shall now present the history of this dispersal 

 year by year and then consider it for information concerning the effects of 

 climate, topography, and other factors which have influenced or controlled 

 this dissemination. 



iS^g. — The first recorded appearance of this beetle east of its original 

 habitat was for the year 1859 by Hazen (1865). According to this author 

 "the potato-bug (Z. decemlineata) was first discovered in 1859 about 100 

 miles west of Omaha City, Nebraska, whence they have been marching east- 

 ward annually." No other record of this beetle for this year is known to 

 me. I do not know on what evidence Hazen made his statement, but Walsh 

 and Riley (1868) accepted it as correct. The delaying of the publication 

 of this record for six years detracts somewhat from its value. However, it 

 is of interest to note that this first discovery was in the line of the old 

 emigrant trail from Omaha to Denver, Colorado. 



1 86 1. — The first authentic records are found in the annals of this year. 

 At Atchison, Kansas, it was numerous and destructive. Colman (1862), 



