44 DISTRIBUTION AND DISPERSION OF LF.PTINOTARSA. 



natural gateways, and has followed in its advance many of the highways 

 that have been mentioned in the preceding pages. 



In following the history of the spread of L. decemlineata it is evident that 

 when a natural highway is encountered the beetles respond at once to the 

 change and the control of their direction of movement. They either go or 

 do not go along a certain path, and the choice is made at once and not after 

 generations of indecision and futile trials. It is clear that in this dispersion 

 the routes followed were natural ones, and were almost entirely independent 

 of man, notwithstanding the frequent statements to the contrary. Human 

 advance nearly always follows natural highways, but this in nowise alters 

 or invalidates the conclusions regarding the part played by these topographic 

 features in the dissemination of L. decemlineata. 



Other Factors Which Have Controlled the Direction of Movement. 



We shall now consider some of the habits of L. decemlineata and the various 

 climatic factors of its environment which have been of importance in directing 

 the lines of its most rapid advance and aiding in its general dispersal. 



The life cycle and habits of this beetle are such as to aid greatly in any 

 migration in which it may be engaged. All through its range it is double- 

 brooded in the summer, the second brood passing the winter as an imago. 

 In Minnesota, Lugger (1895) records three broods in one year, which I believe 

 to be incorrect. All through its habitat the beetles appear on the first warm 

 days of spring, flying about or crawling along the sunny sides of fences. 

 They are thus active for some time before their food plants are up and egg- 

 laying can begin, and during this period they undoubtedly do a considerable 

 amount of traveling. During the eastward migration this period between 

 the emergence from the ground and the time for egg-laying to begin was 

 occupied in extending its bounds. When the number of beetles which hiber- 

 nated was large, so that the mortality resulting from the vicissitudes of the 

 winter would leave a goodly number to emerge alive in the following spring, 

 a considerable and important advance would result. The best example of 

 this occurred in the spring of 1871. The beetle had been numerous and 

 destructive in Michigan and Ohio in 1870, when there is recorded a rapid 

 spread northeast into the Province of Ontario. There was a similar condi- 

 tion in 1872 and 1873 in the Lancaster County, Pennsylvania, colony, when 

 there followed a rapid spread down the shores of Chesapeake Bay. 



The spring dissemination, however, is not of as great importance as that 

 which comes later in the season. As soon as the food plants appear above 

 the ground the beetles cease their wandering habits, and late in May begin 

 to lay their eggs. The first brood reaches maturity in about 35 days, or 

 about the first of July, and after feeding for a few days they pair and deposit 

 the eggs for the second brood, which reaches maturity about the middle of 

 August. The last brood does not pair until the following spring, although 



