THE TRIBE OF THE WHITES 85 



the white butterfly emerges to expand and dry its wings 

 before it flies away for its leisurely life. There are two 

 or more broods each season, the number varying with the 

 latitude. There is a decided variation in the length of 

 time required for the completion of the cycle from egg to 

 butterfly. In hot weather the insect may mature in 

 about three weeks while in cooler weather it may require 

 as much as five weeks. 



Its Introduction and Dispersal 



While it is well known that a large proportion of our 

 most destructive insects have been imported from Europe, 

 it is only in comparatively few cases that man has been 

 able to make careful records of the times and places where 

 the insects were introduced and to follow the spread of the 

 pest from these original centres. The Imported Cabbage 

 butterfly is one of the few species of which this is true. 

 This insect has been known for centuries in Europe, where 

 it feeds freely upon the leaves of cabbages and turnips. 

 So far as known it was first introduced into North Amer- 

 ica about 1860, when it appeared in Quebec. Eight years 

 later it was again introduced into the region of New York 

 City. From these two points the insect spread gradually 

 in various directions until in 1871 it covered the whole of 

 New England and various parts of New York and New 

 Jersey. From then on it spread even more rapidly and 

 was evidently accidentally introduced into various parts 

 of the country which became new centres of distribution. 

 Of course it would be very easy for this to happen through 

 the shipment of cabbages from one part of the country to 

 another. Within thirty years of the time of its first in- 



