Three white butterflies of approximately the same size are found widely distributed over the United States. The most abundant species is the White or Imported Cabbage butterfly. The next in abundance is probably the Checkered White, and the rarest in most localities is the Gray-veined White which is a northern form.
The White or Imported Cabbage Butterfly
Pieris rapae
There is probably no butterfly which one can generally find so easily in its early stages as the White or Imported Cabbage butterfly which is found practically wherever cabbages are grown and is generally so abundant that caterpillars and chrysalids are readily discovered. In the Northern states the insect passes through the winter within the chrysalis, coming forth rather early in spring as the familiar white butterfly with black dots upon the wings and blackish front angles of the fore wings. (See plates, pages [64-65] and [81].)
The butterflies that thus appear in spring flit freely about over fields, meadows, and gardens, sipping the nectar of various early flowers through their long, coiled tongues and stopping occasionally to alight upon the leaf of a cabbage or other plant of the mustard family to deposit the small, pale yellow eggs which remain attached by a sort of glue. The adult butterflies continue their leisurely life for a fortnight or more, thus extending the laying of the eggs over a considerable period.
About a week after being deposited the egg hatches into a tiny green caterpillar that begins feeding upon the tender surface of the cabbage leaf. It is commonly called the cabbage worm and it is doubtless the most generally destructive insect affecting this crop. It continues to feed for several days before the first moult, after which it becomes decidedly larger and begins to eat again more voraciously than before. It undergoes several successive moults during the next two or three weeks before it becomes full grown as a caterpillar. Unlike most butterfly larvae it has changed very little in its general appearance during its growth. It is always of a pale green color, strikingly like the glaucous green of the cabbage leaf, a fact which doubtless helps to conceal it from the eager eyes of birds and other animals.
When the caterpillar is thus full fed it is likely to leave its food plant and find shelter elsewhere. Sometimes it will stop on the lower surface of the outer leaves, but more commonly it will find a piece of board, an overhanging stone, a fence-post, or the side of a building, where it will prepare for the change to the chrysalis. It will do this by spinning a silken thread upon the surface in which to entangle its hind legs and a loop of silk near by with which to hold its body. When these preparations are completed the insect will cast its last caterpillar skin, emerging as a grayish or brownish chrysalis, the color usually varying with the color of the surrounding surface.
A week or more later the chrysalis skin bursts open and 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 America 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 introduction it had become a serious pest over practically all the United States and Canada.
The introduction and spread of such a pest is of interest in itself, but in this case there is to be noted the additional fact that the presence of this foreigner has practically led to the extinction of two native species of butterflies, both closely related to each other and to the invader and both feeding upon the same plants. An almost pure white butterfly—the Gray-veined White—was formerly exceedingly abundant in many of the Northern states, while farther south there was another species, the Checkered White, which was also abundant. Both of these have now so completely disappeared that in some localities they are almost never seen, while their imported relative has become perhaps the most abundant of all American butterflies.