_Pieris rapae_ (Linn.)
In the New England States there are three broods of this insect in a
year, according to Mr. Scudder, the butterflies being on the wing in
May, July, and September; but as the time of the emergence varies, we
see them on the wing continuously through the season.
[Illustration: FIG. 12.]
The expanded wings, Fig. 12, male, measure about two inches, are white
above, with the base dusky. Both sexes have the apex black and a black
spot a little beyond the middle, and the female, Fig. 13, has another
spot below this. The under side of the fore wings is white, yellowish
toward the apex, and with two black spots in both sexes corresponding
to those on the upper side of the female. A little beyond the middle
of the costa, on the hind wings, is an irregular black spot on the
upper surface, while the under surface is pale lemon yellow without
marks, but sprinkled more or less with dark atoms. The body is black
above and white beneath.
[Illustration: FIG. 13.]
The caterpillars of this insect feed on the leaves of cabbage,
cauliflower, turnip, mignonette, and some other plants.
The female lays her eggs on the under side of the leaves of the food
plants, generally, but sometimes on the upper sides or even on the
leaf stalks. They are sugar loaf shaped, flattened at the base, and
with the apex cut off square at the top, pale lemon yellow in color,
about one twenty-fifth of an inch long and one fourth as wide, and
have twelve longitudinal ribs with fine cross lines between them.
The eggs hatch in about a week, and the young caterpillars, which are
very pale yellow, first eat the shells from which they have escaped,
and then spin a carpet of silk, upon which they remain except when
feeding. They now eat small round holes through the leaves, but as
they grow older change to a greenish color, with a pale yellow line
along the back, and a row of small yellow spots along the sides, and
eat their way down into the head of the cabbage.
[Illustration: FIG. 14.]
Having reached its full growth, the caterpillar, Fig. 14, a, which is
about an inch in length, wanders off to some sheltered place, as under
a board, fence rail, or even under the edge of clapboards on the side
of a building, where it spins a button of silk, in which to secure its
hind legs, then the loop of silk to support the forward part of the
body.
It now casts its skin, changing to a chrysalis, Fig. 14, b, about
three-fourths of an inch in length, quite rough and uneven, with
projecting ridges and angular points on the back, and the head is
prolonged into a tapering horn. In color they are very variable, some
are pale green, others are flesh colored or pale ashy gray, and
sprinkled with numerous black dots. The winter is passed in the
chrysalis stage.
After the caterpillar changes to a chrysalis, their minute parasites
frequently bore through the outside and deposit their eggs within.
These hatch before the time for the butterflies to emerge, and feeding
on the contents, destroy the life of the chrysalis.
Birds and spiders are of great service in destroying these insects.
The pupæ should be collected and burned if the abdomen is flexible;
but if the joints of the abdomen are stiff and cannot be easily moved,
they should be left, as they contain parasites.
Several applications of poisons have been used, the best results being
obtained from the use of pyrethrum as a powder blown on to the plants
by a hand bellows, during the hottest part of the day, in the
proportion of one part to four or five of flour.
As the eggs are laid at different times, any application, to be
thoroughly tested, must be repeated several times.