_Plutella cruciferarum_ (Zell.)
The cabbage leaf miner is not a native of this country, but was
imported from Europe.
[Illustration: FIG. 25.]
The perfect moth, Fig. 25, f, with the wings expanded (h, with the
wings closed, g, a dark variety), measures three-quarters of an inch.
The fore wings are ashy gray, and on the hinder margin is a white or
yellowish white stripe having three points extending into the gray,
thus forming, when the wings are closed, three diamond-shaped white
spots. Generally there is a dark brown stripe between the white and
the gray. There are also black dots scattered about on the anterior
part of these wings.
The hind wings are leaden brown, and the under side of all the wings
is leaden brown, glossy, and without any dots.
The antennæ are whitish with dark rings, and the abdomen white. There
are two broods of this insect in this region, the moths of the first
appearing in May, and those of the second in August. They hibernate in
the pupa stage.
The caterpillars, Fig. 25, a (b, the top and c, the side of a
segment), appear in June or July and September; they are small and
cylindrical, tapering at both ends, pale green, and about one-fourth
of an inch long. The head has a yellowish tinge, and there are several
dark stiff hairs scattered over the body.
When ready to transform, this caterpillar spins a delicate gauze-like
cocoon, Fig. 25, e, made of white, silken threads, on the under side
of a cabbage leaf. The pupa, Fig. 25, d, and i, the end of a pupa, is
commonly white, sometimes shaded with reddish brown, and can be
distinctly seen through the silken case.
The first brood is more injurious than the second, as it feeds on the
young cabbage leaves before the head is formed, and this must surely
stunt the growth and make weak, sickly plants; while the second brood
feeds only on the outside leaves. The caterpillars are very active,
wriggling violently when disturbed, and falling by a white silken
thread.
Hot dry weather is favorable to them and enables them to multiply
rapidly. Advantage has been taken of this fact, and spraying the
plants thoroughly with water is strongly recommended. Prof. Riley
states that the insects are very readily destroyed by pyrethrum. There
are two species of spiders and a species of ichneumon fly that destroy
them.