818 60.

j

ENT ed States Department of Agriculture.

BUREAU OF ENTOMOLOGY.

L. O. HOWARD, Entomologist.

THE IMPORTED CABBAGE WORM.
(Pontia rapex Jinn.)

By F. H. Cuirrenpen,

In Charge of Breeding Experiments.
GENERAL CHARACTERISTICS.

The worst of the many destructive enemies of cabbage and other cru-
cifers is the larva or caterpillar of a white butterfly known to science as
Pontia (Preris) rape Linn. The caterpillar, called the imported cab-
bage worm, is altogether
too well known by farm-
ers throughout this coun-
try as well as in the Old
World, and the adult in-
sect, the imported cab-
bage butterfly, called also
the white or rape butter-
fly, is a familiar object to
nearly everyone. It is
generally recognized that
the butterfly is the par-
ent of the ‘““worms.”’

This cabbage worm is
velvety green in color,
much like the cabbage
on which it feeds; and
the surface of the body,
if viewed through an or-

: ; - Fic. 1.—Pontiarapx: a, female butterfly; 6, above, egg as
cinairy Gandtlens;is’'seenm 6.3 som above: below. sae nateeah feonieaidge | lasers
to be somewhat rough natural position on cabbage leaf; d, suspended chrysalis—
paemeclmmunteda with: 2 eee 7 more Sulezeed CoxiginaD):
small black spots. There is a faint yellow stripe down the middle of
the back and a row of yellow spots along each side in line with the
spiracles or breathing pores. It measures, when full grown, about an
inch and a fourth, presenting the appearance shown in figure 1, ¢c.

The butterfly has a wing expanse of nearly two inches, and is white,
marked with black near the tips of the fore wings, as shown at figure
1, a, representing the female. In this sex there are two conspicuous
black spots on each fore-wing; the male (fig.2) has only one. Both

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sexes have.a corresponding smaller black spot at the upper extremity
of the hind wings. The body of the female is whitish; that of the male
is usually darker above. The male is generally a little the smaller.
The under surface of the hind wings is uniform satiny straw-yellow
and there are generally two black spots showing through, in both sexes.

The eggs are fusiform, pale yellowish, and strongly ribbed (fig. 1, 5).
They are large enough to be readily seen with the unaided eye, and are
deposited singly, usually on the lower surface of the outer leaves of
cabbage and other food plants.

When the caterpillar or ‘““worm’’ becomes full grown it attaches
itself to a cabbage leaf or other nearby object, fastens itself by means
of a string-like girdle, and often within the same day transforms to a
chrysalis of the appearance indicated at figure 1,d. The chrysalis

Fic. 2.—Imported cabbage butterfly: male, wings expanded, at left; wings folded in natural resting
position at right—somewhat enlarged (original).

is of variable color, seeming to be influenced in this respect by the

object upon which it is placed. The color thus varies from dirty

gray to yellow, green, and dark gray. The length of the chrysalis

is a little less than three-fourths of an inch.

NATURE OF INJURY.

This cabbage worm has been rightly termed the bane of the cabbage
grower and the dread of every careful cook and housewife. It begins
work early in the season, and the principal damage is therefore usually
to young plants. After riddling the outer leaves, which continue
afterwards attached to the stalk, it attacks the more tender inner
leaves as they form, frequently secreting itself in the heads, which are
made most unsightly by its dark-green excrement, and where it is
difficult to reach with insecticides. As a result, cabbages before being
sent to market must be carefully examined and the damaged leaves
plucked away; and before cooking it is frequently necessary to tear
the heads apart to see that no disgusting ‘‘worm”’ is lurking within.
Even after the dish is cooked there is danger of an admixture of animal
with the vegetable food. In cool weather the caterpillar frequently
feeds in free exposure on the upper surface of the leaves in the sunshine.

3

As early as 1869, when this cabbage worm was restricted to limited
areas in Canada, New England, and New York, it did great damage.
At St. Albans Bay, Vt., in that year it caused the total destruction of
a crop of 3,000 cabbage plants. The worms made their appearance
about the first of September, and there were from 10 to 50 on a head.
The Abbé Provancher estimated the same year a loss of $240,000 in
the vicinity of Quebec alone. One farmer near Montreal lost in a sin-
gle season over 12,000 heads of cabbage. The following year, according
to Mr. Angus, the entire crop of cabbage and cauliflower in some places
about New York City, where the insects had appeared only the year
before, was destroyed. The loss was estimated at half a million dollars.

An interesting feature connected with the establishment of this
insect in America is the practical extirpation, at least as a pest, of the
so-called southern cabbage butterfly (Pontia protodice Boisd.*) in
many regions. This is accounted for in part by the earlier spring
appearance of the imported species, enabling the caterpillars to obtain
possession of the best feeding places and crowd out the less hardy
native form.?

ORIGIN AND DISTRIBUTION.

The imported cabbage butterfly was introduced from Europe per-
haps half a century before the date of present writing,° and was first
recognized from a capture at Quebec, Canada, in 1860. It was not
again seen until two years later, in the same locality. For several
years after this it was observed at intervals in other portions of
Canada. In 1865 its first appearance in the United States was noticed
in Maine. The following year it was seenin northern New Hampshire
and Vermont. By 1868 it had reached New York, and a few years
later began to attract attention, as each year thereafter it was found in
some new locality. In 1875 it appeared in Cleveland, Ohio, and two
years later in Illinois. By 1880 it had penetrated southward to the
Gulf region. This distribution has continued until now the species is
known practically in every State and Territory of the Union, although
it is occasionally reported as new to some more or less isolated locality.

It does not appear to be an inhabitant of any particular life zone,
seeming to be as much of a pest in the Gulf States as in Canada and
New England.

In the Eastern Hemisphere this butterfly ranges fromthe Atlantic to

aThis native species and the imported cabbage butterfly are very generally referred to in
literature as Pieris protodice and P. rap:x, respectively.

6 It iswell known that nearly all species introduced from the Eastern Hemisphere are more
hardy than native American forms and survive conditions which would be unfavorable to
the latter. In the writer’s experience, the southern cabbage butterfly is more susceptible to
disease than the imported species. Occasionally the latter returns in moderate numbers to
localities from which it has apparently been absent for long periods.

¢Riley believed it was introduced in 1856 or 1857 (see Rept. Dept. Agr. for 1883, p. 108).
A monographic account of this species is given by Scudder (Butterflies Eastern U.S. and
Can., Vo) II, pp. 1171-1190, 1205-1218.)

4

the Pacific, in most localities between the thirtieth and sixtieth
degrees of latitude. In Switzerland it occurs on the level of perpetual
snow, and in this country it has been taken on the barren subalpine
heights of the White Mountains.

FOOD AND OTHER HABITS.

The imported cabbage worm feeds on the foliage of nearly all cru-
ciferous plants, is particularly fond of cabbage and cauliflower, and
somewhat less destructive to turnip, radish, mustard, and horseradish.
It also does considerable injury to ornamental plants, such as nastur-
tium, mignonette, sweet alyssum, and spider plant (Cleome).¢

The butterflies sip the nectar of flowers of many kinds and are to be
seen any time hovering over them. They are especially fond of the
white blossoms of crucifers, also of such plants as white aster, lavender,
and thistle. Like other butterflies, this species is diurnal, and is on
the wing from early morning until near dusk. It is a comparatively
slow flyer, but capable of extended flight for long distances. The
butterflies sometimes congregate in immense swarms, as has frequently
occurred where the insect has emigrated from the continent of
Europe to England. Its occurrence in mid-ocean has been recorded.

LIFE HISTORY.

The butterflies appear as early as March, even in the Northern
States, and thereafter can usually be seen flying about cabbage fields
and elsewhere throughout the year, until after several severe frosts in
October. In the Gulf States they occur throughout the season.

Egg deposit commences soon after the butterflies appear; in fact,
pairing and egg-laying may begin within a day or two after the adult
issues from the chrysalis. The duration of the different stages varies
according to temperature. The observed egg period is from four to
eight days.

The larva eats voraciously and grows with greatrapidity, attaining
full maturity in from ten days to two weeks after hatching. It molts
four times; hence there are five distinct stages, the first molt taking
place (in warm weather) in about two days from the hour of hatching,
the second stage lasting two or three days; the third and fourth, one or
two_days each, and the fifth, four or five days.? The duration of the
chrysalis stage is from seven days to a fortnight during the summer
time, but the last chrysalides formed in the more northern States
remain undeveloped during the winter until the following spring.

The life cycle occupies periods varying between twenty-two days
and five weeks. Even in New England this species is credited with
being triple-brooded, but in the District of Columbia and vicinity there

at isa decided pest on this plant in the District of Columbia.

6 Technical descriptions of the different larval stages with particular reference to wing
development are given by W. F. Mercer (Jour. N. Y. Ent. Soc., Vol. VIII, pp. 1-20, March,
1900).

7)

5

must be one or more additional generations, and there is a possibility
of at least six in the extreme South. The first generation develops on
wild plants.

NATURAL ENEMIES.

Were it not for certain natural checks this species would be a still

greater pest than it is. The most important of its insect enemies are
small parasites, all introductions from Europe. One of them, a Bra-

Fia. 3.—A panteles glomeratus: a, adult fly; b,cocoon; ©¢, flies escaping from cocoons—4a, b, highly magni-
fied; c, natural size (original).

conid, Apanteles giomeratus Linn. (fig. 3), was purposely imported in
1883 from England. During the autumn of 1904 this species held its

Fig. 4.—Parasitized cabbage worm (Pontia rapx), showing cocoon mass of A panteles glomeratus below
(original).

host under complete control at Washington, D. C., killing every
“worm” which came under the writer’s observation. A larva which
has been destroyed by this parasite is shown in figure 4, together with
the parasite’s cocoons. A secondary parasite, a minute chalcis fly,
Tetrastichus microgastri Bouché, was also present, but did not militate
against the effectiveness of the Apanteles. Of similar importance to

6

this Apanteles is a chalcis fly, Pteromalus puparum Linn. (fig. 5),
noticed in this country in 1869, having evidently been imported with
its host.

Wasps, particularly Polistes pallipes Lepel., and related forms are
also of great service in reducing the numbers of this and other cabbage
worms, appearing to prefer them to other prey.

A small, evil-smelling plant-bug, Phymata wolff Stal. (fig. 6),
secretes itself in flowers, such as thistle and goldenrod, and destroys
numbers of the butterflies, capturing them and sucking out their vital
fluids.

Fia. 5.—Pteromalus puparum: female at left; male at right—highly magnified (original).

Numerous other enemies attack this cabbage worm in addition to the
ones that have been enumerated,? and it is sometimes subject to a con-
tagious bacterial disease? similar
to that of the cabbage looper.“ It
is, however, less susceptible to this
malady, and is seldom destroyed in
any numbers by it. The parasitic
flies can be encouraged by collect-
ing the larve and chrysalides of
Fia. 6.—Phymata wolffiti: a, dorsal view; b, lat- the cabbage butterfly and placing

eral view; c, front leg; d, snout—a, b, enlarged; them in barrels or boxes covered
c, d, more enlarged (from Riley). : é : :

with coarse mosquito netting, which

will prevent the butterflies from escaping when they develop and per-

mit the much smaller parasites to issue and continue their good work.

REMEDIES.

In the treatment of this species it should be kept in mind that other
‘“worms’’ and other cabbage pests are more often present than not.

Arsenicals.—The best remedy is one of the arsenicals; and Paris
ereen and arsenate of lead are preferred to othersincommonuse. Ifthe
former is used it may be applied either dry or wet, preferably, however,

aAmong other predaceous enemies observed in this country are the wheel bug (Aridus
cristatus Linn.) and the armed soldier bug (Podisus spinosus Dall.). The cabbage worm is
also parasitized by the Tachina fly Zrorista vulgaris Fall. The writer has a list of 10 addi-
tional European parasites.

b Micrococcus pieridis Burrill.

¢ Autographa (Plusia) brassice Riley.

7

as a spray, at the rate of about 1 pound of the poison to 150 gallons of
water. It should be applied when the plants are first set out, so as to
insure the poison reaching the young larvee before they have burrowed
far into the heads. Other applications should follow as required.
These can be made with absolute safety until the heads are about half
formed, and, for that matter, even later, as the poison disappears from
plants almost completely within three or four weeks after application.¢

Bran mash.—A mixture of bran with Paris green, one of the standard
remedies for cutworms as well as grasshoppers, is, according to the tes-
timony of some who have used it, successful against ‘ cabbage worms,”’
and should be tried against the present species. Any arsenical can be
employed in the preparation of this mixture. It is best to mix the
bran with water and sugar before adding the poison. The proportions
are 2 or 3 ounces of sugar or other sweetening, and about 1 pound of
bran, to a gallon of water; so as to make, when stirred, a mixture that
will easily run through the fingers. In its application it is merely
sprinkled, either wet or dry, over the affected plants.

The hot-water remedy.—As long ago as 1883 water, at a temperature
of about 130° F., was advised as a remedy for this cabbage ‘“‘ worm.”’
It does practically no harm to the plants and kills all insects with which
it comes in contact. It is scarcely applicable to large fields, however,
on account of the difficulty of maintaining the proper temperature.

Kerosene emulsion is not as effective as the arsenicals, because in its
application it is necessary for the spray to come into direct contact
with the larve in order to kill them.

Pyrethrum is not so useful as an arsenical. Of its effectiveness Dr.
James Fletcher, who still advocates itsuse in Canada, wrote in 1891 that
“diluted with four times its weight of common flour, and then kept
tightly closed for twenty-four hours (before use), leaves nothing to be
desired, and thousands of dollars are yearly saved to small growers, who
most need assistance.’ Pyrethrum is rather costly and, though said to
discolor the leaves, has the advantage of not being poisonous to human

a Harmlessness ot arsenicals when properly ap plicd.—Chemical analysis has shown that cab-
bage which has been dusted or sprayed with an arsenical in the way prescribed, and then
prepared for cooking in the usual manner a week later, has not even a trace of arsenic remain-
ing. The use of arsenicals against cabbage worms is almost universal in the United States,
although growers are sometimes loath to acknowledge the fact for fear of the loss of cus-
tomers who are not fully acquainted with the harmlessness of the remedy. There are no
authentic recorded instances known to the writer of poisoning from the consumption of
cabbage treated with an arsenical. According to Gillette, 28 cabbage heads, dusted in the
ordinary way, would have to be eaten by a human being at one meal in order to produce
poisonous effects. Still it is preferable, in order to avoid all danger, to use other insecticides
in the case of vegetables that are to be eaten soon. On this head it might be well to cite the
experience of a Virginia market gardener who dusted his cabbages with Paris green and
flour, omitting to inform his family of the fact. A day or two later he ate heartily of this
cabbage, as did others, and afterwards was questioned by his wife in regard to the peculiar
powdery substance on the heads. Although poisoning was anticipated, no ill results fol-
lowed.

STITUTION LIBRARIES

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beings or domestic animals. If it is not used in liberal quantity,
however, a proportion of the caterpillars are merely numbed by this
mixture and recover. Younger caterpillars are more susceptible.

Hand-picking.—F or the kitchen garden where, for obvious reasons,
it is undesirable to use arsenicals, hand-picking is sometimes practiced,
especially when plants are first set out.

The corn-meal remedy.—According to Prof. L. Bruner, corn meal
dusted on cabbage causes the ‘‘worms”’ to drop off, and protects the
crop until the meal is washed away by rain. It is applied in the morn-
ing while the dew is on and is said to act as a deterrent.

Clean farming and trap crops.—lf cooperation in clean methods of
farming and in the use of arsenicals could by any possibility be secured,
much of the loss due to the ravages of this pest might be averted. The
practice of leaving cabbage stalks in the field after the main crop is off
isareprehensible one. All remnants should be gathered and destroyed
with the exception of a few left at regular intervals through a field as
lures for the females to deposit their eggs. Such stalks, being useless,
should, where feasible, be freely poisoned with arsenicals so that the
last generation will have no place to develop in the fields.

The utilization of natural enemres.—It 1s matter of common obser-
vation, frequently recorded, that the two parasitic enemies of this
species (Pteromalus puparum and Apanteles glomeratus) do excellent
service in reducing the numbers of their host. The means by which
they can be encouraged is described on page 6.

Attempts have also been made to utilize diseases of this insect for its
control, but without very encouraging results.

Hellebore will kill this ‘‘ worm,” but is not as satisfactory a remedy
as the arsenicals.

Trap leaves.—A remedy suggested by one of our correspondents was
tested by the writer against this and other cabbage ‘‘worms,”’ includ-
ing the looper, and found worthless. It consists in placing fresh cab-
bage leaves over the heads at night, the supposition being that the
“worms’’ will gather under these traps and can be destroyed next
morning.

Approved:
JAMES WILSON,
Secretary of Agriculture.

WasHINGTON, D.C., April 28, 1905.

O