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Historic, archived document

Do not assume content reflects current
scientific knowledge, policies, or practices.

CIRCULAR No. 116. Issued February 7, 1910.
United States Department of Agriculture,

BUREAU OF ENTOMOLOGY,

L. O. HOWARD, Entomologist and Chief of Bureau.

THE LARGER CORN STALK-BORER.

(Diatrea saccharalis Fab.)

By Grorcre G. AINSLIE,
Associate Professor of Entomology, Clemson Agricultural College of South
Carolina.
INTRODUCTION.

In many southern cornfields a heavy wind late in the season, be-
fore the corn is matured, does great damage by breaking the plants
off at the surface of
the ground, thus
ruining them. An
examination of these
broken stems will, in
most cases, show that
they have been
greatly weakened by
the burrows of a
larva or caterpillar.
This larva (fig. 1) is
known as “the lar-
ger corn stalk-borer ”
(Diatrea sacchara- Fig. 1.—The larger corn stalk-borer (Diatrea saccharalis) :
lis). Its work is a, Summer form of larva; b, c, hibernating forms of
"ig hehe ca ee nerpermape eter

g Se al seg
stem of the plant and from above; g, same from Side. @, b, c, Enlarged; d, e, f,
is 0 concealed that, still more enlarged. (Redrawn from Howard.)
in most cases, unless weather conditions make it conspicuous, the
presence of the insect passes unnoticed.

eo atten’ A

@This is practically a revision of Circular No. 16, prepared many years
ago by Dr. L. O. Howard. Mr. Ainslie was formerly in the employ of this

Bureau as an agent and expert in cereal and forage insect investigations, and

this pest was one of the subjects of investigation assigned to him. . He after-
wards did some work upon the species for the South Carolina Agricultural
Experiment Station in cooperation with this Bureau.

21731—No. 116—10

2

This insect seems to have been originally an enemy of sugar cane
and to have first transferred its attention to corn in the southern
part of this country, where corn and cane are grown over the same
territory. It occurs in many countries where sugar cane is the staple
crop, and has caused great damage in the West Indies, British Guiana,
Australia, and Java. The bulk of the evidence goes to show that it
was first brought into this country with the importation of sugar-
cane cuttings from the West Indies and Central and South America,
where, since early times, it has interfered with the production of this
staple.

In the United States this borer is found almost universally through-
out the South, from Maryland to Louisiana and westward to Kansas.
Among other localities it has been reported to the Bureau of Ento-
mology from Bennettsville, S. C., as destroying corn, especially that
planted early in the season. From Waynesboro, Ga., in 1909, reports
were received that in some fields the corn was “at least one-third
destroyed ” by an insect which later proved to be this species. In
Virginia it has been found recently at Nathalie, where it was studied
by Mr. J. A. Hyslop, of this Bureau, at Allenslevel, at Church Road,
and at Farmville. In late October, 1909, Mr. E. G. Smyth found
that nearly one-half of the cornstalks at Diamond Springs, Va.,
were infested, often as many as three larve being found in one
stalk, boring from the surface of the ground down to the base of
the root; and while the author has frequently found as many as
a dozen larve in a single stalk, there are never more than two or
three pups in the same stalk. In each case it had damaged the corn,
and especially that planted early in the season. Detailed investi-
gations of this insect have been conducted by the author during the
last two years, chiefly in South Carolina.

NATURE OF DAMAGE.

Corn is damaged by these caterpillars in two ways. First, in the
early part of the season, while the plants are small, they work in the
“throat” of the young corn, and if the tender growing tip within
the protecting leaves is once damaged all chances that the plant
will become a normal productive specimen are gone. In many sec-
tions of the South this is commonly known as “ bud-worm ” injury,
and though there are several other insects which cause a similar
mutilation of the leaf, a very large proportion of the so-called “ bud-
worm” damage may be charged to this insect. The effect of its
work on the leaves of the young corn plants is similar to that re-
sulting from attacks by the corn billbugs (Sphenophorus spp.) and
is evidenced by the familiar rows of small circular or irregular holes
across the blades of the plant (fig. 2).

The other form of serious damage chargeable to this pest occurs
later in the season. The larvee, having then left the leaves and

[Cir. 116]

3

descended to the lower part of the stalk, tunnel in the pith. (See
fig. 3.) If the larvee are at all numerous in the stalk, their burrows
so weaken the plant that any unusual strain will lay it low and
destroy all chance of its maturing. While frequently ten or more
larvee may live and mature in one plant, it must be remembered that

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Fic. 2.—Work of larger corn stalk-borer, showing mutilation of leaves of corn by larve.
Greatly reduced. (Original.)

any infestation, however light, will lessen in some degree the vitality

of the plant and cause a corresponding loss in the quality and quan-

tity of the harvest. |
HABITS OF THE LARVE.

Immediately upon leaving the egg in spring, the young larva of
the first generation, spinning a silken thread behind it, wanders
down into the throat of the plant as far as the water or dew usually
standing there will allow it to go, and begins to feed on the leaves,
going back and forth through the yet unfolded clusters and soon
riddling the more tender leaves with aimless burrows. If the bur-
row reaches the tender terminal bud where the future joints are
being formed, further growth at that point ceases and the plant be-
comes stunted and misshapen, with no tassel. As the plant continues
to mature, the larva “ grows out,” as the farmers say. It is more

[Cir. 116]

4

likely that it is the evidences of its work and not the larva itself
that “grow out;” but for whatever reason, the caterpillar soon leaves
the more leafy portion of the plant and attacks the stalk at or near
the ground. Here a hole is cut through the outer wall of the stalk
and the larva burrows upward for a short distance, after which it
seems to run aimlessly through the pith, frequently even leaving the

Fic. 5.—The larger corn stalk-borer: Larva in lower part of corn plant preparatory to
hibernation. Reduced. (Original.)
stalk entirely and reentering it at another point. Turning upward,
the caterpillar, when fully grown, bores toward the outside and
cuts a circular hole in the outer wali of the stalk. Then, after
spinning a few loose threads across this opening to keep out un-
desirable visitors, it retreats a short distance, plugs the burrow below
with digested pith, and in the chamber thus created slowly changes
to the next or pupal stage (fig. 4, ¢).
[Cir 4161

5

Seldom is the stalk damaged above the third joint from the ground,
although the larve, when small, are found in the large midribs of
the lower leaves and later in the season, when the food supply is
restricted, even in succulent nubbins farther up. They sometimes
also penetrate the underground part of the stalk in feeding and enter
some of the larger brace roots for a short distance.

The larve of the second generation work in a similar manner, except
that at the time they appear the tassel has been formed; hence the
damage is now confined altogether to the lower stalk. Thus, instead
of arranging to pass the pupal stage in the upper stalk, they pene-
trate to the root to hiber-
nate and there, as larve,
pass the winter in a qul-
escent state (fig. 3).

SEASONAL HISTORY.

During the winter this
enemy of corn is- to be
found as a robust, creamy-
white larva of the second
generation in the lower
part of the stalk—or of
the stubble, if, as is usu-
ally the case, the corn
has been cut. In this lo- Fie. 4.—The larger corn stalk-borer: a, Female
eation the larva forms a moth; b, wings of male; c, pupa. All somewhat
smalltcavity below the surf) 4) 01s |)
face of the ground, well protected from birds, predaceous insects,
and unfavorable weather conditions. From the time the corn is
mature in the fall until about corn-planting time in the spring this
caterpillar remains inactive. About the time the ground is being
prepared for corn, from March 15 to April 30, depending on the
Joeality, this larva changes into a reddish-brown pupa or chrysalis
(fig. 4, ¢). After a further period of ten or more days’ inactivity the
adult insect emerges from the pupa case as a pale brownish-yellow
moth (fig. 4, a, 0) ,with a spread of wings of about an inch and a quarter.
The moths then mate and the females begin at once to deposit eggs
on the underside of the leaves, the larvee hatching from these eggs
forming the first generation. |

The eggs hatch in from seven to ten days and the young larve
begin their destructive work in the upper leafy portion of the plant,
later descending to the base of the stalk, where they attain full
growth. This period, from egg to full-grown larva, requires from
twenty to thirty days, depending largely on the weather conditions

[Cir. 116]

.

6

and the vigor of the plant. The larve when full grown pupate in
the staik, usually in the second or third joint from the ground, and
in from seven to ten days the adult moths of the first generation
emerge. |

The eggs for the second generation are laid in similar positions
on the lower leaves or on the stem, and the larvee, after feeding for
a short time on the leaves, go directly to work in the stalk, completing
their larval growth in the pith of the lower stalk as did the larve
of the first generation. No damage is done to the upper part of the
plant by larvee of the second generation.

By the time the larve of the second generation are full grown
the corn is rapidly nearing maturity, and, instead of pupating in
the stalk, they turn downward, penetrate to the extreme lower tip
of the taproot, and there form a small cavity in which to pass the
winter. At this time the larve lose the darker markings of the ear-
her forms, and as overwintering larve are creamy-yellow in color.
They are plump and active in the fall, but flabby and sluggish after
fasting throughout the winter. The only way in which the insect
passes the winter is in the form of this overwintering larva, found
below the ground in the extreme lower tip of the corn roots. Two
generations a year appear to be the rule, although it is possible that
in the far South and on sugar cane a partial third generation may

occur.
DESCRIPTIONS.

Egg—tThe eggs are flat and scalelike, almost circular in outline,
and are placed in rows or irregularly, overlapping one another
shingle fashion. From two to twenty-five eggs are laid in one place
on the underside of a lower leaf or occasionally on the upper side
and on the stem. Creamy-white when first laid, they gradually
change to a reddish-brown, and in seven to ten days a minute, bristly,
reddish caterpillar cracks the shell and crawls out through a narrow
slit at one end. The eggs are about three one-hundredths of an inch
(7.6 mm.) long and about two-thirds as wide. After hatching, the
white papery shells are soon washed off the leaves.

Larva—tThe larva of the first generation (fig. 1, @) when full
grown is a robust, dirty-white caterpillar 1 inch in length, thickly
covered with round or irregular dark spots, each of which bears a
short, dark bristle. When the larva is small these markings are
almost contiguous, giving the whole insect a dark color and a hairy
appearance. The head and thoracic plate of all the stages are
brownish-yellow. The overwintering larva of the second generation
(fig. 1, 6, ¢) gradually loses the darker markings of the body and
after the last molt remains unspotted and lght yellow in color,
except for the head and the thoracic plate, which retain the brownish-
yellow of the earler stages.

[Cir. 116]

7

Pupa.—When first formed, the pupa (fig. 4, ¢) is light honey-
yellow in color, soon changing to a rich mahogany-brown. It is
about seven-eighths of an inch in length and is able to contort itself
violently when disturbed. It lies in the cavity usually with the head
up. On emerging, the moth leaves the brownish shell of the pupa
case, partially withdrawn from the hole.

Adult.—The female moth (fig. 4, a@) varies in color from almost
white to smoky yellow. The fore wings, which spread to about 14
inches, are darker than the hind wings, and bear faint markings.
When at rest the wings are held close to the body, forming an acute
triangle. The egg laying is done for the most part either at night or
in the dusk of evening, the moths flying rapidly from plant to plant.
The male moth (fig. 4, 6) is usually somewhat darker in color than
the female and always smaller.

FOOD PLANTS.

Besides corn and sugar cane, this borer has been reported as feed-
ing on sorghum, Johnson grass, guinea corn, and grama grass. The
injury to the four last-mentioned plants is never severe, but in plan-
ning methods of control they must be considered and an examination
made to determine whether or not they are harboring the pest.

NATURAL CHECKS.

The larger corn stalk-borer has very few natural enemies. A
minute Hymenopterous parasite (Trichogramma pretiosa Riley) has
in a very few instances been found living in and destroying the eggs.
In one case ten of these minute parasites were reared from two eggs.
The larva of a brown, velvety beetle (Chauliognathus pennsylvanicus
DeG.) sometimes enters the holes in the stalks of stubble after the
corn is cut and devours the caterpillars found therein. This larva
has been found to be of great value in reducing the numbers of the
borers in fields of sugar cane. The termites or white ants (Zermes
flavipes Koll.), locally known as “ wood lice,” have been observed
destroying the larvee in the stubble in the winter, although apparently
only when the presence of the larve interfered with the work of the
ants. In a few cases bodies of the borers have been found in the
stubble killed by a fungus, as yet undetermined, which envelops their
bodies in a white mold. Fungi, however, are too dependent on
weather conditions to be of any practical value in controlling the pest.

PREVENTIVE MEASURES.

Rotation is one of the best general preventives of injury from in-
sects affecting field crops. Experience has shown that where corn
has followed itself upon the same field for two or more years there
has been a much greater loss from the borer than where an annual —
change of crop has been practiced. This is especially noticeable

[Cir. 116] ;

8

where stalks or stubble from the previous year have been allowed to
remain undisturbed throughout the winter. The moths, upon emer-
gence in the spring, finding themselves surrounded by the young
corn, commence egg laying at once and escape the dangers encountered
in searching for another field of corn. A forced journey in search of
young corn results in many of the females being eaten by birds or

being destroyed because of rain, cold, or failure to find the object of |

their quest. A few moths will always succeed in their search, but the
successful proportion will be greatly decreased by persistent crop
rotation.

Another remedy, probably the best for this insect, is the thorough
destruction, some time before the period of emergence of the moths
in the spring, of all the stalks and stubble remaining in the field from
the preceding crop. If all this trash can be disposed of before the
opening of spring, the numbers of the pest must be greatly diminished
if not almost exterminated, for the only form in which the insect
passes the winter is that of the caterpillar, and the only known loca-
tion is in the lower tip of the corn root, snugly hidden. Some few
may, however, be found to survive in the roots of the larger grasses
mentioned above, and care should be taken in such cases to treat these
in the same way. The method employed in disposing of the stubble
and stalks will depend largely on the conditions in individual cases.
If the stubble is cut low and the land is moderately heavy, a
thorough deep plowing may suffice, an inch or two of well-settled soil
being sufficient to prevent the escape of the adult moths. Bringing
the stubble to the surface where it can dry will kill some of the con-
tained larvee, but this method depends too much on the state of the
weather to be trusted. By far the most effective plan is to remove
the stubble from the field with a rake and burn it.

In the cane field the methods of treatment must be adjusted to
correspond with the methods of handling that crop. The larve com-
monly spend the winter in the trimmings and tops which have been
discarded at harvest time because of immaturity. This refuse, left
on the ground throughout the winter, becomes dry and inflammable
and, if thoroughly burned before spring, enough larve will be killed
to insure at least temporary relief from the ravages of the borer.

Any method which will insure the complete destruction of the over-
wintering larvee, if persisted in and carried out simultaneously over
large sections of the country, will effectually preclude serious damage
from the insect.

Approved:

JAMES WILSON,
Secretary of Agriculture.

Wasuineton, D. C., December 14, 1909.
[Cir. 116] _
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