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Digitized by the Internet Archive
in 2011 with funding from
University of Illinois Urbana-Champaign —

http://www.archive.org/details/armyworm 14flin

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. STATE OF ILLINOIS
DEPARTMENT OF REGISTRATION AND EDUCATION
DIVISION OF THE
STATE NATURAL HISTORY SURVEY

STEPHEN A. ForBES, Chief

ENTOMOLOGICAL SERIES
CIRCULAR 7

THE ARMY-WORM

By W. P. FLINT

——

SS

PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS

URBANA, ILLINOIS
Revised Edition
April, 1921

U. OF ILL. UB.

Asiuy a) History Survey,

Tl threrw

STATE OF ILLINOIS -
DEPARTMENT OF REGISTRATION AND EDUCATION
W. H. H. MILLER, Director

BOARD OF
NATURAL RESOURCES AND CONSERVATION
W. H. H. MILLER, Chairman

JOHN W. ALvorD, Engineering

JOHN M. Coutter, Forestry Kenpric C. Bascock, Representing the
Rouuin D. SALIsBuRyY, Geology President of the University of Illi-
WILLIAM A. Noyes, Chemistry NO1S8

WILLIAM TRELEASE, Biology

THE NATURAL HISTORY SURVEY DIVISION

STEPHEN A. ForBEs, Chief

eo

ILLINOIS PRINTING CO., DANVILI E. ILL,
47396—10M

THE ARMY—-WORM*

By W. P. Fuint, ENTOMOLOGIST,
Sratm Naturat History Survey, Ursana, ILL.

The army-worm is one of the most destructive of the insect pests
of the cereal crops and forage grasses, but its injuries may be
promptly arrested, and in many cases almost completely prevented,
by simple measures easily applied by any farmer. This insect be-
longs to the great family of moths which includes all of our common
cutworms, and it occurs throughout the state each year. Ever since
Illinois was first settled there have been, at irregular intervals, more
or less general outbreaks of the army-worm covering, one season,
from four to five counties, and another, nearly the entire state, and
there is seldom a year during which it does not become destructively
abundant in some Illinois locality. These outbreaks are usually con-
fined to small or comparatively restricted areas, here and there,
ranging in size from a few acres up to several thousand. As a rule,
there is a period of at least five or six years between local outbreaks.

SEASONAL History

The winter is passed in the partly grown, worm or larval stage,
the worms being buried in the ground or well hidden and protected
in thick matted grass. There is a slight possibility that a few of the
- insects may overwinter in the pupal or resting stage. The worms
become full-grown very early in spring—by the middle of March or
the first of April in the southern part of the state,.and from the
middle to the last of April in central Illinois—enter the ground to a
depth of about two inches, and there change to brown pupae, the
resting stage, in which they do not eat, or move except for a slight
wiggling of the tip of the abdomen. The worms remain in the pupal
stage from two to four weeks, the exact time depending on the tem-
perature, and emerge as fawn-colored moths.

GENERAL APPEARANCE OF THE MoTH

The front wings of the moth have a few irregular dark mark-
ings, and a small white dot near the center. The hind wings are a
little lighter than the front wings, show a few more irregular dark
lines and markings, and lack the central white dot. When the wings
are folded, the moth is about 1 to 114 inches long, and about 34 inch
across the ends of the wings.

*Known scientifically as Cirphis unipuncta.

4

s

Hapits or THE Moru

The moths fly mostly during the night, hiding by day under rub-
bish and trash in the fields. They are only slightly attracted by
lights, but very strongly attracted by the odor of decaying fruits or
of sweets. They are strong fliers, and with a moderate wind behind
them they travel a number of miles. For depositing their eggs they
generally seek a place where the vegetation is dense and. the grasses
are closely matted together, frequently laying them in the rank
growth around old straw-stacks or in fields of oats, wheat, or rye
where growth has been very vigorous and the grain has lodged. They

‘

Fic. 1. Army-worm moths showing position of the white dot
on front wings. (Twice natural size.)

also frequently lay their eggs in the rank-growing grass of timothy
meadows, blue-grass pastures, or orchards. The eggs are deposited
under the sheaths of the leaves and around the bases of various
grasses, from 25 to 100 or more in a place. As the moths are very
prolific—a single female laying from 500 to 800 eggs—and somewhat
gregarious, enormous numbers of eggs may be laid in a small area.
The worms hatching from these eggs very soon exhaust the limited
food supply at hand and move out in large numbers or armies to
search for fresh food plants. It is this migration that has given the
insect the popular name of army-worm.

5

GENERAL APPEARANCE OF THE WORM

Ten days or two weeks after egg-laying small colorless worms
with dark heads hatch from the eggs. They have the looping habit
of the measuring-worms, but remain closely hidden by day about the
roots of grasses, feeding at night and growing rather slowly for the
first ten days or two weeks. At the end of this time they begin to
feed more ravenously and to grow rapidly, becoming full-grown in

Fic. 2. Full-grown army-worm, the lower one showing eggs of tachinid fly
attached to back. {Twice natural size.)

from three to four weeks, and are then from 11% to 2 inches long.
They are gray or blackish, with a narrow, broken whitish stripe
down the center of the back. There are three stripes of nearly equal
width on each side, the lower stripe being light yellow with whitish
edges, the middle one dark, with a somewhat lighter center, and the
upper one a little darker than the lower, but with narrow white
lines along its edge. When full-grown they enter the ground,
change to the resting or pupal stage, and emerge as moths in about
twenty days.

.

NuMBER OF GENERATIONS AND TIME OF THEIR APPEARANCE
e
There are three generations of this insect each year in the
southern two-thirds of the state, and probably throughout Illinois.

6 i

In southern Illinois moths of the first generation usually make
their appearance about March 20, in central Illinois about April 1,
and in the extreme northern end of the state about April 10; but the
dates vary greatly in different seasons, being earliest, as might be
expected, when an early spring follows a mild winter. As a rule,
worms of the first generation do the greatest damage. They may be
looked for in the southern end of the state from May 20 to May 30;
in the central part of the state from June 1 to 15; and in the north-
ern end of the state from June 15 to 30. The second generation of
the worms rarely appears in sufficient numbers to do damage in this

Fic. 3. Pupae or resting stage. This stage is passed wholly
in the ground. (Twice natural size.)

state, although there have been a few local outbreaks the last of July
and in August. The third generation is a little more likely than the
second to be abundant. When it-is, it generally occurs from the
first to the middle of September.

Foop PLANTS

The larvae prefer the grasses, and in this state have been found
feeding upon practically all the wild grasses, and on blue-grass,
timothy, corn, rye, oats, barley, millet, wheat, and sorghum, all be-
longing to the great family of grasses. They will at times feed on
legumes, such as clover and alfalfa, but they greatly prefer the

¢

grasses and generally do not damage legumes unless the grasses are
scarce. The greater damage done by the worms is by eating the
leaves and blades of corn, oats, and other small grains. If they are
abundant early enough in the season, they will eat the heads of
wheat, rye, and barley, but if they do not appear until the wheat
heads are fairly well matured, very little of the grain is injured.

During nearly every army-worm outbreak there is also an
abundance of the variegated cutworm.* This pest will be found in
the same situations as the army-worm and associated with it. It
feeds readily upon clover and alfalfa, and often causes considerable
of the injury to these plants which is attributed to the army-worm.

NATURAL ENEMIES

Owing to the fact that the army-worm is preyed upon by a large
number of insect parasites and predacious enemies which become

Fic. 4. Corn field, Piatt county, June, 1919. About ten acres
of corn, twenty inches high, has been completely
eaten by the worms.

very abundant during excessive outbreaks and greatly reduce the
numbers of the worms, the generation succeeding an outbreak is not
abundant enough to cause any damage. In fact we do not have a
single record in this state of damage by more than one brood during
any one season, and outbreaks rarely occur in the same locality in
consecutive years. One of the most abundant of the insect enemies
of the army-worm is a grayish fly, a little larger than the house-fly.
which deposits its eggs on the front third of the worm’s body. The

*Peridroma margaritosa.

8

maggots which hatch from these eggs feed on the body contents of
the worm, and always kill it before it reaches the moth stage.
Sometimes these eggs are mistaken for those of the army-worm,
but it should go without saying that the army-worm deposits eggs
only when it is in the full-grown or moth stage.

ConTROL MEASURES

The most effective means of controlling an outbreak is by poison-
ing the worms in the fields where it originated or around the mar-
gins of such fields as the worms are leaving. This can best be done
by the use of a bait made as follows:

25 lbs. dry bran
1 lb. white arsenic
or 4
1 lb. Paris green

Mix dry. Add water (about 3 gallons) to make a stiff mash,
in which has been stirred 2 quarts of cheap molasses, black strap
preferred. The mash should be of such consistency that it will just
hold together when tightly squeezed in the hand. If possible to
obtain it, white arsenic should be used instead of Paris green, as
it can be bought for about one fifth the price and is just as effective
for baits of this sort.* The bait should be sown broadcast over the
infested field at the rate of 8 or 10 pounds to the acre. This is
about as thickly as one can scatter this quantity and have it cover
the ground. The mixture will be much more effective if put on in
this way than if thrown out in lumps or placed in lines or ridges
over the fields. The application should be made during the latter
part of the afternoon or early in the evening, as the worms do the
greater part of their feeding at night, and the bait is much more
attractive when fresh. This mash has been used successfully in a
number of states and in hundreds of instances in this state, and
when properly mixed and correctly applied it has never been known
to fail in killing from 60 to 90 per cent. of the worms in the fields.

It is not advisable to use this bait in the vicinity of buildings,
where chickens will have access to it. It may, however, be sown in
pastures if it is applied thinly over the entire area at the rate
recommended in this circular, and not put out in lumps or lines.

Another method for controlling the army-worm is to surround
fields or areas in which it is abundant by ditches, in which a log
should be dragged until a layer of fine dust is formed, the worms
being trapped and killed in post-holes dug in the bottoms of these
ditches. Such ditches are best constructed by plowing a deep dead-
furrow, and then dragging a heavy log back and forth a number of
times, until the sides of the furrow are pulverized to a fine dust.
After this, the post-holes should be dug at intervals of about one rod

*White arsenic can not be used in sprays which are applied to the foliage of
plants, as it has a very caustic effect on leaf-surfaces.

9

along the bottoms of the furrows. If a large number of worms are
attempting to cross such a furrow, their continued attempt to climb
the sides will, after a time, wear down the dust, making it necessary
to use the drag and clean out the post-holes each day that the barrier
is maintained. The worms move out of the field where the out-
breaks have originated as soon as their food supply is exhausted.
This movement generally begins in the latter part of the afternoon
and continues during the night, which makes it necessary to have
the dusty furrows in good condition each evening. The worms may
be killed in the post-holes or the bottom of the furrow by sprinkling
lightly with kerosene or with a 20% kerosene emulsion. Those that
have accumulated in the furrows may be killed by scattering liberal
quantities of the poisoned bran mash among them.

It is highly important to use the bait as soon as the worms are
detected in large numbers, as they will feed upon it as readily when
small as when nearly full-grown, and if they are detected and the
bait applied before they reach the half-grown stage, nearly all
damage from them may be avoided.

Tar-line barriers, such as are used against the chinch-bug, have
not proved very effective in controlling the army-worm.

If one will learn to recognize the army-worm moth, this will aid
greatly in foretelling outbreaks of this insect. When moths are
abundant, the worms may be expected to appear about three weeks
later, and a close watch should be kept of all places. on the farm
where the moths are most likely to lay their eggs.

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