Control of the earworm in sweet corn by fumigation


Material Information

Control of the earworm in sweet corn by fumigation
Physical Description:
7 p., 7 leaves of plates : ill. ; 27 cm.
Barber, G. W ( George W )
United States -- Bureau of Entomology and Plant Quarantine
U.S. Dept. of Agriculture, Bureau of Entomology and Plant Quarantine
Place of Publication:
Publication Date:


Subjects / Keywords:
Insect pests -- Control   ( lcsh )
Helicoverpa armigera -- Control   ( lcsh )
Sweet corn -- Diseases and pests -- Control   ( lcsh )
federal government publication   ( marcgt )
non-fiction   ( marcgt )


General Note:
"E-485 July 1939."
General Note:
Caption title.
Statement of Responsibility:
by Geo. W. Barber.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 030269199
oclc - 607920677
System ID:

Full Text

E-485 July 1939

United States Department of Agriculture-7 a* '6,-
Bureau of Entomology and Plant Quarantine ?t^4, ^1^


By Geo. W. Barber,
Division 'of Cereal and Forage Insect Investigations


Introduction............. . ................ ............ ...... ................ 1
Discussion of the problem .............. ....... ... .. ... ..... ......... ............ 1
,:. t J ials used in fumigation ....... ...... . ..... ..... ... ............ ................ 2
Tool for applying tablets of the fumigant .... .............. . ............................ 3
Method of applying fumigation .. .......... ...... ...... ..... .................... ......... 4
When fumigation should be applied ........................... .......................................... 5
Cost o f fum igat ion ..................... ................................................. .............. .. .. ....... ..... 5
Effectiveness of the treatment ............................................................................... 6


The possibility of control of the corn earworm (Heliothis armigera
(Hbn.)) in sweet corn by fumigation was first investigated during the period
February and March, 1936, in southern Florida. Attention was directed to
this method because of the failure of dusts or sprays, consisting of what-
ever material and however applied, to protect corn ears against attack by
earworms. The process of ear fumigation is described at this time so that
interested persons may study and evaluate this control measure. It is not
recommended for use by commercial growers.

Discussion of the Problem

Although the corn earworm moth prefers to lay her eggs on fresh corn
silk, eggs are laid on leaves, stalks, or other plant parts when corn plants
are young and before silks have appeared'. The larvae that hatch from these
eggs feed usually in the bud or growing point of the plant and on developing
tassels before silk exposure occurs, but migrate to the ears as soon as
these have silked. Thus, where the insect is abundant many of the ears may
become infested before dusts or sprays can be applied. Depending on the
length of the husk beyond the tip of the cob and whether the internal silk
is loosely or tightly surrounded by the husk, a larva on gaining access to
the ear tip may crawl readily downward between the silk strands towards the
cob or may more slowly eat its way in that direction. The number of larvae
that reach the developing cob is influenced by the length and tightness of


the husk beyond the tip of the cob, and injury to the kernels varies from
a few tip kernels to nearly total destruction of ears, such as that shown
in figure 1.

A study of the problem indicated that if a barrier of toxic gas could
be set up in the portion of the, silk mass surrounded by husk leaves, and
if this barrier of gas could be maintained for a period of about 10 days
to 2 weeks, the probable result would be the destruction of larvae that had
entered the ear previous to application of the fumigant and the prevention
of other larvae from entering the ears subsequent to treatment. The experi-
mental work with fumigation was therefore directed toward this goal,

In the earliest work, cones of roofing paper, containing paradichloro-
benzine in their tips, were placed over the ends of corn ears after their
silks had wilted. An elastic band placed about a cone held it fast to an
ear while allowing for expansion as the cob grew in length and diameter.
Waxed paper drinking cups were soon substituted for the cones. A higher
percentage of uninfested or uninjured ears resulted from this treatment than
from any other control measure studied up to that time. However, the husks
and silks of treated ears were burned or rotted by the fumigant, and the
cones or cups were expensive and could not be handled rapidly.

By the winter of 1937-38 continuous study of ear fumigation had led
to the method herein described. Among the fumigants investigated only one,
hexachloroethane, could be used without injury to the corn silks or husks.
Results similar to those obtained by capping the ears were obtained by use
of a simple, inexpensive, specially designed wire clip, which could be
handled rapidly.

Materials Used in Fumigation

The materials needed for ear fumigation are two, (1) the fumigant
and (2) the clips.

Hexachloroethane (C2C16) at ordinary temperatures is a colorless,
crystalline material having a pungent, camphorlike odor. Having a boiling
point of 185 C. (about 365 F.), it evaporates slowly. Its vapors are
reported as having a narcotic action and a toxic effect if inhaled over a
long period. I_/

For ease of handling in fumigating corn ears this material was made
by machinery into tablets of a suitable shape. The tablets produced were
cylindrical, 5 millimeters in diameter and 13 millimeters in length, flat
at the top end, rounded at the other, and weighed about 0.5 gram apiece.

1/ "Hexachloroethane: Properties and Specifications. No. 1 of a
Series of Bulletins on Chlorine Products." R. and H. Chemicals Dept.,
E. I. Du Pont de Nemours & Co. Inc., Wilmington, Del. (No date.)


The cylindrical shape and rounded end of the tablets facilitated their
easy entrance into the tips of ears, while the flattened end afforded a
suitable surface against which pressure could be applied to force the
tablets into the ears.

Although hexachloroethane may be satisfactorily tabletized without
employing binding material, it was found that the fumigant could be more
easily handled, especially during damp periods, when mixed with about 10
percent of corn starch. The addition of starch did not affect the efficiency
of the fumigant. When stored in tight containers the tablets retained their
effectiveness for as long as 2 years with little loss in weight.

The metallic clips were made of 5-inch lengths of 16-gage crimping
wire, or other inexpensive wire of sufficient stiffness. The length of wire
is bent at its center in a hairpin shape, so that the two arms of equal
length lie parallel and in contact with each other. The terminal one-fourth
inch of one arm is then bent vertically at right angles to it, to serve as a
fastener over which the straight arm may be lifted, caught, and held in a
closed position. Figure 2 shows the appearance of a clip. When applied
to an ear the two arms of the clip exert a continuous squeeze or pressure
on the husk and result in closing the ear and keeping it closed.

These clips could be made by a farmer and his helpers during spare
time. A pair of pliers is the only tool needed. The 16-gage crimping
wire is marketed in 12-pound rolls and costs about 8 cents per pound. From
an average roll, 233 dozen clips have been made by one employee, who could
cut the wire into 5-inch lengths and bend it as described at the average
rate of about 125 dozen clips per 8-hour day. It is possible to use the
clips more than once, provided they are straightened to the original con-
dition after being used.

Experience has not indicated the least danger to consumers of roasting
ears because of fumigation of ears with hexachloroethane. In corn varieties
having husks which extend for an average of about 4 inches beyond the tips
of the cobs at the roasting-ear stage of growth, little gas reaches the
kernels, little or no odor was detected on the ear after the husk was re-
moved, and no odor or impairment of flavor was detected in the cooked ears,
although these frequently were eaten by persons who were unaware that the
ears had been fumigated. Furthermore, in experiments in which fumigated
ears, as well as hexachloroethane crystals mixed with a regular ration, were
fed to rats, not the slightest adverse effects of such feeding could be

Tool for Applying Tablets of the Fumigant

In order that ears may be treated rapidly, it is desirable that both
hands of the operator be free to apply both tablets and clips. To avoid
handling a special tool, involving expenditure of extra motion and time,


a finger-ring pusher was designed for use in forcing the tablets into the
ears. A section of wire 1/8 inch in diameter and 14 inches long was mounted
radially on an aluminum finger ring. This is worn on the ring finger of
the right hand, with the wire extending outward from the back of the hand.
The method of using this tool is shown in figure 5.

Method of Applying Fumigation

The method of applying the fumigant to corn ears is illustrated in
its several steps by a series of photographs, figures 3 to 7 inclusive.

It was found convenient to carry the tablets and clips in an apron
having two pockets, as shown in figure 3. Upon selecting an ear to be
treated, both hands are thrust into the apron pockets simultaneously; a
tablet is grasped and withdrawn with the right hand and a clip with the
left (fig. 3). The tablet is inserted into the tip of the ear (fig. 4)
and thrust into place by the ring pusher (fig. 5). The clip is opened
sufficiently to straddle the ear tip just above the location of the tablet
(fig. 6), is then forced over the ear tip, and the straight arm of the
clip is pushed over the bent end of the other arm, where it is caught and
held (fig. 7). The hands may be immediately thrust into the pockets of the
apron again, and the process repeated.

The primary reasons for using the fumigant as a tablet are convenience
and rapidity in handling. The tablet provides a uniform dosage that can be
placed where desired in the ear more quickly than by any other means of appli-
cation. In most corn varieties a tablet is thrust into an ear to the length
of the wire mounted on the ring pusher. Here the fumigant evaporates slowly,
and the gas pf.rmeates through the spaces between the silk strands, usually to
a distance uf at least 1 inch below, and for a shorter distance above,
the location of the tablet. But this is governed by the prevailing temper-
ature, which causes the fumigant to evaporate faster or more slowly as the
weather becomes warmer or cooler, and by the degree to which the silk strands
are compacted by the husk. The function of the clip is (1) to prevent the
fumigant tablet from being thrust out of the ear as a result of the growth or
adjustment of the ear parts during the time when the cob is lengthening and
filling, and (2) to prevent the escape of the gas through the ear tip. In
brief, the function of the tablet is to set up a barrier of toxic gas
between the tip of the cob, which it is desired to protect, and the husk
tip, where most earworms enter. The function of the clip is to maintain
this barrier in the desired location in the ear. The amount of fumigant
used in the tablet is much more than will evaporate between the time of
application and ear harvest. This insures a continuous volume of gas within
the barrier. The clip is quite efficient in maintaining its position on the
ear, and the writer has rarely found one that had become displaced by the
time of roasting-ear harvest. After treatment of most ears, the location
of the tablet should be about 1l inches below the husk tip. The clip
should be located just above the tablet or about 1 inch below the husk tip.


Ears having husks that extend for less than 1 inches beyond the tips
of the cobs at the time of roasting-ear harvest, and, in particular, ears
having husks no longer than the cobs at this stage of development, cannot be
successfully fumigated. Corn varieties having husks that are reasonably
uniform in length, extending at least 4 inches beyond the cobs at the time
of roasting-ear harvest, are ideal for successful application of this method
of fumigation. Ears in which the husks are wrapped with medium tightness
about the silks are especially satisfactory, since in such ears the larvae
penetrate downwards towards the cobs rather slowly, while the toxic gas can
permeate and diffuse for satisfactory distances about the tablets to form
good barriers. The operator should examine the husk characters of the
variety of corn it is desired to fumigate. In ears of varieties having very
long husks it is desirable to thrust the fumigant tablets farther into the
ears than is desirable in the case of varieties having ears characterized
by shorter husks.

When Fumigation Should Be Applied

It was found that where fumigation was applied to corn ears before
fertilization had taken place, the fumigant, the mechanical act of insert-
ing the tablets into the ears, the action of the wire clips, or all three
interference, prevented fertilization. Therefore ears should be treated
only after fertilization has occurred. Usually ears become fertilized with-
in about 3 days after silking begins, and they may be recognized by their
wilted, but not dried, silks. However, since the appearance of silks is not
always the same at the same stage of ear development in different varieties,
and because the silks may be prematurely wilted by rains, dry winds, or
other factors, the operator should observe ears, marked for identification,
of varieties which he intends to fumigate in order to learn to distinguish
the appearance of the silks when fertilized.

The length of the period during which fumigation can be applied with
satisfactory results depends on the rapidity with which the larvae penetrate
into the ears. This in turn is dependent on the tightness of the husk. Too
long a delay after silking before applying fumigation may allow the larvae
to penetrate too far into the ears to be reached by the toxic gas. Thus
delay is an important limiting factor in the success of fumigation. The
ears of some varieties can be protected when the fumigant is applied between
the third and seventh days after silk exposure. The fumigant should be
applied as promptly as possible in fields where partly grown earworms are
found migrating from the tassels to the new ears, as is often the case in
fields of very early corn, because larger larvae usually penetrate toward
the cobs more rapidly than do smaller larvae.

Cost of Fumigation

Until tablets and clips have been manufactured in quantity or extended
experience in field application of fumigation has been secured, conclusive
figures on the cost of this treatment cannot be given. It was found that


ears could be treated experimentally at an average rate of about 4 per
minute. At a labor cost of $3 per 8-hour day, the cost of treating ears
would be approximately l1 cents per dozen. Hexachloroethane is quoted at
26 cents per pound in 100-pound kegs. Allowing ample margin for the cost of
tabletization, for distribution, and for reasonable profit, it seems likely
that the tablets could be marketed for less than 1 cent per dozen. The cost
of clips was quoted by the manufacturer at less than one-half cent per
dozen. Thus it seems likely that ears could be fumigated for a total cost
of about 3 cents per dozen. However, this cost would be largely determined
by the rate paid field labor. The cost of fumigation would be least with
varieties having ears which silk uniformly within a brief period. In such
corn a large percentage of the ears could be treated in a single trip through
a field. When treating ears of varieties that silk over longer periods, a
condition characteristic of open-pollinated varieties, several trips through
a field might be necessary to fumigate a large percentage of the ears, and
the cost of treatment would be correspondingly greater.

Effectiveness of the Treatment

The aim of fumigation is the production of a large percentage of
uninfested, uninjured ears in the more valuable corn, in areas where the corn
earworm is especially abundant, or where it has been impossible heretofore
to produce usable sweet corn because of attack by earworms. Examples of
such protected ears are shown in figure 8. These ears were protected by
fumigation at a time when earworms were very abundant in southern Florida,
and when all untreated sweet-corn ears were infested and injured, the more
severely damaged ears appearing as those shown in fig. 1.

The proportion of uninfested or uninjured sweet-corn ears (fig. 8)
that have been obtained by fumigation, even where all untreated ears were
infested and seriously injured (fig. 1), has usually baen somewhat more
than 90 percent. In some instances, such as when the variety used bore
ears having husks that were not suitably long, control has not been so

Ear fumigation can be applied and remains effective under all weather
conditions. Control is obtained by a single treatment per ear. Fumigation
does not change the appearance of the ear materially, or alter the flavor
of the kernels. Experience has indicated that hexachloroethane, used as
described, does not endanger in the least the consumer of roasting ears or
the operator applying the control.

In some sections of the country corn ears are attacked by cater-
pillars having habits somewhat similar to those of the earworm. The most
common of these are the fall armyworm (Laphygma frugiperda (S. & A.)),
occurring principally in the South, but occasionally invading the North
during the fall, and the European corn borer (Pyrausta nubilalis (Hbn.)),
occurring only in the Notheast. The larvae of these insects often enter


corn ears by way of the silks; but they also feed among the husk leaves
or in the shank and reach the kernels by boring directly through any part
of the ear. Fumigation has proved effective against larvae of these species
when they enter ears by way of the silk, but it is not effective against
those larvae that bore through the husk or shank. Often, within their range,
the larvae of these species occur coincidentally with earworms, and in the
fall of the year in the Northeast all three species may be present in ears
of late corn. When evaluating the effectiveness of fumigation one should be
certain that he is dealing with earworms only, and that injury occasioned
by other species is not attributed to this insect.

Digitized by the Internet Archive
in 2013


Figure 1.--Sweet corn ears showing severe injury caused
by the feeding of earworms. Dade County, Fla., Feb-
ruary 1938.

Figure 2.--Wire clip, approximately natural size.

Figure 3.--Tablets of hexachloroethane and wire clips
are carried into the field in an apron having two
pockets, the right-hand pocket holding tablets, the
left holding clips. The hands are thrust simulta-
neously into the pockets. One tablet and one clip
are withdrawn.



a^ I



Figure 4.--The tablet is thrust among the silk strands
at the tip of a corn ear within from 1 to 3 days
after fertilization has taken place.




Figure 5.--The tablet is forced into the tip of the
ear from 1 to 1 1/2 inches by force of a ring pusher.


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Figure 6.-The clip is opened sufficiently for the open
end to encircle the tip of the ear Just above the lo-
cation of the tablet, and forced onto the ear tip.



Figure 7.--The clip is then fastened by pressing the open
ends together and slipping the straight member over
the bent member.

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Figure 8.-Appearance of ears protected by fumigation
under conditions when all untreated ears were "in-
fested and injured.


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