The use of oil or oil containing insecticides for earworm control in sweet corn


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The use of oil or oil containing insecticides for earworm control in sweet corn
Physical Description:
Barber, G. W ( George W )
United States -- Bureau of Entomology and Plant Quarantine
U.S. Department of Agriculture, Bureau of Entomology and Plant Quarantine (Washington, D.C )
Publication Date:

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All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 30270783
oclc - 778784175
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Full Text

E-525 DEPARTMENT January 1941


By Geo, W, brber, Division of Cereal and Forage Insect Investigations


Roasting-ear corn may be protected against injury by the corn earworm
(Heliothis armizera (Hbn.)) by the use of oil or oil containing insecticides
applied to the silks. The usefulness of oil was first indicated in investi-
gations made during the summer of 1936. That the effectiveness of the oil
is increased by the addition of pyrethrins was first shown in southern
Florida during February and March 1939. In the summer of 1939 it was
discovered that the effectiveness of oil could be still further increased
by the addition of dichloroethyl ether instead of pyrethrins. These inves-
tigations have not been concluded, and a discussion of the use of oil, both
alone and containing insecticides, as a means of control, is presented herein
as a progress report rather than as a recommendation for large-scale use by

The problem of protecting corn ears against earworm attack presents
many difficulties and an important peculiarity. To protect a given ear it
is necessary to effect 100 percent control of the insect. If 100 eggs were
laid on the silks of an ear, it does not suffice to prevent even 99 percent
of the resulting earworms from entering the ear, since the one remaining
worm might cause injury almost as objectionable as would be the case had
many entered it. Hence, to be successful, a control measure must attain
perfection in each ear. Earworms destroy one another within the ear, and
rarely do more than one or two worms mature in an ear or survive suffi-
ciently long to feed on the kernels.

1 This supersedes publications E-476, "The Use of Oil for Earworm
Control in Sweet Corn," which was issued in May 1939, and E-497, "The Use of
Oil oa Oil Containing Pyrethrins for Earworm Control in Sweet Corn," which
was issued in February 1940.


The protection of sweet corn is especially difficult because most
varieties are characterized by ears having loose husks, which allow the
worms to reach the kernels quickly and penetrate below the tips of the cobs,
where feeding causes particularly objectionable damage. In such corn one
worm can render an ear unfit for use. Furthermore, sweet-corn varieties
are especially attractive to the insect and consequently sustain severe
infestation. Their ordinarily loose husks allow the worms to disperse,
and hence more individuals survive and severe kernel injury results.

Silking Habits of Corn Ears

At first the corn ear is a closed package. But to receive pollen the
silks are thrust out of its tip, where they grow for a day or two and,
weather favoring, are fertilized during the first few days of exposure. The
new silk is extremely delicate and wilts readily. If perfect ears are to be
obtained, each strand of silk must receive its pollen grain, and hence a
successful method of control of earworms must not interfere with fertiliza-
tion. A few days after exposure the silk becomes limp and begins to dry.
As by this time all except the tip kernels will have been fertilized, control
measures can then be applied without danger of preventing pollination.

The period elapsing from silking to harvest of sweet-corn ears is
usually from 16 to 21 days. If oil is applied to the silks as herein
described, during the first week after they appear, the period during which
a control measure must be effective is not more than from about 10 days to
2 weeks.
Habits of Corn Earworms

Most earworm eggs are laid on the silks of a given ear during the week
following first exposure of the silks, particularly while they are fresh and
unpollinated. The newly hatched worms, which emerge from the eggs about 3
days after they are laid, feed very little on the exterior silk but immedi-
ately crawl into the tips of the ears among the silk strands, where they
begin eating. Where the insect is abundant, eggs are sometimes laid on parts
of the plants other than the silks, often before the silks have become
exposed. The resulting worms, which often feed on the tassels, are ready to
enter the ears immediately after the silks become exposed, frequently as
partly grown individuals. In these circumstances the application of insecti-
cides to the exterior silks may be ineffective because larvae may already
have entered the ears before the application is made.

Usually most of the earworms enter the ears by way of the silks, but
some, usually migrating individuals, bore through the husk to reach the
kernels in a way that is characteristic of the fall armyworm (LAphygma
fru ierda (S. & A.)) and the European corn borer (Pyrausta nubilalig
(Hbn.)). Unless all the ears of a field are treated, irrespective of their
marketability, in order to kill as large a percentage of the earworm popula-
tion occurring in the field as possible, migrants usually move from the
untreated to the treated ears in numbers somewhat proportionate to the
density of the population. Under these conditions as many as from 5 to 10

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percent of treated ears may be attacked by migrating earworms, usually
individuals in the fourth to sixth instars, which, being probably repelled
from the treated tips, bore directly through the husks to the kernels.

Determination of the Rate of Infestation in Corn Ears

The grower of sweet corn needs to know two facts regarding the earworm
infestation in his field before applying the type of control described here-
in; first, that the percentage of infested ears will be sufficiently high to
warrant any expenditure for control; and second, at what time during the
development of the ears such control can be applied most advantageously.

In the Southern States (approximately the area south of the 38th
degree of latitude, or, a line running through Maryland, northern Kentucky,
and central Missouri) the insect usually is sufficiently abundant to warrant
application of control measures to the ears of all sweet-corn or other vari-
eties used for roasting ears. In much of the area north of this line, how-
ever, the abundance of earworms fluctuates from year to year and during
different seasons of the year. In this northern area not all sweet corn is
attacked, and a satisfactory means is needed for determining what degree of
infestation will occur in a given field. The presence or abundance of eggs
on the silks has not proved to be a satisfactory index to this, however,
particularly for use by farmers. The eggs are small (somewhat larger than
the period at the end of this sentence) and, although they may be seen
rather easily when laid on red or pink silks, their detection is not nearly
so easy or sure when laid on green silks, such as those of the variety
Golden Cross Bantam. Where only one or two eggs are deposited in the mass
of exterior silk of the ear, they may be easily overlooked. Furthermore,
eggs often are laid on the husks or on other parts of the plants where they
may escape detection. Even if all the eggs could be found, their presence
would not necessarily indicate the rate of infestation of the ears because
many eggs are devoured by predators, especially by the small bug Orius
insidiosus (Say), and they sometimes become dislodged from the silks.

It is much more practical for the grower to determine infestation by
observation of worms in the interior silk of the ears. This can be done as
soon as the corn has silked by cutting the silks off cleanly with a sharp
knife, together with the terminal inch of the husk, and then splitting open
the clipped tip of the husk to expose the interior silk. If larvae are
present they may be seen lying among the interior silks, where their numbers
may be determined. When only first-instar individuals are present, a mag-
nifying glass may be needed to locate them quickly and without error. These
examinations also enable the observer to determine how far within the ears
the worms have penetrated. This depends largely on the prevailing stage of
development of the ears and the size of the worms. Where the worms consist
of small individuals (first- or second-instar larvae), most of them are
usually contained in the clipped terminal inch of the husk, and in this
case the cut surface of the interior silk mass will appear clean and without
discoloration. Where larvae have penetrated farther than 1 inch into the
ears, their presence may be detected by brown spots on the cut surface of


the interior silk. These spots consist of excrement and damaged silk, and
the number of such spots present indicates the number of worms that have
penetrated more deeply than 1 inch into an ear. If they have penetrated
deeper than 1 inch, an examination of the remaining part of the interior
silk above the tip of the cob will disclose the exact position in which
worms are feeding.

Such examinations of ears in one or more lots of 10, so selected as
to represent the entire cornfield, should be made every 2 or 3 days during
the first 10 days after ears have begun to expose silks. These examinations
will give dependable information on the percentage of ears attacked, the
size of the contained larvae, the population present per ear, and the
progress of penetration of the larvae into the ears. From information thus
gained, the grower may judge whether infestation is present at a rate
sufficient to warrant expenditure for control, and at what time during
development of the ears control may be applied most effectively. Whether
or not the ears of a given field should be treated for earworm control will
depend on the price the grower expects to receive for them. If the expected
price is a profitable one, control usually may be applied advantageously
when 20 percent or more of the ears are infested.

Use of Oil Alone

The effect of applying oil alone to the tips of corn ears is to place
a barrier of oil in the mass of silk that is enclosed by the husks above
the tips of the cobs. The small larvae that are already in the silks at
the time of treatment are killed, probably by being smothered, and small
larvae that enter the silks after the oil has been applied also succumb
or are repelled. Against larger larvae, especially those that migrate to
the ears from the tassels in early corn, the oil alone was found ineffective.

Vegetable and mineral oils of many sorts were studied. White mineral
oils were found to be most satisfactory. These oils are standardized
products, pass the U. S. P. copper and acid tests, and are colorless, odor-
less, and tasteless. They are used medicinally, in small quantities are
harmless to persons, and they do not adversely affect the flavor of green

When oil alone is used, white mineral oils of from 150 to 250 Saybolt
viscosity at 1000 F. were found to be most satisfactory, because these
grades did not disperse too deeply within the ear but established the desired
barrier film among the interior silk strands between the tip of the husk and
the tip of the cob, in which the small larvae were killed.

Use of Oil Containing Pyrethrins

White oil with the addition of pyrethrins is an efficient contact
insecticide that kills larvae of all sizes rather quickly. Because of the
effective character of the mixture as a contact insecticide, establishment
of an oil barrier in t!'e interior silk at the tip of the ear is no longer


the most desirable objective. It becomes important that the oil containing
pyrethrins disperse thoroughly throughout the interior silk so that it will
come into contact with, and kill, all the contained larvae. On the other
hand, it is not desirable that the oil disperse so freely within the ear
that much of it penetrates to parts where no larvae occur. For the purpose
mentioned, white mineral oils of Saybolt viscosity from 100 to 150 proved
to be the most effective carriers of pyrethrins, as well as being less
expensive than the heavier oils formerly used.

Oils containing pyrethrins added by the manufacturer, and oil to which
pyrethrum extract was added by the grower, proved equally effective. It is
probable that the use of oils containing pyrethrins prepared by the manufac-
turer will prove to be more convenient, if not more economical, than those
prepared by the grower.

The effectiveness of oil containing pyrethrins as a contact insecti-
cide in protecting corn ears against injury by caterpillars is somewhat
proportionate to its percentage content of pyrethrins. Considering the
results of many field experiments and toxicity tests, and the cost of pyreth-
rum, it is believed that a content of 0.2 percent of pyrethrins in oil will
give reasonably satisfactory earworm control under usual conditions. Pyreth-
rum extract having a 20-percent pyrethrin content is available on the open
market. To obtain a 0.2-percent pyrethrin content, 37.9 cubic centimeters,
or approximately 1 fluid ounces, of the pyrethrum extract should be added
per gallon of oil.

Use of Oil Containing Dichloroethyl Ether

Dichloroethyl ether is a colorless liquid having a boiling point of
2930 F., which mixes readily with white oil when shaken or stirred thor-

The results of many field experiments, toxicity tests, and tasting of
treated ears by many persons showed that 2 percent of dichloroethyl ether in
oil is the most satisfactory concentration for use in earworm control in
sweet corn. Oil containing up to 3 percent of dichloroethyl ether caused
injury to the corn ears only infrequently, but greater proportions caused
burning of the husk or rotting of the interior silks or cob tips.' Although
somewhat better protection from earworms was obtained by use of 3 percent of
dichloroethyl ether in oil, the study of the occurrence of residue on the
kernels, as discussed later under the heading "Effect on the Flavor of
Kernels", showed that a 2-percent concentration was safer. The use of more
than 2 percent of dichloroethyl ether in oil, therefore, is not recommended.
Dichloroethyl ether is available on the open market. To obtain a 2-percent
mixture, add 21 fluid ounces of this per gallon of oil.

1 Pepper, Bailey B., and Barber, Geo. W. Dichloroethyl Ether in
Mineral Oil for Corn Earworr Control in Sweet Corn. Jour. Econ. Ent.
33(3): 584-585, 1940.


The effect of the oil-dichloroethyl ether mixture on earworms is
different from that of either the oil alone or of oil containing pyrethrins.
Although this mixture acts as a contact insecticide, the volatilizing di-
chloroethyl ether acts as a fumigant. As it evaporates, the gas penetrates
throughout the interior silk and about the tip of the cob, reaching and
killing larvae that have not been wetted by the mixture. It is believed
that this is the reason why the oil-dichloroethyl ether mixture gives a
higher rate of protection to corn ears than the oil-pyrethrum mixture.

As was the case with the oil-pyrethrum mixture, it is desirable that
the oil-dichloroethyl ether mixture penetrate thoroughly through the interior
silk of corn ears so that all the larvae may be wetted by the mixture or
reached by the evaporating gas. Therefore it is believed that white mineral
oil of viscosity from lO0 to 1500 Saybolt will prove to be the most effec-
tive carrier of dichloroethyl ether.

When Oil Should be Applied

Because the oils interfere with fertilization of the ears when
applied to unpollinated corn silks, they should not be applied until the
ears have been fertilized; that is, not before the silks have become wilted
or before about the third day after silk exposure. Since oil containing
pyrethrins or dichloroethyl ether is effective against larvae in all stages
of growth, it may be applied efficiently several days later than is satis-
factory for oils alone. However, any of the mixtures should be applied
before the larvae reach the kernels at the tips of the cobs, as determined
by the method of sampling previously described. Applications of oil con-
taining insecticides are usually most successful when made during the
period between the time when the silks have wilted and the time when their
tips have turned brown. Since inclusion of insecticides in the oil permits
successful treatment of ears later than was satisfactory when oil alone was
used, or up to about a week after silks are first exposed, a longer time can
be allowed to enable the ears to become completely pollinated before they
are treated. Whether oil alone or oil containing an insecticide is used, the
small kernels at the tips of the cobs, which in some varieties often become
fertilized much later than the uniformly sized kernels of the remainder of
the ears, occasionally fail to develop. It is not believed that this failure
of the tip kernels to develop detracts greatly from the appearance of the
ears or affects adversely their use as roasting ears.

Method of Applying Oil

Oil may be applied to ears as a spray, by atomization, or by use of
an oiler, with results that are reasonably similar. Both spraying and
atomization are wasteful, as they result in the loss of a large percentage
of the oil through run-off. This is important because the oil and insecti-
cides used are comparatively expensive, and if the treatment is to be eco-
nomical the materials must be conserved. For this reason an apparatus has
been developed by means of which oil in measured quantities can be applied to
corn ears without loss of material through run-off. This equipment consists

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of a force oiler of about 1 or 2 gills' capacity, equipped with a long,
slender spout, connected to a 1-gallon can by a hose. The 1-gallon can holds
sufficient oil to treat about 5,000 ears, or as much as can be applied by
one man in half a day, and it serves as a supply tank which feeds oil into
the oiler by gravity. This assembled equipment is shown in figure 1. The
complete equipment may be purchased from manufacturers, or it may be assembled
by growers.

The oilers, which serve in place of nozzles, are variously constructed
by different manufacturers. The oiler shown in figure 1 has been found to be
very satisfactory. It is provided with an interior force pump controlled
by an exterior trigger. When the trigger is squeezed it forces down a
plunger which exerts pressure on a column of oil in a cylinder, and this
oil is forced out through the long spout. Release of the trigger causes
the plunger to rise through the action of a spring located beneath it, caus-
ing oil to flow into the cylinder from the body of the oiler through a
flutter valve which opens only inward, located in the base of the cylinder.
One squeeze of the trigger forces the ejection of a measured quantity of
oil. Oil is prevented from returning from the spout by a ball-and-socket
valve which opens only outward. An adjustable setscrew, bearing a spiral
spring to hold it in proper adjustment, attached to the cover of the oiler
beneath the trigger, regulates the length of the stroke of the latter, and
this in turn controls precisely the quantity of oil ejected by the oiler at
each squeeze of the trigger. It is possible to place marks on this setscrew
indicating the proper position to cause ejections of oil ranging from 0.5 to
1.0 cubic centimeter per trigger squeeze.

A rectangular oil can, such as those in which lubricating oil is sold,
may be used as part of the equipment, and serves as a supply tank. To allow
connection of the supply can to the oiler, a section of A-inch brass pipe
about 2 inches long is let into the base of the oiler and soldered. A
similar piece of pipe is let into the base of the supply can. The oiler and
supply can are then connected by means of a piece of hose about 3 feet long,
preferably of oil-resistant construction. A small hole is punched in the
side of the supply can just under the top rim to serve as a vent. A piece of
fine wire screening may be soldered about this vent if desired. Trash can
not easily enter a vent so placed. The proper location and screening of the
vent are important because drying parts of pollen sacks of the corn tassels,
which are easily shaken loose, may interfere with operation of the oiler if
then enter the oil supply. With one or two straps, pieces of rope, or strips
of cloth attached to the handle of the supply can it may be carried over one
shoulder or in the middle of the back, thus freeing both hands for facili-
tating ear treatment.

To aid in applying the oil to each ear as uniformly as possible, a
metal disc or washer about 3 inch in diameter may be mounted on the spout
of the oiler about inch from the tip. This serves as a gauge of the depth
to which the tip of the spout should be inserted into the ear tip before
injecting the oil. This method of application results in less loss of
material through run-off than occurs when it is applied to the exterior

ST 1y3I X wP


silk, and places the oil where it will be most effective.

When in use the oiler may be filled by simply lowering it whenever
necessary until oil flows into it from the supply tank under the force of
gravity. Experience will enable one to judge by the weight of the oiler
approximately how much oil it contains and when it is necessary to lower it
for refilling. When the ears are located below the level of the supply can
the oil will naturally flow into the oiler more or less -,onstantly, but when
the ears are located at or above the level of the supply can more frequent
lowering of the oiler for refilling is necessary.

The described oiler fits the hand somewhat like a pistol grip and is
not tiring to use for hours at a time. One hand remains free to adjust the
corn plants as the ears are oiled.

Growers having considerable acreage usually have provided themselves
with several oilers assembled as described, and have sent the workers into
the fields in groups under a leader to apply oil to the ears. If the acreage
is large, it may be advisable to plant portions of it at intervals a few days
apart so the ears will come into the proper stage of growth for treatment
over a period sufficiently long to enable the workers to apply the control
to ears in a uniform stage of growth.

Some growers who have small fields have found that oilers can be used
satisfactorily without connection to supply cans. A container of oil is
placed at each end of the field, and the oiler is filled after completing the
treatment of the ears in each row. Oilers that have been used in this way,
some of which have a capacity of about a half pint, are shown in figure 2.
This procedure probably would be satisfactory for treating small plots of
sweet corn grown for home consumption.

Quantity of Oil Needed Per Ear

The oil, or oil containing an insecticide, when applied to the tips
of ears, seeps quickly into the interior silk. The introduction of too much
oil into an ear is likely to cause rotting, especially if it penetrates
too deeply about the kernels. On the other hand, when too little is used
it fails to penetrate through the interior silk sufficiently to reach all
the larvae. Dependable results were obtained when from -1 to 1 cubic centi-
meter (about 4- teaspoonful) of oil was used per ear. This quantity is
sufficient if it is applied by means of an oiler so that little is lost
through run-off. When oil is applied by atomization or spraying there is
no means of determining how much seeps into a given ear and how much is
lost through run-off. By these methods of application, the ears receive
variable doses, resulting in less uniform control and occasional injury
to the ears from the insecticides.


Cost of Applying Oil Containing Insecticides

Studies of the time consumed in oiling showed that, on a conservative
average, ears could be oiled at a rate of 1,500 per hour, or 12,000 per
8-hour day per man. Assuming an average of 9,000 ears per acre, and the
application of 1 cubic centimeter of oil per ear, an acre of corn could
be oiled by one man in 6 hours, using about 2 gallons of oil.

Using oil of 200/2100 viscosity alone, costing about 80 cents per
gallon, and labor at $2.50 per 8-hour day, the total cost of treating one
acre of sweet corn would be $1.60 for oil and $1.88 for labor, or a total
cost of $3.48. With 9,000 ears per acre, or 750 dozen, the cost of oiling
would be about 4 cents per hundred ears, or half a cent per dozen.

At the prevailing price of about $15 per pound for pyrethrum extract
having a 20-percent pyrethrin content, the addition of 0.2 percent of pyreth-
rins to oil increases the cost by about 86 cents per gallon. If white oil
of from 1000 to 1500 viscosity is used, the total cost of the material
should not exceed $1.60 per gallon. With oil containing 0.2 percent of
pyrethrins costing $1.60 per gallon and labor at $2.50 per 8-hour day, the
cost of treatment per acre would be $3.20 for oil containing pyrethrins and
$1.88 for labor, or a total cost of $5.08. With 9,000 ears per acre the
cost of treatment with oil-pyrethrum would be about 51 cents per hundred
ears or about two-thirds cent per dozen.

Dichloroethyl ether is obtainable at a cost of about $2.50 per gallon.
At this cost the addition of 2 percent to oil would increase the cost of
the latter by 5 cents per gallon. Used with oil of from 1000 to 1500
viscosity, the total cost of material would be about 80 cents per gallon,
and the cost of treatment per acre would be approximately that mentioned for
oil alone.

The costs of treatments mentioned are necessarily only approximate,
since the cost of materials will depend on cost of shipment to various loca-
tions, and the cost of labor will be variable. Moderate increases in cost
of the materials, however, will not materially affect the approximate costs
of treating ears per unit of 100 or per dozen that have been mentioned.

The cost of oiling depends particularly on the variety of corn to be
oiled. Thus approximately 80 percent of the ears of the hybrid variety
Golden Cross Bantam can be oiled in one trip through a field because of
the uniformity in time of silking. Under these conditions the cost of
applying oil is least. With open-pollinated varieties, having less uniformity
in time of silking, several trips through a field are necessary in order to
treat ears when in the proper stage of development. Under such conditions
the cost of applying oil becomes correspondingly greater.

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Effect on the Flavor of Kernels

White oil, being colorless, odorless, and tasteless, leaves no
detectable residue on the kernels of treated ears.

Although the addition of 0.2 percent of pyrethrins causes the oil to
become slightly brown, this does not change the appearance of the silks or
kernels of treated ears. A slight odor of pyrethrum may be detected occa-
sionally in the interior silks, but no alteration of flavor of the kernels
has been detected when oil-pyrethrum was used as described herein.

Although dichloroethyl ether can be detected by odor and by taste if
a residue remains on the kernels, if properly used the insecticide usually
evaporateF completely from the ears between treatment and harvest under
usual summer conditions. However, certain precautions should be observed
in the use of this material, and growers should familiarize themselves with
the hazards involved before undertaking treatment of a large acreage with
oil-dichloroethyl ether. The rate of evaporation of dichloroethyl ether
depends on temperature, and applications should be timed so that at least
10 days intervene between treatment and harvest of roasting ears, except
during very warm weather. In locations where, and during periods of the
year when, the average mean temperature of the period of treatment falls
below 650 F., or the night temperature often falls below 600 F., oil-
dichloroethyl ether should be used with caution, and treated ears should be
examined by smelling the silks and tip kernels in order to reject those
having detectable residue before marketing. In general, it has been found
that oil-dichloroethyl ether may be used with less danger of residue remain-
ing on ears at roasting-ear harvest in slowly developing varieties, such as
Maule's Lead-all, than in varieties that develop more rapidly, such as
Golden Cross Bantam. Usually, in the cooler periods of the year, oil-
pyrethrum will prove to be a safer treatment than oil-dichloroethyl ether.
When oil containing an insecticide is applied at the dosage rate and loca-
tion on the ears described herein, residues will probably be detected rarely
and then usually will affect only the tip kernels of treated ears.

Effectiveness of Oiling

Where oil alone is used its effectiveness is correlated with the husk
character of corn ears. In general, the degree of protection against earworm
infestation is comparatively low where husks are short or loose and increases
with the length and tightness of the husks. In ears having tight husks the
earworms penetrate downward through the interior silk largely by eating
their way, whereas in ears having loose husks they may crawl downward between
the silk strands. In tight-husked ears there is a better opportunity for
the oil to come in contact with and kill them. In ears having long husks a
suitable place for establishing and maintaining an oil barrier is provided,
and the oil tends to remain near the tip as a film about the silk strands,
whereas in ears having short husks there may be no place for the oil barrier
to ecome established, and nothing to prevent the oil from spreading downward
about the kernels.

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With the use of insecticides in the oil, extreme husk development
becomes a less important factor than is the case when oil alone is used.
This is because the oil containing pyrethrins kills the worms by contact,
and oil containing dichloroethyl ether kills them by contact or by fumiga-
tion, rather than by smothering them in a barrier of oil located at the tips
of the husks. Therefore when oil containing pyrethrins or dichloroethyl
ether is used it is desirable that it penetrate throughout the interior silk
of ears so thoroughly as to reach all the contained larvae. This is espe-
cially the case where the ears are oiled after the tips of the silks have
become browned, since by this time the worms will have become more widely
dispersed in the interior silks than they are immediately after pollination
has occurred, which was the most satisfactory time for applying oil alone.
The use of oil containing an insecticide has been most successful in varieties
such as Golden Hybrid 3839, in which the husk is uniformly, but not exces-
sively, developed and of an average length of between 2 and 3 inches beyond
the tips of the cobs at roasting-ear stage of growth. In varieties in which
the husks of the ears are excessively developed, such as in the variety
Kancross, the mass of interior silk is so great that, after larvae begin to
disperse within the interior silk, a dosage of 1 cubic centimeter or more
of oil containing an insecticide may be needed per ear to reach and kill a
large percentage of them. In general, it seems that the use of oil contain-
ing an insecticide gives as good earworm control in ears of sweet corn
varieties having husks of moderate length as was obtained with oil alone in
varieties having long husks. Although ears with long husks are not necessary
for successful use of oil containing an insecticide, uniformity of husk
development is very important, for if a portion of the ears have very short
or very loose husks the worms will probably reach the kernels of those ears
before the oil containing an insecticide can safely be applied without inter-
fering with pollination.

Oil alone, oil-pyrethrum, or oil-dichloroethyl ether may be applied
to the ears, and remains effective under all weather conditions, and control
is obtained by a single application per ear.

Although oil alone was not so effective against large as against small
earworms, the oil-pyrethrum or oil-dichloroethyl ether is effective against
larvae of any size. But, since migrating larvae which are repelled from
entering the tips of ears through the treated silks sometimes bore directly
through the husks to reach the kernels, it is important to kill as large a
percentage as possible of the earworms present in a cornfield. It is believed
that this can be accomplished, and the number of migrating larvae can be
kept low, by oiling all the ears in a field, irrespective of whether all of
them will be marketable.

From the results of many field experiments in New Jersey and Florida
during several years, and observations of practical use by growers of sweet
corn, it is believed that under usual conditions the average expectancy of
earworm control in ears of Golden Cross Bantam or similar varieties, when
all ears are infested, is about 60 percent for oil alone, about 75 percent
for oil containing 0.2 percent of pyrethrins, and about 85 to 90 percent
for oil containing 2 percent of dichloroethyl ether.

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In some sections of the country corn ears are attacked by caterpillars
having habits somewhat similar to those of corn earworms. The most common
of these are the fall armyworm, occurring principally in the South but
occasionally invading the North during the fall, and the European corn borer,
occurring in the Northeast. The larvae of these insects often enter corn
ears by way of the silk, but they also feed among the husk leaves or in the
shank, and many of them habitually reach the kernels by boring directly
through the husks. The oil containing insecticides is as effective against
larvae of both these species as against earworms when they enter the ears
through the silks, but it does not reach them, and therefore is not effec-
tive, when they burrow through the husks or other parts of the ears. 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
oiling, one should be certain that he is dealing with earworms only, and that
injury caused by other species is not attributed to this insect.


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