The use of oil for earworm control in sweet corn


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The use of oil for earworm control in sweet corn
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Barber, G. W ( George W )
United States -- Bureau of Entomology and Plant Quarantine
U.S. Dept. of Agriculture, Bureau of Entomology and Plant Quarantine ( Washington )
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aleph - 30268674
oclc - 778632212
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E-476 May 1939
STATE PLANT BOA JA ited States Department of Agriculture
Bureau of Entomology and Plant Quarantine


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


That roasting-ear corn might be protected against injury by the
corn earworm (Heliothis armigera (Hbn.)) by the use of oils was first indi-
cated in investigations made during the summer of 1936. Concentrated study
was given this subject in New Jersey during the summer of 1938 and in south-
ern Florida during the preceding and following winters. The investigations
have not been concluded, and a discussion of the oiling method of control
is presented at this time as a progress report rather than as a recommenda-
tion for its large-scale field use by growers.

The problem of protecting corn ears against earworm attack presented
many difficulties and an important peculiarity. To protect a given ear
it was necessary to effect 100 percent control of the insect. If 100 eggs
were laid on the silks of an ear, it would not suffice to prevent even 99
percent of the resulting earworms from entering the ear, as the one remaining
larva would cause injury almost as objectionable as would be the case had
many entered it. Earworms destroy one another by cannibalism within the
ear anyway, and rarely do more than one or two larvae mature in an ear
or survive sufficiently long to feed on the kernels. Thus, to be success-
ful a control measure must attain perfection in each ear.

The protection of sweet-corn ears is especially difficult because
most varieties are characterized by ears having loose husks, which allow
the larvae to reach the kernels quickly and penetrate to the middle of an
ear, where feeding causes particularly objectionable damage. In such
corn one larva can render an ear unfit for use. Furthermore, sweet-corn
varieties are very attractive to the insect, usually receive a large number
of eggs, and consequently sustain high rates of infestation. Their ordi-
narily loose husks allow the larval population to spread out, and hence
more individuals survive, resulting in severe kernel injury.

Silking Habits of Corn Ears

The corn ear is, at first, a closed package. But to receive pollen
the silks are thrust out of its tip, where they grow for a day or two, and,
when the weather is favorable, are fertilized during the first few days of
exposure. The new silk is extremely delicate and is easily caused to wilt.


Thus, if perfect ears are to be obtained, each strand of silk must receive
its pollen grain, and hence any method of control of earworms, to be suc-
cessful, must not interfere with fertilization. Within a few days after
exposure the silk, having served its purpose, becomes limp and begins to
dry. As by this time all except the tip kernels have been fertilized, con-
trol measures can then be applied without danger of preventing fertiliza-

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 de-
scribed on the third to fifth day after they appear, the period during which
a control measure must be effective is from about 2 to 21 weeks.

Habits of Corn Earworms

Most corn earworm eggs are laid on the silks of a given ear during
the first week after silk exposure, particularly while the silk is fresh
and unpollinated. The newly hatched larvae feed very little on the ex-
terior silk but immediately crawl into the tip of the ear among the silk
strands, where they settle down to feed. Where the insect is abundant,
eggs are sometimes laid on parts of the plants other than the silks, often
before these have become exposed. The resulting larvae, 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 circum-
stances the application of insecticides to the exterior silks may be in-
effective because larvae may already have entered them. On the other hand,
earworms usually do not enter ears in any other way than by way of the silks.

The effect of oiling the ears is to place a barrier of oil in the
mass of silk that is enclosed by the husk above the tip of the cob. Small
larvae already in the silk at the time of oiling are killed, probably by
being smothered, while larvae entering the silk after oil is applied are
either killed or prevented from entering.

Type of Oil Used

Vegetable and mineral oils of many sorts were studied. The most
effective were found to be the light mineral oils of from 150 to 250 vis-
cosity Saybolt at 1000 F. These oils pour at minimum temperatures of
from 100 to 150 F., pass the U. S. P. copper and acid tests, and are color-
less, odorless, and tasteless. Besides being used for summer spraying of
plants, they are used medicinally and, in small quantities, are harmless to
persons. When purchased in 50-gallon drums, these oils cost from about 70
to 85 cents per gallon. Within the range mentioned, it was found that the
viscosity of the oil is less important than other factors.

The results of preliminary experiments in Florida during February and
March, 1939, showed that the addition of 1 percent of pyrethrins, especially
when used as oleoresin of pyrethrum, considerably increased the efficiency
of the oil.

When Oil Should be Applied

Because these 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 until about the third day after
silk exposure, or as soon as the silks become wilted. Because the silks
of different corn varieties have a somewhat different appearance on ears
of similar age, and because some silks may be wilted owing to rains, dry
winds, or other factors before being fertilized, the operator will need some
experience in judging the age of ears by the appearance of their silks. This
can be obtained readily by watching the development of silks on selected

To be successful, oiling not only must be done after the ears are
fertilized, but also before larvae have penetrated deeper than from 12
to 2 inches into the silk. Usually a period of several days intervenes be-
tween fertilization of the silk and deep penetration of the larvae, during
which oil may be applied with effective results.

Method of Applying Oil

Most recent studies seem to show that oil may be applied to ears
as a spray, by atomization, or by the 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 used is comparatively expensive, and if treat-
ment is to be made at low cost this material 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 of a force oiler of about 1 gill capacity,
equipped with a long, slender spout, connected to a 2-gallon knapsack
tank by a hose. The 2-gallon tank holds as much oil as can be applied by
one man in a day, and it serves as a supply tank which, when slight pressure
is provided therein, keeps the oiler filled. This assembled equipment is
shown in figure 1.

The oiler, which serves in place of a nozzle, is provided with an
internal force pump controlled by an external 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, creating a vacuum in
the cylinder, and causing oil to flow into it 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.

To allow connection of the knapsack tank to the oiler, a brass pipe
of 10 millimeters outside diameter, 75 millimeters long, was let into the
cover of the oiler and soldered, as shown in figure 2. Since these oilers


depend on the force exerted in the internal pump only to eject oil, they are
not constructed to stand pressure without leakage, and a leak about the
trigger plunger occurred. This leakage was prevented by sweating a nipple
onto the inside of the screw cover of the oiler at the point where the trigger
plunger passes through it, and attaching a packing box thereto by means of
an adjustable screw. This improvement is shown in figure 3. Finally an
adjustable setscrew, attached to the cover of the oiler beneath the trigger,
regulated the length of the stroke of the latter, and this in turn controlled
precisely the quantity of oil ejected by the oiler per trigger squeeze.
This addition is shown in figure 4. It was possible to place marks on this
setscrew indicating the proper positions to cause ejections of oil ranging
from 0.5 to 1 cubic centimeter per trigger squeeze.

The described oiler fits the hand somewhat like a pistol grip and
is not tiring to handle for hours at a time. One hand remains free to ad-
just the corn plants as the ears are oiled. Less loss of oil occurs when
the spout of the oiler is inserted into the tips of ears from 1/2 to 1 inch
before the oil is ejected.

Quantity of Oil Needed to Protect Ears

The oil, when applied to the tip of an ear, seeps quickly into the
internal silk, sets up a film about the internal silk strands from the tip
of the husk downward, and penetrates at least l- to 2 inches. The intro-
duction of too much oil into an ear is likely to cause rotting, while the
use of too little fails to set up a barrier. Investigations have shown that
dependable results are obtained when from 1/2 to 3/4 cubic centimeter of oil
is used per ear. The use of this quantity assumes the use of the described
knapsack oiler and very little or no loss of oil through run-off.

Cost of Applying Oil

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 appli-
cation of 3/4 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. With oil at 75 cents
per gallon and labor at $3 per 8-hour day, the cost per acre would be $1.50
for oil and $2.25 for labor, or a total of $3.75. With 9,000 ears per acre,
or 750 dozen, the cost of oiling would be approximately a half cent per
dozen ears.

Actually the cost of oiling ears is variable, depending 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 the 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 cor-
respondingly greater.

Effectiveness of Oiling.

It has been found recently that the effectiveness of oil is cor-
related with the husk character of corn ears. In general, as husks are
longer and tighter the degree of protection against earworm infestation
or injury is greater, while as husks are shorter or looser the oil is less
effective. The reasons for the difference in effectiveness of the oil
seem reasonably simple. In ears having tight husks the earworms penetrate
downward through the internal silk largely. by eating their way, while in
ears having loose husks they are able to crawl downward between the silk
strands. In tight-husked ears there is a better opportunity for the oil to
contact and kill them. In ears having long husks a suitable place for es-
tablishing and maintaining an oil barrier is provided, and the oil tends
to remain near the tip as a film about the silk strands, while in ears
having short husks there may be no place for the oil barrier to become es-
tablished or maintained, or the oil may seep downward about the kernels
and thus provide no barrier against the entrance of earworms,

In sweet corn the greatest percentage of uninfested or uninjured
ears obtained experimentally with this method of control was 93.4 and the
least was 48.2 under conditions where all untreated ears were infested and
injured. Under similar conditions an average of about 60 percent of the ears
of Golden Cross Bantam were protected. Of the commercial varieties studied,
the greatest degree of protection against earworm attack or injury was ob-
tained by oiling ears of the variety Kancross. The percentage of unin-
fested or uninjured ears was 89.2.

Many seed corn breeders at present are engaged in an attempt to im-
prove the husk character of sweet-corn ears. Possibly varieties having great-
ly improved husks will be commercially available to growers before long.

Oil may be applied to ears, and remains effective, under all weather
conditions. Control is obtained by a single application per ear. The oil
does not change unfavorably the appearance of the ear or alter the flavor
of the kernels. It is not harmful to consumers or to those applying it.

Oil is not so effective against large as against small earworms,
particularly when the former enter ears after they have been oiled. Thus
under conditions of high earworm population, which is the condition under
which migrating larvae are usually found in numbers, it is important to
reduce the number of larvae that migrate, as partly grown individuals, from
one ear to another. It is believed that the number of migrating larvae
can be kept low by oiling all ears in a field, irrespective of whether all
of the ears will be marketable.

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 (Laphygma frZg, erda (S. & A.)), occurring
principally in the South, but occasionally invading the North during the
fall, and the European corn borer (Pvrausta nubilalis (Hbn.)), 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 reach
the kernels by boring directly through any part of the ear. The oil method
of earworm control seems to be effective against fall armyworm larvae which
enter ears by way of the silk, but it is not effective against many corn
borer larvae which enter ears this way. 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 occasioned
by other species is not attributed to this insect.

'Figure l.-Complete assembly of knapsack oiler. A knapsack pressure tank

of 2-gallon capacity connected to an oiler with improvements.

Figure 2.- Improvement of oiler; showing pipe let into the top of the oiler
to allow connection by means of hose with the knapsack pressure tank.


Figure 3.-Iprovement of oiler; showing nipple and packing box placed
about plunger to prevent leakage when oil in placed under pressurer.

Figure 4.-Improvement of oiler; showing adjustable setscrew, which is

attached to the cover of the oiler and against which the trigger

strikes to control the amount of oil ejected per trigger-squeeze.


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