..**::.:*::i* :* LU R .A R Y
Xfy l4 STATE PLANT BOARD
^y.may 1944 3-619
United States Department of Agriculture
Agricultural Research Administration
Bureau of Entomology and Plant quarantine
STYMUE D1BROMIDB: A SUBSTITUTE FOR PYMTHIUM IN ITISECTICIDAL OIL USED
I FOR CONTROL OF EARWOMS IN SWEZT CORN
Py G. V. Barber, Division of Cereal and Forage Insect Investigations,
and J. Wilcox, Division of Truck Crop and -Garden Insect Investigations
42? A widely used method of control .of the corn earworm in sweet
corn is the mineral oil treatment (i).!I As usually recommended, this
method consists of applying about 374 cubic centimeter of highly
.refined mineral oil, ranging from 100 to 150 seconds Saybolt viscosity
at 1000 F. and containing 0.2 percent of pyrethrins, to the tips of
eaxr about one week after silking, or at a time when the silks have
'. red or their tips have become brown. Scarcity of pyrethrum for use
as an insecticide on cultivated crops, owing to war conditions, has
stimulated search for a satisfactory substitute for use in mineral oil.
Among many chemicals that have been tested as substitutes for
pyrethrnm in oil, styrene dibromide (alpha-beta-dibromethyl-benzene)
has been found to be the most satisfactory. This chemical has been
tested in New Jersey and Texasby Barber (2), in Ohio by Davidson (1),
and in California by Wilcox. b)
Styrene dibromide is a colorless, crystalline chemical having a
mild naphthalenelike odor. It is soluble in and imparts no color to
mineral oil. At concentrations of from 0.5 to 1.5 grams per 100 cc. of
white oil the mixture kills earworms by contact, but they succomb more
slowly than when oil containing 0.2 percent of pyrethrins is used.
Half-grown rabbits ate the silks and tips of the cobs of several ears
treated with 3/4 cc. of oil-styrene dibromlde mixture daily 'v'ithout
The chemical was studied by Barber at concentrations ranging
from 0.5 to 1.5 grams per 100 cc. of oil, by Davidson at a concentra-
tion of 1 gram per 100 cc. of oil, and by Wilcox at concentrations
of 1, 2, and 3 grams per 100 cc. of oil. The average results of apply-
ing oil containing 1 percent of styrene dibromide or oil containing 0.2
percent of pyrethrins to ears of sweet corn were, in percentage of pro-
tected ears, 64.6 and 64.0 percent respectively, in New Jersey in 1941;
78.7 percent for each material in Texas in 1942; 91.7 and 81.6 percent,
respectively, in Ohio in 1943; and 77.1 and 90.8 percent, respectively,
in California in 1943. These results showed that oil containing 1 per-
cent of styrene dibromide gave practically the same protection to sweet
corn against earworms, on an average, as va% obtained by use of oil
containing 0.2 percent of pyrethrins.
y/ Underscored numbers in parentheses refer to Literature Cited, p. 3.
2/ Wilcox. J. Unpublished data from experiments in California during
- 2 -
Although a mild odor of styrene dibromide may be detected in
the silks of treated ears at roasting-ear harvest, tests have shown
that no residue that can be detected either by taste or odor occurs
on the kernels. The mixture caused no discoloration of any part of
the ears and does not cause an increase in the length of the undevel-
oped tip beyond that caused by the oil containing other insecticides.
Styrene dibromide is more satisfactory than dichloroethyl
ether (which has been recommended at a concentration of 2 percent in
oil) because it does not present the hazard of leaving a detectable
residue on the tips of treated ears, such as may be deposited by oil-
dichloroetbyl ether when the temperature of the interval between treat-
ment and harvest is not sufficiently high to cause evaporation of the
dichloroethyl ether from them.
Small quantities of styrene dibromide will be available in 194
for use in mineral oil for earworm control in commercial plantings or
Victory gardens, and this mixture is being prepared for sale by several
concerns dealing in insecticides. It should be applied to the tips of
ears not earlier than 1 week after the silks have been exposed, at a
dosage of 1/2 to 3/4 cubic centimeter per ear, as described in Circu-
lar 657, copies of which may be obtained from the U. S. Department of
In victory gardens, when force oilers which are used to make
accurately measured applications in commercial fields are not obtainable,
the ears of small plantings of sweet corn moy be treated by use of
ordinary medicine droppers, the capacity of which usually is 1 cubic
centimeter. The level of oil in the medicine dropper should be marked
when it contains 20 drops (1/2 cubic centimeter) or 30 drops (3/4 cubic
centimeter), that is, when it is approximately one-half or three-fourths
full. These are the proper amounts of oil containing styrene dibromide
for application to small or large sweet-corn ears, respectively.
In a sweet-corn ear having a husk extension of about 2 inches at
the time of roasting ear harvest, the 1/2-cc. dosage will' usually result
in a barely perceptible increase in the number of undeveloped kernels at
the tip of the cob. Larger dosages per ear or applications to ears
having a shorter husk extension will often cause a small increase in
length of the undeveloped tip of a half inch or more. Although a
moderate increase in the undeveloped tip of the cob does not affect the
marketability of the crop, this loss may be avoided by choosing a sweet-
corn variety that has a uniform husk extension of at least 2 inches, and
by not applying more than 1/2 to 3/4 cubic centimeter of the oil contain-
ing styrene dibromide per ear, depending on whether the ears of the
variety used are small or large. An ear, of course, should never be
oiled more than once.
- 3 -
The rate of control obtained has been found to depend on
several factors, among whichh a vigorous, regular growth of the plants
is the most important. Consequently,' the pl;-nits should be ,given suffi-
cient plant food and. frequent watering in orOder that the.- nay reach
their greatest perfection. In well-watered anid fertilized fields when
every ear is attacked. by several earforms, an average of about 90
percent of the ears have been protected by aiylication of the mineral-
oil treatment. In proportion as the fields are less well fertilized
or suffer from drought, the rate of control is progressively less.
(1) Barber, G. W.
1942. Mineral-oil treatment of sweet corn for earworm control.
U. S. Dept. Agr. Circular 657, 15 pp.
(2) Barber, G. W.
1943- Styrene dibromide as a substitute for pyrethrumw in oil
for corn earworm control. Jour. Econ. Ent. 36: 330-2.
(3) Davidson, Balph H.
1943k The relative effectiveness of some corn earworm, control
measures in sweet corn. Jo-ur. Econ. Ent. 36: 938.
UNIVERSITY OF FLORIDA
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