United States Department of Agriculture
Agricultural Research Administration
Bureau of Entomology and Plant Quarantine
C01PARISM OF FOUR INSECTICIDES, INCLUDING DDT, IN MIIV J C>
FOR CONTROL OF THE CON1 EARWC IN DENT SMED CORMT
By R. A. Blanchard and A. F. Satterthwait,
Division of Cereal and Forage Insect Investigations
Inbred lines of corn used in making commercial single crosses
are often severely damaged by the corn earworm (Heliothis amAo), as well as by diseases that develop in the ears allowing earworn attack, The extent of such damage is dependent' upon the susceptibility of any particular line to earworm and disease, and also upon the number of larvae entering the ear and the time of their entrance.
In Illinois the earworm may cause damage to corn either in the roasting-ear stage or in the maturing ears. Diseases are more often associated with the first type. Loss from the second type is due to feeding by the larvae on the outer parts of the kernels, or to their burrowing beneath the kernels so that they are lost in harvesting. Kernels with any appreciable surface feeding must be discarded.
The method recommended by Barber and coworkers (1-_4) and
Davidson (5) for the control of the earworm in sweet corn grown for roasting ears or for seed is the injection of mineral oil containing pyrethrum, dichloroethyl ether, or styrene dibromide into the silk mass just above the tips of the ears at about the time the silks begin to turn brown. The degree of control obtained by this treatment in dent corn being grown for seed has been questionable. Tests comparing DDT (l-trichloro-2,2-bis(p-chlorophenyl)ethane) with the other insecticides were therefore made at Decatur, Ill., in 1544, in an effort to obtain a greater reduction of worm damage. It was already known, from experiments conducted by Ivy (6), Johnson (2), and Stevenson, Sheets, a Breazeale (9), that DDT is highly toxic to the corn earworm. It was also known that the degree of control from the injection of oil plus ar insecticide depends on the length and tightness of the husks as well as
on the amount of oil injected. On the other hand, it was kno- tht excessive amounts of oil prevent development of kernels on the tips of the ears. The experiments were therefore designed to take all these factors into account.
Eight inbred lines, four long.-husked and four short-husked,
with husks ranging from loose to very tight, were planted in triplicate. All the plots were planted and cared for by the Bear HKbrids Corn Company on its seed-production farm at Decatur. On the same farm the long- and short-husk conditions were simulated in one inbred line, U.S. 197-2, by planting it in duplicate and clipping back the husks of one planting to the tips of the ears. One replication of all eight lines was destroyed by chinch bugs (Blissus leucopterus (Say)).
Two highly refined mineral oils were used, Bayol (90-90 seconds Saybolt) and Superla No. 13 (120-125 seconds Saybolt). Each insecticide was tested in both oils, in the following concentrations: DDT 2 percent, pyrethrum extract (0.2 percent pyrethrins), dichloroethyl ether 2 percent, and styrene dibromide 1 percent. They were apnlied at two dosages, 0.6 and 1.2 ml. of solution per ear.
Each plot was 6 hills long, with a maximum of 3 plants per hill.
The ears of 3 hills were treated with the insecticide dissolved in Bayol, and the others with the insecticide in Superla No. 13. The number of ears in each replic-ftion ranged from 4 to 9, with an average of 6.3. As is shown later, the type of oil did not appear to influence the results, and thus the average size of the sample in each replicp.tion that could be used for comparing the various dosages was increased to 12.6 ears.
The corn was harvested at the normal time for gathering seed.
Ears from each plot were examined to determine the number of immature kernels per ear damaged b v the earworm and disease, and the number of mature kernels per ear damaged by the earworm. The effect of DDT on germination of the seed was also determined. Table 1 summarizes the average percentage of infested ears per plot per inbred and the average loss of kernels on all inbred lines in each treatment and in the untreated checks. It also gives the combined averages of both oils and the two rates of treatment. In table 2 the results obtained fromi the four insecticides inboth oils and at two rates of treatment are averaged according to inbred line and husk type in order to show the effects of length and tightness of husk on control.
Table 1.-Effect of injection of insecticides in mineral oil into ok
corn for control of the corn earworm. Small-plot oxeortmentL,
and 1.2 ml. of oil solution per ear, 1944
Estimated kernels lost per ear due to Ears infested arworm and disease Earworm in
Treatment 6 1 2 ml in green corn maturing corn
0.6 ml.,1.2 l 0.6 ml. 1.2 ml. 0.6 ml. 1.2 ial. 0
Percent Percent hNumber Number Number Number ?-i
Bayol 16.7 12.3 2.~4 2.5 0.04 0.1 0.
Superla 17.2 11.4 3.1 2.6 0 0
Av. 14.4 2.7 .04
Bayol 149.7 49.3 12.2 8.7 1.0 1.8
Superla 51.1 47.0 14.1 8.8 .9 .7
Av. 49-3 11.0 1.1
Bayol 68.1 58.1 15.1 8.8 1.4 2.3
Superla 67.6 58.0 12.0 8.5 .5 1.4
Av. 63.0 11.1 1.4
Bayol 59.2 35.0 10.7 5.3 .5 0.6
Superla 61.2 49.3 16.4 6.3 1.0 1.7
Av. 51.2 9.7 1.0
Checks (untreated 92.12J 19.0 2.3
l/ Average represents larvae on one replicaticn.
2/ An average of 106.9 ears of each inbred line examined. There e
plots in each replication.
Table 2.-Zffects of length and tightness of the husk covering of dent corn on
control of the corn earworm by injection of insecticides in oil into the
silk. Average of all ears injected with the four insecticides in both
oils at both dosages.
Tyne of husk TInfested ears Kernels damaged by earworm
coverage Inbred lines I and disease
Treated Untreate eduction Treated1Utreated Feduction
Percent Percent Percent Number Number Number
Long, loose U.S. '17-2 61.3 97.7 37.3 17.2 -39.1 56.0
Ill. R4 1/ 25.0 9S.2 71.5 .I.0 15.5 74.2
Av. 447 98.0 54.4 10.6 27.3 65.
Short, loose Ind. 38-11 53-3 96.9 39.7 10.1 20.1 49.8
ilm9- 8. 51.2 6,6 15056,0
u.S. 17-2 2/ '.0 96.4 29.5 21.7 35.6 09.0
Av. 53.7 98.2 39.8 12.8 23.6 14.3
Long, tight Kans. K 6,4 93-5 93.2 .7 11.6 94.0
Long, moderately tight Ind. Wf9 44.5 97.1 148.9 7.1 i.4 61.4
Short, moderately tight Ill. Ht 55.2 97.3 43.3 10.0 20.6 51.2
Average, all lines 44.6 92.3 51.7 9.7 22.0 55.9
l/ Although classed as loose-husked, the husk sticks to the ear surface in places
2/ Husks clipped bacl-Ae tips of ears.
Commercial-Scal e Experiment
The Bear Hybrids Corn Company also treated by injection the ears on
two rows in each of two commercial foundation seed fields with 2 percent of DDT in Superla No. 13 oil and on two other rows with 3 percent of dichloroethyl ether in the same oil. Two rows in each field were left untreated as
checks. The treatments were made by the regular crew in the course of
their application of dichloroethyl ether in oil to these fields. The solutions were injected at the rate of 0.75 ml. per ear.
At harvesttime five 25- to 30-ear samples were taken from each area
and examined in the same manner as in the small-plot experiment. In addition the company processed the seed from each area separately, and determined the relative amounts of labor involved in sorting the ears, as well
as the approximate amounts of damaged corn. The results are shown in
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DDT was the only insecticide which protected the corn until. harvesttine. DDT in mineral oil gave almost complete control of the corn earwori in both the green and the maturing stages of the
corn ears in both experiments, Some of the slight damage shown undoubtedly resulted from larvae already established before the insecticide in oil was applied. In the small-plot experiment most of the damage in the DDT plots occurred in one inbred line, U.S. 197-2. Pyrethrum, styrene dibromide, and dichioroethyl ether were all less effective than DDT. Styrene dibromide ranked second to DDT.
In the small-scale experiment the 1.2-ml. dosage was, in
general, superior to the 0.6-mi. dosage. The greatest increase in control resulting from the larger dosage occurred with styrene dibromide in long, loose-husked lines such as U.S. 197-2. None of the insecticides adversely affected germination.
The 0.6-mi. dosage did not appear to reduce the development of the tips of the esrs appreciably, whereas the 1.2-ml. dosage caused some loss of tip kernels, especially in the tight-husked line Kansas V)4. This loss is thought to have been due to the oils, as none of the insecticides tested appeared to retard kernel development. The dosage of the solution should be limited to 0.6 mi. in very tight-husked lines and increased in lines with loose husks. The best control was obtained in the long, tight-husked line 1)4. The
reduction in numbers of kernels damaged -was somewhat greater in the long than in the short, loose-husked lines. Tightness of husk seemed more important than length for a high degree of control. There did not appear to be any significant difference in the control obtained with the two oils.
In 19)44 by far the greatest saving in corn resulted from
prevention of the diseases that enter the ears following earworm attack. This does not apply to those diseases that enter the ears
through the shank.
Of the four insecticides tested, DDT appears, at present, to be the most ideally suited for use in controlling the earworm in dent corn grown for seed. This insecticide produced no visible effects on the operators, whereas styrene dibromide is very irritating to some persons*
(1) Barber, G. W.
1942. Mineral-oil treatment of sweet corn for e~
control. U.S. Dept. Agr., Cir. 657, 15 pp.
(2) ------19433 Styrene dibromide as a substitute for pyrethrum in
oil for earworm control. Jour. Econ. Ent. 36:330-332.
(3) --- and Shull, W. E.
(n.d.) Control of the corn earworm in seed sweet corn.
Idaho Agr. Col. War Cir. 20, )4 pp., illus.
(4) ---------- and Wilcox, J.
1944. Styrene dibromids: A substitute for pyrethrum in
insecticidal oil used for control of earworms in
sweet corn. U. S. Bur. Ent. and Plant Qpar. 1-619,
3 pp. (Processed.)
(5) Davidson, R. R.
1943. The relative effectiveness of some corn earworm control
measures in sweet corn. (Scientific Note) Jour. Econ.
(6) Ivy, 3. z.
19441. Tests pith DDT on the more important cotton insects.
(Scientific Note) Jour. con. Ent. 37'142.
(7) Johnson, G. V.
1944. MDT for control of the tomato fruitworm. (Scientific
Note) Jour. Econ. Ent. 37:151-152.
(8) Stevenson, W. A., Sheets, L., and Breazeale, J. M.
194. Tests with DDT against pentatomids, mirlds, the bollworm,
and the cotton aphid. (Scientific Note) Jour. Econ.
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