2007 Extension Research Report
Insect Management
EFFICACY OF SILK CHANNEL TREATMENTS WITH OIL FOR
CONTROL OF CORN EARWORM IN SWEET CORN
Alton N. Sparks, Jr., University of Georgia Cooperative Extension, Department of Entomology,
Tifton, GA 31793, asparks@uga.edu
from each treatment date were examined for
damage to the tip of the ear, presence of
caterpillar pests and quality of the ear. Damage
was rated on a 0 to 3 scale as follows: 0 = no
damage at the tip of the ear (referred to none), 1
= less than 5 kernels damaged at the tip (referred
to as light damage), 2 = greater than 5 kernels
damaged but damage restricted to the outer one
inch of the ear (referred to as tip damage), 3 =
damage extending from the tip down greater than
one inch (referred to as ear damage). Any
caterpillars present were identified, classified as
small (<1/4 in), medium (>1/4 in and < ½ in), or
large (>½ in), and counted. Oil placed into the silk
channel has been reported to effect pollination and
ear development. To evaluate this potential effect,
ears were rated for ‘quality’ on a 1 to 4 scale as
follows: 1 = kernel development to the tip of the
ear; 2 = obvious impact on kernel development at
the tip of the ear but overall minor impact; 3 =
numerous kernels undeveloped, likely making ear
unmarketable; 4 = less than half of the kernels
developing. Data were not analyzed as true
replication was not conducted.
Introduction
Corn earworm is the primary pest of
sweet corn in Georgia, with the most severe
damage occurring on tips of the corn ears. Moths
lay eggs on emerging silks and the hatching larvae
eat their way down the silk channel to the tip of
the ear. Once inside the ear they are protected
from common control strategies, and ear
protection is generally provided by frequent
insecticide applications for control of the larvae
prior to entering the ear. Application of oil into
the silk channel has been recommended as an
alternative to insecticide applications for this pest.
Although this technique is labor intensive, it may
offer an alternative for organic growers and others
with limited pesticide options. This demonstration
was conducted to gain experience with this
technique for corn earworm control.
Materials and Methods
This test was conducted at the University
of Georgia’s Tifton Vegetable Park in Tifton,
Georgia. The corn plants used in the experiment
were originally used for a pre-tassel insecticide
efficacy trail. The corn, var. Serendipity, was
planted 18 Aug, 2006. Plots were treated during
the whorl stage with a variety of treatments, with
the last application on 20 Sept. First silk
emergence was noted on 1 Oct. Oil treatments
were applied on 3, 6 and 9 Oct. Individual ears
were not monitored for exact date of silk
emergence; thus, treatment dates are indicated as
approximate days after silk emergence. On each
date, approximately 60 ears were treated. Each ear
had 0.5 ml of corn oil placed at the top of the silk
channel with a repeating pipette. Each ear was
marked with a tag indicating the date of treatment.
At harvest maturity (25 Oct.), 50 ears
Results and Discussion
Silk treatments with oil appeared to
provide minimal protection of the ears from corn
earworm, with the possible exception of the 8
days after silk emergence treatment (Table 1). The
8 DASE treatment had only 8 of 50 eras rated
with tip or ear damage, while all other treatments
had 19 or 20 ears rated with this type of damage.
Efficacy of the two earlier treatments may have
been impacted by an irrigation event on 5 Oct.
This irrigation may have removed some oil from
the 2 DASE treatment and may have placed water
into the silk channel that may have interfered with
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the 5 DASE treatment. However, corn earworm
data indicate a possible effect with both the 5 and
8 DASE treatment, with lower corn earworm
densities in these treatments (Table 2). Fall
armyworm densities showed some trend for
similar reductions with the 5 and 8 DASE
treatments. The most obvious effect of treatments
was on ear quality. The earlier treatments were
applied, the higher the number of ears rated with
significant damage (rated 3 or higher)(Table 3).
The potential for oil treatments to protect
corn ears from corn earworm was obvious from
field observations. It was not uncommon to find
larvae that had eaten down the silks to the location
of the oil and had feed no farther or had actually
died. Additionally, efficacy of this approach is
reported to improve with addition of B.t.
insecticide to the oil. The potential impact of
irrigation or rain would an obvious concern given
the water requirements of this crop. The most
obvious effect in this test was that of negative
impacts on ear quality if the oil treatment is
applied too early in ear development. This
approach is generally recommended with
application at 5 to 7 days after silk emergence,
which showed much less negative impact on ear
development. The minimal impact on ear quality
would likely have little impact on ear acceptance
in the markets in which this approach would be
considered (organic production, road-side
markets, etc.). This approach is far too labor
intensive and provides inadequate control for
typical commercial production.
Table 1. Ear damage ratings in sweet corn, silk channel oil treatment test, Tifton, Georgia, 2006.
Treatment
Timing*
Number of ears (of 25) within tip damage rating category
No damage
Light damage
Tip Damage
Ear Damage
2 DASE
30
1
12
7
5 DASE
28
2
12
8
8 DASE
40
2
6
2
2
9
10
None
29
* approximate days after silk emergence.
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Table 2. Caterpillar densities in silk channel oil treatment test, Tifton, Georgia, 2006.
Treatment
Timing*
Corn earworm larvae per 25 ears
Fall armyworm larvae per 25 ears
Small
Med.
Large
Total
Small
Med.
Large
Total
2 DASE
2
6
3
11
3
7
6
16
5 DASE
1
2
2
5
0
3
9
12
8 DASE
0
2
1
3
1
4
7
12
None
2
6
3
* approximate days after silk emergence.
11
1
9
13
23
Table 3. Ear quality ratings, silk channel oil treatment test, Tifton, Georgia, 2006.
Treatment
Timing*
Number of ears (of 25) within Ear Quality Ratings
1
1.5
2
2.5
3
3.5
4
2 DASE
1
4
4
2
22
3
14
5 DASE
0
1
3
28
18
0
0
8 DASE
3
6
15
16
10
0
0
14
5
3
0
0
None
9
19
* approximate days after silk emergence.
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