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Efficacy of Insecticides to Control
Alfalfa Weevil Larvae in Illinois,
2000 |
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John T. Shaw and Kevin Steffey |
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| Summary |
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A trial
to evaluate the efficacy of insecticides
for control of alfalfa weevil larvae
was established at the University of
Illinois Crop Sciences Research &
Education Center, Urbana, Illinois,
in 2000. The numbers of alfalfa weevil
larvae per sweep in the untreated control
plots were significantly higher on all
three days after treatment (DAT) than
the numbers of alfalfa weevil larvae
in all insecticide-treated plots. No
significant differences in numbers of
alfalfa weevil larvae occurred among
insecticide-treated plots. All treatments
reduced the population of alfalfa weevil
larvae by 86 to 95% within 3 DAT. Baythroid
2EC, Mustang 1.5EC, and Warrior T 1SC
all provided greater than 90% control
for the duration of the study. |
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| Plot
Information and Methods |
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Plot
Location
University of Illinois Crop Sciences
Research & Education Center, Field
NC-900, Urbana, Illinois
Plot Size
40 ft x 50 ft for each treatment in
each replication, with 20-ft alleys
between each replication.
Experimental Design
Randomized complete block with four
replications, with an untreated control
in each replication
Alfalfa Cultivar
Target II
Pre-treatment Infestation
All plots were sampled on April 29,
just before treatment. We found an average
of 1.3 alfalfa weevil larvae (second
and third instars) per stem (n = 100
stems picked at random), and an average
of 98.75 larvae per 20 sweeps (n = 80).
The average stem height on April 29
was approximately 21 in.
Environmental
Conditions at Time of Application
Wind - light with 6'8 mph gusts
Canopy - wet
Air temperature - 72oF.
Method of Insecticide Application
A 20-ft boom was mounted by a 3-point
hitch to the rear of an International
140 tractor (boom height = 17 in above
the crop canopy). Twelve flat fan spray
tips (XR TeeJet 8002VS, Spraying Systems
Co.) were spaced 20 in apart on the
boom. A compressed-air system was calibrated
to deliver 14.4 gpa at a pressure of
28 psi at a speed of 2.5 mph.
Post-Treatment Samples
Twenty sweep-net samples were taken
from each plot 3, 7 and 10 days after
treatment (DAT). The standard 15-in-diameter
sweep net was used for all sampling.
Phytotoxicity
By 7 DAT the NuFos 4E treatment had
turned many of the alfalfa plants white.
Most plants survived but were stunted
by 14 DAT.
Weather Information
Refer to Appendix A, Table
A.1
Statistical Analysis
Data were analyzed with Agriculture
Research Manager (ARM) version 6.1.6,
from Gylling Data Management, Inc. (GDM).
Means were separated by Duncan's New
Multiple Range Test MRT (P = 0.05).
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| Results
and Discussion |
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Before
the application of insecticides, the
mean number of alfalfa weevil larvae
(second and third instars) per stem
in the trial area was 1.3. Numbers of
alfalfa weevil larvae in the designated
plot areas did not differ significantly.
The mean numbers of alfalfa weevil larvae
per sweep for all treatments throughout
the duration of the study are presented
in Table 1.1. Percentages of control
of alfalfa weevil larvae are presented
in Table 1.2. The numbers of alfalfa
weevil larvae in the untreated control
plots increased from 120.5 per sweep
on 3 DAT to more than 220 per sweep
on both 7 and 10 DAT (Table 1.1). The
numbers of alfalfa weevil larvae per
sweep in the untreated control plots
were significantly higher on all three
sampling dates than the numbers of alfalfa
weevil larvae in all insecticide-treated
plots. No significant differences in
numbers of alfalfa weevil larvae occurred
among insecticide-treated plots. By
3 DAT, all insecticides had reduced
densities of alfalfa weevil larvae by
86% or more (Table 1.2). Baythroid 2EC,
Mustang 1.5EC, and Warrior T 1SC all
provided greater than 90% control for
the duration of the study |
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| Table
1.1. |
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Mean number
of alfalfa weevil larvae (all instars
combined) per 20 sweeps 3, 7 and 10
days after treatment (DAT), Urbana,
Illinois, 2000 |
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Treatment |
Rate1
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Larvae
per 20 sweeps2
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| 3
DAT |
7
DAT |
10
DAT |
| Warrior
T 1SC |
0.025
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6.25
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b |
10.25
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b |
10.25
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b |
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Mustang 1.5EC |
0.044 |
8.00
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b |
8.75
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b |
7.75
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b |
| Baythroid
2EC |
0.037
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8.50
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b |
3.00
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b |
6.00
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b |
| NuFos
4E |
0.500
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16.75
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b |
49.00
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b |
35.75
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b |
|
Control |
-- |
120.50
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a |
2210
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a |
227.50
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a |
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1Rates
are specified as lb (AI)/acre.
2Means in a column followed
by the same letter are not significantly
different (P = 0.05; Duncan‰̃ªs New
MRT). |
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| Table
1.2. |
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Percentage
control of alfalfa weevil larvae (all
instars combined) for 3, 7, and 10 sampling
days after treatment (DAT), Urbana,
Illinois, 2000 |
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Treatment |
Rate1
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% Control2
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3 DAT
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7 DAT
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10 DAT
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| Warrior
T 1SC |
0.025
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95
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95
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95
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| Mustang
1.5EC |
0.044
|
93
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96
|
97
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| Baythroid
2EC |
0.037
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93
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99
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97
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| NuFos
4E |
0.500
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86
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78
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84
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| Control
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0
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0
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0
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1Rates are specified
as lb (AI)/acre.
2Percentage control is based
on the numbers of larvae per sweep compared
with the number of larvae in the untreated
control on each of the DAT. |
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