Can
Aphid Control Reduce Barley Yellow Dwarf Incidence In
Wheat? A Case Study (Caldwell Co., KY 1998-99)
Doug
Johnson and Lee Townsend
Department of Entomology
Pioneer
2510 wheat was planted using a no-till planter on Oct 22, 1998 following a corn
crop on the
Significant
differences in percentages of plants displaying BYD symptoms, as related to
insecticide treatments, were detected (F (3,12 df) = 3.83, Pr>F =0.039) (Table 1). Although very few
aphids were seen before the final insecticide application; they were widespread
and numerous during the spring.
|
TABLE
1. MEAN PERCENTAGES (± S.E.) OF WHEAT PLANTS SHOWING BYD SYMPTOMS IN PLOTS
TREATED WITH WARRIOR INSECTICIDE ON SELECTED DATES TO CONTROL APHID
VECTORS OF BARLEY YELLOW DWARF VIRUS. | |
|
Time
of Application |
%
of Plants Showing BYD Symptoms ± SE1 |
|
No
Insecticide |
13.2
± 5.0 a |
|
24
Nov 98 |
5.6
± 1.0 ab |
|
24
Nov 98 and |
1.6
± 0.4 b |
|
17
Feb 99 |
3.2
± 1.2 b |
1Means
followed by the same letter are not significantly different. p = 0.5. Ryan-Einot-Gabriel-Welsch Multiple range test.
Variations
in plant stands among plots due to establishment problems prevented valid yield
comparisons. The variation due to stand difficulties would not have allowed a
fair comparison of the yield effects.
The
November treatment, often made as an 'insecticide only' application, costs about
$11.00 per acre. The February insecticide application is often made in
conjunction with other inputs, so the application cost may be saved. Therefore,
in this location and in this year, the fall, winter, and combination treatments
would have cost $11.00, $6.00 and $17.00 respectively.
Assuming
the entire difference in percentage of plants showing BYD symptoms was a result
of insecticide timing, and that a damaged plant would have about a 20% yield
loss, we can compare the relative merits of treating -vs- not treating.
No
Insecticide Treatment
Using
an estimate of 13.2% damaged plants with a 20% yield reduction for each damaged
plant, the effective yield loss was calculated to be
2.64%. If this were 100 bu/acre
wheat, the resulting loss would be 2.6 bushels. At a price of
$2.50/bushel, the untreated acre of wheat would bring about (97.4 bu at $2.50/bu) $243.50 or a loss of $6.60 per acre due to
this aphid-vectored disease.
Nov 24
& Feb 17 Insecticide Treatment
The
best insecticide treatment (two applications) contained an average of 1.65%
damaged plants. This indicates that about 88% of the loss to BYD was prevented
by the two treatments. As calculated above, this is a 0.3% yield loss per acre.
For 100 bu/acre wheat, this loss would be 0.3 bushel,
leaving a per acre yield of 99.7 bushels. At $2.50/bu the resulting loss would
be $0.75, bringing a per acre return of (99.7 bu at
$2.50/bu) $249.25. However, this level of protection was obtained by making two
insecticide applications, at a cost of about $17.00 per acre. Reducing the per acre return by this cost leaves a net return of
($249.25 - $17.00) $232.25.
Nov 24
Only Insecticide Treatment
The
Nov 24 treatment had 5.6% damaged plants. Assuming the standard plant yield
loss, this is the equivalent of a 1.1% yield loss per acre. For 100 bu/acre wheat, this loss would be 1.1 bushels, leaving a per
acre yield of 98.9 bushels. At $2.50/bu the resulting loss would be $2.75,
bringing a per acre return of (98.9 bu at $2.50 /bu) $247.25. However, this level of protection was obtained
by making an insecticide applications which would cost
about $11.00 per acre. Reducing the per acre return by
this cost leaves a net return of ($247.25 - $11.00) $236.25.
Feb 17
Only Insecticide Treatment
The
incidence of damaged plants in the Feb 17 treatment was 3.2%. For 100 bu/acre wheat, this loss would be 0.6 bushels, leaving a per
acre yield of 99.4 bushels. At $2.50/bu the resulting loss would be $1.50
bringing a per acre return of (99.4 bu at $2.50/bu)
$248.50. However, this level of protection was obtained by making an insecticide applications which would cost about $6.00 per
acre. Reducing the per acre return by this cost leaves
a net return of ($248.50 - $6.00) $242.50.
SUMMARY:
Under
these test conditions, the insecticide applications did cause statistically
significant differences in BYDV symptom expression. However, it is clear that
the assumed associated protection of yields resulting from this level of symptom
reduction was not cost effective. If all other things are equal, the cost of the
insecticide applications was greater than the reduction in damage (Table 2).
|
TABLE
2. NET RETURN ($/AC) FROM PLOTS TREATED AT SELECTED TIMES WITH AN
INSECTICIDE APPLICATION TO CONTROL APHID VECTORS OF BYDV IN | ||||
|
Treatment |
No-Insect. |
Nov
24 |
Nov
24 |
Feb
17 |
|
Net
Ret/ac |
$243.50 |
$232.25 |
$236.25 |
$242.50 |
The
circumstances and yield potential on your farm will alter these figures. As
prices and yields decline and treatment costs increase, the insecticide
treatments will look even less appealing. However, a rise in prices and yields
coupled with a lower treatment costs will make the returns from insecticide
applications look much more favorable.
Choosing
a 100 bushel per acre yield as a basis for comparison may be misleading.
'Intensive Wheat Management' has used 100 bushels as a benchmark; however, many
fields will not support this level of production. When yields
change so do the level of expenses that can be supported. Using the
percent damage estimates, and assumed costs of control from the previous
examples we have calculated the necessary value of a bushel of wheat needed to
support the three treatments at various yield levels, using the BYD intensity
seen in the 1998 experiment (Table 3).
|
TABLE
3. THE VALUE ($) OF A BUSHEL OF WHEAT REQUIRED TO OFFSET
| |||
|
Potential
Yield |
Fall
Treatment |
Winter
Treatment |
Fall
& Winter Treatments |
|
100 |
7.23 |
3.00 |
7.35 |
|
90 |
8.03 |
3.33 |
8.17 |
|
80 |
9.04 |
3.75 |
9.19 |
|
70 |
10.33 |
4.29 |
10.49 |
|
60 |
12.06 |
5.00 |
12.23 |
|
50 |
14.47 |
6.00 |
14.66 |
|
40 |
18.09 |
7.50 |
18.47 |
|
30 |
24.12 |
10.00 |
24.64 |
There
is no consistently successful strategy to reduce losses to BYD virus by trying
to control their aphid vectors with insecticidal sprays. While sprays may kill
many aphids and reduce the percentage of infected plants, potential yield
savings may not pay for the chemical and application. There are many other
factors that impact the relative effect of BYDV
infections.
BYDV
infections developed very late in the 1998-1999 crop,
probably because of very low aphid numbers during the fall. The aphids that were
present did not arrive until December. The late aphid flight probably resulted
from the late summer-early fall drought that affected
The
lateness of the aphid/BYDV infections is illustrated by the fact that the late
winter (Feb. 17) application was just as effective at reducing BYDV symptoms as
either of the other two applications (Table 1). A larger than "normal" portion
of the infections occurred after Feekes GS 3. Because
of this, the data presented in Table 3 must be used very carefully. If you
consider only Table 3, it appears that the most appropriate time to make an
insecticide application is in the late winter. While this was true in 1998-99,
this may not be the case in most years. If both aphids and BYDV had been present
very early in the fall, the percentage of infected plants and the relative
damage to each would have been much greater. While late infections may be
important in a year of good prices and low costs, an early fall infection is
always a more important consideration.
ACKNOWLEDGMENTS:
The
authors express their gratitude to Dr.'s Don Hershman
(Plant Pathology) and Lloyd Murdock (Agronomy) for their review of this
publication. We also especially appreciate the time and work of Dr. Dick Trimble
(Ag. Economic) in proofing and challenging our economic
arguments.
From