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Trial: 2002-1SBAseedearl,
2002-1SBAseedlate
Seed applied insecticide control of the Soybean Aphid (Aphis glycines) and Bean Beetle (Cerotoma trifurcata).
Bruce
Potter, University of Minnesota
Background
The soybean aphid (SBA)
is a new pest of Minnesota soybeans. It was first identified during the late
summer of 2000 in SE MN. SBA subsequently spread throughout the soybean growing
areas of MN during the 2001 growing season.
The bean leaf beetle (BLB)
is a native soybean insect pest whose populations have increased over the
past few years in response to favorable wintering conditions. This insect
is capable of transmitting bean pod mottle virus (BPMV). This virus has previously
been documented but at low levels in Southwest Minnesota soybeans.
A trial was conducted during
the 2002 growing season to: 1) Evaluate rates of two neonicotinoid insecticide
seed treatments, Cruiser (thiamethoxam, Syngenta Crop Protection, Inc.) and
Gaucho (imadochloprid, Gustafson LLC), on SBA and BLB 2) Compare seed treatment
SBA control to foliar applications of pyrethroid (Warior, l-cyhalothrin, Syngenta
Crop Protection, Inc.) and organophosphate (Lorsban 4E, chlorpyrifos, Dow
AgroSciences, LLC) insecticides.
Site and application information
The
trial was placed near a grove with a high density of buckthorn, Rhamnus spp. on the University of Minnesota
Southwest Research and Outreach Center (U of M SWROC). This area was also
presumed to provide an overwintering site for SBA. Additionally, the trial
was near an area where SBA “hotspots” occurred in August 2001. The wooded
area was expected to produce a high level of overwintering BLB adults. This
field has a history of iron deficiency chlorosis and soybean cyst nematode
and an attempt to chose a soybean cultivar tolerant to these conditions was
made.
Two
planting dates for the insecticide seed treatments were used to: 1) evaluate
the duration of control and 2) try to minimize expected overwintered bean
leaf beetle pressure in the late planting.
Cooperator: University of
Minnesota Southwest Research and Outreach Center
County: Redwood County
Nearest
town: Lamberton, MN
Soil
type: Canisteo clay loam and Knoke silty clay loam
Cultivar: NK S18-U9
Row
spacing: 30”
Seeding
rate: 175,000 with cones on JD Maxemerge units.
Planting
date: May 2, 2002(early planting) and May 23,2002 late planting
Experimental
design: randomized complete block, 4 replications
Plot
size: 10’X30’
Application
date: Post emerge insecticide application date (July 23,2002 late planting)
Crop
stage at post application: R3
Soybean
aphid population at application: 0.8 aphids/upper expanded trifoliate
Insecticide
treatments were applied with a CO2 pressurized backpack sprayer, 8002XR flat
fan nozzles on 18-inch spacing, 20 gallons/acre, 35 PSI.
Results and discussion
In
spite of variety selection, soil property and soybean cyst nematode induced
chlorosis and stunting of soybeans occurred in this trial. Two of the 4 replications of both the early
(south rep 3 and 4) and late (north rep 1 and 2) planting dates were affected
by chlorosis and the reduced growth doubtless increased the variability in
this study.
Plant growth:
No
phytotoxicity was observed for any of the seed treatments.
Soybean
emergence was very slow in the early-planted portion of the trial, beginning
12 days after planting with emergence continuing over the next 21 days. Faster
emergence was observed for both compounds at all rates at the early planting
date. Although a trend still existed for greater stand where seed applied
insecticides were used, statistical differences no longer present at the VC
stage after additional plants emerged. Stand differences were not observed
with the late planting date (Table 1, Figure 1).
The basis for these stand differences is unclear but may be due to
seed corn maggot as damage from these insects was observed in the trial.
In
the early planting date portion of the trial differences in height were observed
between untreated and insecticide treated plots. The higher rate of Gaucho
and Cruiser were taller than lower rates (Table 1, Figure 2). Bean pod mottle virus was detected in these
trials and is discussed under bean leaf beetle. Height differences were fairly
uniform within plots and are likely related to a factor(s) other than BPMV.
Height differences were not observed in the later planted portion of the trial
and were not rated in those plots.
Soybean aphid
Soybean
aphid was first detected in these plots on June 19, one of the first 2002
SBA observations in Minnesota. Soybean
aphid populations, however, were slow to increase in this study.
Aphid
populations were estimated by examining trifoliate leaves from 20 randomly
selected plants in the center two rows of each plot. Leaves were selected from the portion of the soybean canopy with
the greatest SBA density. Aphid densities were greatest on the uppermost expanded
trifoliate for early sampling dates. The exception was the August 16 sampling
date in the late-planted trial. In this case mid canopy leaves (terminal –5
to 7 nodes) were sampled.
Aphid
populations in the early-planted portion of the trial were very low throughout
the growing season (maximum of 1.3 aphids/ trifoliate leaf July 23-31). SBA
populations in the untreated checks at insecticide application averaged <
1 aphid/ upper trifoliate and populations increased late in the season (14
aphids/mid trifoliate August 16) in the late planted portion of the trial.
Aggregated,
low SBA population densities make interpreting this SBA control data difficult.
To facilitate analysis of variance (ANOVA), aphid counts were transformed
to a 1-10 scale. Differences (p<0.1) in aphid populations
between seed treatments and the untreated check were observed on July 23 (R3
stage) of the early planting date. With the possible exception of the low
(20g) rate of Cruiser all seed treatments preformed similarly with 54 to 76
percent less aphids than untreated. These differences had disappeared by July
31(R4 stage) and were not significant at either date when the data were transformed
(Table 2a).
In
the late planting date, the same trend for seed treatments to have lower SBA
populations than the untreated plots was observed (Table 2b). Foliar insecticide
treatments were applied July 23 to this planting date. No differences between seed treatments were
observed on that date and SBA populations were extremely low (1.0 aphids /
trifoliate). Percent control is based on the number of aphids/leaf data (not
shown). Plots with insecticide seed
treatments had 20-77% and 0-30% fewer aphids than untreated plots on August
1 and August 16 respectively. Seed
treatments allowed more aphids than Warrior at 0.015 and 0.025 lbs AI/acre
or Lorsban 4E at 0.05 lbs AI/acre as foliar insecticide treatments. Foliar treatments had 92 to > 99% l and
75 to 95% less SBA than untreated plots at 9 and 24 DAT. Aphid populations in untreated plots had declined
after 24 DAT.
The
20g rate of Cruiser and 62 gram rate of Gaucho had equal or higher levels
of SBA aphid than untreated in late season samples of both planting dates. This indicates that insecticide concentrations
in soybean plants had declined. Early season aphid control may have led to
lower populations of SBA aphid predators and parasites in these plots. In
turn, lower natural enemy populations and low in-plant insecticides might
allow rapid SBA increases during late season.
Bean leaf beetle
Overwintering
BLB adults were assessed by examining 20 plants (2 sets of 5 plants) and the
adjacent soil surface from each of the center two plot rows. Defoliation caused by bean leaf beetle was
estimated visually on the unifoliate and first trifoliate leaves of these
same 20 plants.
Bean
leaf beetle populations were higher in the early (May 2nd) planted
portion of the trial as expected. At no time were beetle populations greater
than 0.3 BLB/plant. Bean leaf beetle
populations were higher in untreated plots (p<0.05), which averaged 0.32
BLB/plant on May 29 and declined thereafter.
Dead BLB were observed in the insecticide treated plots, which ranged
from 0.03 to 0.10 BLB/plant on the May 29 sampling. Evidence of a rate response within products was observed (Table
3,Table 4) with the higher rates of both products allowing less defoliation.
BLB control lessened by the second trifoliate where only the 30g and 50g rate
of Cruiser had less defoliation then untreated (Table 3). No significant differences between insecticide
treated and untreated were detected by the R3 stage.
Bean
leaf beetle populations were at much lower levels in the May 23 planting date.
All insecticide seed treatments had significantly less defoliation than the
untreated (Table 4).
Defoliation
injury levels and BLB populations were well below levels that are assumed
to cause economic injury. However,
at flowering, a high incidence of virus symptoms (Figure 3) appeared in the
May 2nd planted portions of this trial. Fifty randomly selected
plants from the center two plot rows were used to assess virus incidence. Only those plants with obvious symptoms (mottling and distortion
of leaves) were counted as positive. Incidence of virus symptoms was significantly
higher greater in the untreated plots (Table 4). Expression of virus symptoms was lower and
symptoms were subtler in the chlorotic 1st and 2nd replication
and accounts in part for the high CV values. Differences between insecticide treatments
and rates were not observed. The virus
was suspected to be bean pod mottle virus (BPMV) because of its association
with bean leaf beetle activity. Serology
tests, subsequent to the visual ratings, confirmed the presence of BPMV in
symptomatic plants.
Virus
symptoms were at very low levels in the May 23 planted portion of this trial
and as a result not rated. The differences in BPMV incidence by planting date
combined with low levels BPMV symptoms in insecticide treated plots indicates
that transmission probably occurred soon after the beetles moved to the early
planted plots.
Yield
Iron
deficiency chlorosis impacted yields in these trials and caused obvious yield
reduction in two of the four replications.
Yield data for the May 2 planting date is presented in table 5 for
the entire trial Additionally yields/treatment
using only the two non-chlorosis affected replications are shown. With the
former, differences occurred at the 10% but not the 5% level.
As a result of few degrees of freedom, no differences were detected
in the latter.
Yields
in general, corresponded to the plant height differences observed between
insecticide treated and untreated plots and
indicates that early season beetle feeding and/or bean pod mottle virus reduced yield. Analysis
of viral symptoms (seed discoloration) by treatment has not yet been completed.
Aphid pressure was low in this planting date and probably did not reduce
yield
Chlorosis
was more severe in the late planting date. Stepwise regression indicates that
chlorosis scores explain more than 85% of yield. Likewise aphid populations
were negatively correlated with chlorosis scores.
Yields
are presented in table 6 for the two non-chlorotic replications only. For comparison, the SBA per leaf data at 9
and 16 DAT in the same plots is included.
Yields tend to correspond to the 9 DAT aphid populations. Foliar treatments
provide the best aphid control and tended to yield higher. The trend for higher
yield in may also be due to in part to late season bean leaf beetle control.
The high C.V. values are stark indicators of the problems in accurately sampling
SBA populations.
Chlorosis
and low, non-uniform aphid populations caused tremendous variability in this
trial. Therefore, in spite of trends that may appear in these data, it is
best to refrain from drawing many conclusions.
Acknowledgments: Thanks to Scott
Anderson, Derek Erickson, and Tim Lendt, for aphid counting and application
assistance.
Table
1.
Soybean seed applied insecticides effect on stand.
U of M SWROC,
Lamberton, MN 2002
Means within columns followed by the
same letter(s) are not significantly different (p <0.05), least significant
difference (LSD), Duncan’s New MRT.
1 Soybean plant population was estimated from the center two
rows of each plot.
2 Not currently labeled on soybeans
3 Heights based on 5 plants from center 2 rows of each plot
Figure
1.
Figure
2. Height differential between
untreated (left) and insecticide treated (right) soybeans, May 23 planting date.
U of M SWROC, Lamberton, MN 2002.
Means within columns followed by the same
letter(s) are not significantly different (p <0.10), least significant
difference (LSD), Duncan’s New MRT.
Table
2b. Insecticide seed treatment control of Soybean
aphid (05/23/02 planting) - Lamberton, MN 2002
Means within columns followed by the same
letter(s) are not significantly different (p <0.05), least significant
difference (LSD), Duncan’s New MRT.
1 Soybean aphid density was estimated by counting the number
of aphids /trifoliate leaf from 20 randomly selected plants in the center two
rows of each/plot. Leaves were selected from the portion of the canopy most
heavily colonized at each sampling date. Aphid density was greatest on
uppermost-expanded trifoliate leaves for all sampling dates but the August 16
sampling of the May 23 planting date.
In this case, the mid canopy
(terminal – 5-7 nodes) was sampled.
2 Not currently labeled on soybeans
3Aphid counts/trifoliate leaf were transformed to the
following rating scale for analysis of variance: 1) no SBA, 2) >0 -1 SBA, 3)
>1-2 SBA, 4)>2-4 SBA, 5)>4-8 SBA, 6)>8-16 SBA, 7)>16-32 SBA,
8)>32-64 SBA, 9)>64-128 SBA, 10) >128 SBA.
Table
3.Insecticide seed treatments effect on overwintering and F1 BLB (05/02/02)
planting.
Lamberton, MN 2002.
Means within columns
followed by the same letter(s) are not significantly different (p <0.05),
least significant difference (LSD), Duncan’s New MRT.
* Non - homozygous variances, LSD values and
non-transformed data shown for illustrative purposes only.
1 Bean leaf beetle damage was visually estimated on the
unifoliate, 1st trifoliate, and 2nd trifoliate leaves (where expanded) leaves
of 20 randomly selected plants in the center two rows of each/plot ( 2 sets of
5 consecutive plants in each row). F1
beetle damage based on 20 randomly selected plants in the center two rows of
each plot. Upper expanded trifoliates were recorded as positive when BLB
occurred.
2 Not currently labeled on soybeans
Table
4. Insecticide seed treatment effect on over wintering bean leaf beetles and
bean pod mottle virus. Lamberton,
MN 2002.
Means within columns
followed by the same letter(s) are not significantly different (p <0.05),
least significant difference (LSD), Duncan’s New MRT.
* Mean separations
based on transformed data (arcsine sqrt percent).
1 Bean leaf beetle damage was visually estimated on leaves of
20 plants in the center two rows of each plot (2 sets of 5 consecutive plants
in each row). Data shown are average of
unifoliate and 1st trifoliate defoliation.
2 Not currently labeled on soybeans
3 Bean pod mottle virus was assessed visually on 25 plants
from each of center two plot rows. Only plants with obvious virus symptoms
(leaf mottling and distortion) were counted as positive,
Figure
4. Soybean expressing Bean pod mottle virus symptoms on upper foliage.
U
of M SWROC 2002
Table
5.
Soybean seed applied insecticides effect on yield.
U
of M SWROC, Lamberton, MN 2002
Means within columns
followed by the same letter(s) are not significantly different (p <0.10),
least significant difference (LSD), Duncan’s New MRT.
2 Not currently labeled on soybeans
Table
6.
Soybean seed applied insecticides effect on yield. May 23 planting
date. Means calculated from the two non-chlorotic replications only.
U
of M SWROC, Lamberton, MN 2002
Means within columns
followed by the same letter(s) are not significantly different (p <0.10),
least significant difference (LSD), Duncan’s New MRT.
1 Data transformed (arcsine sqrt
percent). Means reported as non-transformed, LSD transformed.
2
Not currently labeled on soybeans
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This page was created 12/9/03 by B. Potter with technical assistance from M. Werner.