Trial: 2003-SBASEED
Seed applied insecticide efficacy against the Soybean Aphid (Aphis glycines) and Bean leaf beetle (Cerotoma trifurcata)
Bruce Potter,
University of Minnesota Department of Entomology and Southwest Research and
Outreach Center
Wayne Hansen, University of Minnesota Extension Service
Background
The soybean aphid (SBA) is a relatively
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 with localized yield limiting populations
in 2002. Bean leaf beetle and bean pod mottle virus transmitted by the beetle
have been a problem in Southwest Minnesota the past couple of years. Management of both insects is problematic. Soybean
aphids are unpredictable in timing and location of occurrence. Additionally, the potential for extremely rapid population increase
makes timely treatment difficult. In the case of bean leaf beetle, multiple
and overlapping generations make timing of scouting and insecticide applications
difficult. It is unclear weather seed applied insecticides will be effective
in preventing early season bean leaf beetle injury and associated virus transmission.
The relationship between aphid populations and soybean yield is not well understood. Quantifying
the relationship between aphid population density and yield loss is a necessary
first step in the development of workable economic thresholds.
Objective
This trial was designed to examine: 1)
soybean aphid control with several seed and foliar applied insecticides 2)
yield impacts from varied soybean aphid populations by manipulating product and
rates of seed applied insecticide (currently not labeled) and foliar applied
insecticides and 3) effect of seed applied insecticides on over-wintering bean
leaf beetle populations.
Methods
The
trial was planted at the University of Minnesota, Southwest Research and
Outreach Center near Lamberton, Redwood County, Minnesota. Site and application
details are shown below. Several farm
woodlots and the adjacent Cottonwood River area contain buckthorn, Rhamnus sp, the winter host for soybean
aphids. For the second year, soybean
aphids, presumed to be the result of overwintered eggs, were observed in June. These very low initial populations were
severely limited by a June 23 hailstorm.
Soybeans suffered an average of 43% defoliation at that slowed
development, produced callusing and loss of apical meristems or much more
rarely mortality. The soybeans, however,
had recovered well by mid-July. The
2003 growing season was very dry late in the season with 2.96 inches of
rainfall in July and August compared to a 7.07-inch average with below normal
growing degree day accumulations.
Beginning
on or near July 18th a large immigration of alatoid (winged) aphids
occurred. Alate populations averaged
nearly 1/plant with associated nymphs causing a rapid increase of populations
(Figure 1).
County: Redwood, MN
Nearest
town: Lamberton, MN
Soil
type: Normania loam
Fertility:
P(bray)- 19 ppm,
K – 173 ppm,
pH 5.8
Previous
crop: Corn
Tillage: Disk 2x Fall 2002
Field
cultivator 2x 4/23/03
Cultivar: Cropplan CR1976 (roundup ready
cyst nematode resisistant)
Planting
date: May 16, 2003
Seeding
rate: 175,000 seeds/acre
Row
spacing: 30”
Weed
control:Trifluralin @ 1.5 pt./acre PPI
4/23/03
Roundup Weathermax @ 22 oz./acre
6/03/03 and 7/9/03 POST
Design:
Randomized complete block with 4 replications
Plot
size: 10’ x 30 ’
Application
date(s): 1) Seed applied (Rival/Allegience, Gaucho 480, Poncho 600, Cruiser 5)
2) Foliar
July 25
Wind
conditions: Calm
Crop
stage at foliar application:
R3
Gustafson
R&D, McKinney, TX, applied insecticide seed treatments to seed supplied by
the SWROC. Foliar insecticide treatments were applied with an offset boom
sprayer (R&D Sprayers) with 8002XR flat fan nozzles on 18-inch spacing, 15
gallons/acre, 28 PSI.
Soybean aphid
populations were estimated by whole plant aphid counts on 5 plants/plot. SBA
densities were initially greatest on the upper canopy but by R4 stage lower
leaves were also being colonized. Data were transformed to a 1-10 scale to
perform an analysis of variance (ANOVA) and means were separated by Duncan’s
new multiple range test.
Aphid-days
or the number of aphids/plant x number of days aphids present were calculated
for each plot to provide an estimate of both intensity and duration of soybean
aphid pressure. Aphid days = [(SBA/plant
sample date 1 + SBA/plant sample date 2)/2 * days between samples]. For
example, 25 aphids/plant for 4 day and 50 aphids/plant for 2 days would both
equal 100 aphid days.
Seed
size was determined by weighing a 200 seed sample from each plot.
This
trial was planted late to avoid bean leaf beetle. However, this insect did occur in this trial and injury was
noted. Bean leaf beetle damage was
estimated based on the percentage of plants expressing bean pod mottle virus
symptoms and the percentage of seed showing bean pod mottle symptoms in a 200 seed
sample. Protein and oil content will also be determined from each plot.
Results and
discussion
All
seed insecticide treatments had less overwintering bean leaf beetle injury
based on % plants with feeding damage and feeding sites/plant than those with
no insecticide. The low (625 mg/kg rate of Poncho) had significantly more
injury than other insecticide seed treatments.
First and second-generation bean leaf beetle populations in this study
were low. Bean leaf beetle injury was recorded June
12th on VC stage soybeans as % plants injured and average number
of feeding sites/plant. Virus symptomatic plants were 1% or less in
all treatments. Differences in bean pod mottle virus transmission,
as measured by mottled seed, were not detected between treatments. This may
be in part due to low incidence (2.5 % or less seed w/symptoms). Additionally,
this method is problematic under high soybean aphid populations as mottled
seed could potentially be caused by soybean mosaic or other viruses transmitted
by aphids.
Soybean
yield and soybean aphid populations are shown in table 2. Aphid assessments
were not continued after August 15th due to severe lower leaf loss
in untreated plots and declining aphid populations. At this date, the July
25 foliar applications of Baythroid and Warrior had significantly fewer aphids
than other treatments. Aphid populations
for both Poncho treatments were not significantly different from the untreated. The
Cruiser and Gaucho treatments were intermediate in performance (Aphid populations
for Cruiser and Gaucho were significantly lower than the untreated, but significantly
higher than the foliar applications. Accumulated aphid days and yield differences
were similar to late season aphid populations with Cruiser and Gaucho intermediate
between foliar insecticides and Poncho treatments. As opposed to early season bean leaf beetle
control, the 1250-mg/kg rate of Poncho did not perform better than the 625
mg/kg rate for SBA control, and the trend was for higher aphid populations
later in the season with both treatment levels. The reason for this is unclear but perhaps it may reflect slightly
better early infestation growth favoring SBA populations.
Soybean
plants in untreated and seed-applied insecticide plots were shorter than those
receiving foliar insecticide treatments. Treatments
that accumulated the largest number of aphids matured earlier.
Yields
related well to estimated aphid – days (Table1) and a regression of aphid –days
on percent yield loss was highly significant with an r2 of > .95
(Figure 3). Forcing the line through 0%
yield loss yields the equation: % yield loss = 0.0012 * aphid-days. If this
relationship is consistent over multiple environments and over SBA
phenology/crop stage relationships, a relatively precise economic threshold
might be determined.
These
data suggest that seed applied insecticide treatments (if labeled) can provide
control of overwintering bean leaf beetle and early season suppression of
soybean aphid. Soybean aphid control
with well-timed foliar insecticides is significantly better than seed applications
tested. Seed applied insecticides
differ in performance against soybean aphid. They probably will not provide
adequate stand-alone protection for soybean aphid long enough in the season,
especially under conditions where aphids continue migrating into the field. Bean
leaf beetle may require a 1st generation treatment in July for
best control of bean pod mottle virus.
Acknowledgments:
Many
thanks to Derek Erickson, Mellisa Olsem, and Mark Anderson for aphid counting.
Yield data would not have been possible without the intrepid SWROC mobile
plot crew of Steve Quiring and Jeff Irlbeck with additional assistance from
Gene “Beancounter” Hildreth.
The
information given in this publication is for educational purposes only.
Reference to commercial products is made with the understanding that no
discrimination is intended and no endorsement by the University of Minnesota
any specific product(s) used in this implied.
Table
1. The effect of seed applied
insecticide on bean leaf beetle. University of Minnesota – Southwest Research
and Outreach Center, Lamberton, MN. (2003). B. Potter and Wayne Hansen.
Table 2.
Relationship of insecticide on soybean aphid populations and
on soybean yield. University of
Minnesota- Southwest Research and Outreach Center, Lamberton, MN (2003).
B. Potter and W. Hansen
Figure 1.
Figure 2.
