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Economic Threshold for Soybean Aphid (Hemiptera: Aphididae) Economic Threshold for Soybean Aphid (Hemiptera: Aphididae)
D. W. Ragsdale
University of Minnesota
, ragsd001@umn.edu
B. P. McCornack
University of Minnesota
R. C. Venette
U.S. Forest Service
B. D. Potter
University of Minnesota
I. V. Macrae
University of Minnesota
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Ragsdale, D. W.; McCornack, B. P.; Venette, R. C.; Potter, B. D.; Macrae, I. V.; Hodgson, E. W.; O'Neal, M. E.;
Johnson, K. D.; O'Neil, R. J.; Difonzo, C. D.; Hunt, Thomas E.; Glogoza, P. A.; and Cullen, E. M., "Economic
Threshold for Soybean Aphid (Hemiptera: Aphididae)" (2007).
Faculty Publications: Department of
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. 297.
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D. W. Ragsdale, B. P. McCornack, R. C. Venette, B. D. Potter, I. V. Macrae, E. W. Hodgson, M. E. O'Neal, K. D.
Johnson, R. J. O'Neil, C. D. Difonzo, Thomas E. Hunt, P. A. Glogoza, and E. M. Cullen
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entomologyfacpub/297
FIELD AND FORAGE CROPS
Economic Threshold for Soybean Aphid (Hemiptera: Aphididae)
D. W. RAGSDALE,
1
B. P. MCCORNACK, R. C. VENETTE,
2
B. D. POTTER,
3
I. V. MACRAE,
E. W. HODGSON, M. E. O’NEAL,
4
K. D. JOHNSON,
4
R. J. O’NEIL,
5
C. D. DIFONZO,
6
T. E. HUNT,
7
P. A. GLOGOZA,
8
AND E. M. CULLEN
9
Department of Entomology, University of Minnesota, 219 Hodson Hall, 1980 Folwell Avenue, St. Paul, MN 55108
J. Econ. Entomol. 100(4): 1258Ð1267 (2007)
ABSTRACT Soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), reached damaging
levels in 2003 and 2005 in soybean, Glycine max (L.) Merrill, in most northern U.S. states and Canadian
provinces, and it has become one of the most important pests of soybean throughout the North Central
region. A common experimental protocol was adopted by participants in six states who provided data
from 19 yield-loss experiments conducted over a 3-yr period. Population doubling times for Þeld
populations of soybean aphid averaged 6.8 d ⫾ 0.8 d (mean ⫾ SEM). The average economic threshold
(ET) over all control costs, market values, and yield was 273 ⫾ 38 (mean ⫾ 95% conÞdence interval
[CI], range 111Ð567) aphids per plant. This ET provides a 7-d lead time before aphid populations are
expected to exceed the economic injury level (EIL) of 674 ⫾ 95 (mean ⫾ 95% CI, range 275Ð1,399)
aphids per plant. Peak aphid density in 18 of the 19 location-years occurred during soybean growth
stages R3 (beginning pod formation) to R5 (full size pod) with a single data set having aphid
populations peaking at R6 (full size green seed). The ET developed here is strongly supported through
soybean growth stage R5. Setting an ET at lower aphid densities increases the risk to producers by
treating an aphid population that is growing too slowly to exceed the EIL in 7 d, eliminates generalist
predators, and exposes a larger portion of the soybean aphid population to selection by insecticides,
which could lead to development of insecticide resistance.
KEY WORDS yield loss, population dynamics, invasive species
The soybean aphid, Aphis glycines Matsumura
(Hemiptera: Aphididae), is native to Asia, and it has
caused substantial damage to soybean, Glycine max
(L.) Merr., in North America since its conÞrmed oc-
currence in August 2000 (Ragsdale et al. 2004). At
present, the soybean aphid is the most signiÞcant in-
sect threat to soybean production in North America.
In China and in other parts of Asia, this insect is only
an occasional pest of soybean, and when plants are
colonized by soybean aphid in early vegetative growth
stage, yield loss in excess of 50% can occur (Wang et
al. 1994). In Minnesota, soybean aphid outbreaks are
associated with a reduction in plant height, pod num-
ber, seed size and quality, and yield (Ostlie 2001). The
damage potential at low-to-moderate aphid densities
is less clear, but soybean aphid feeding is known to
disrupt the photosynthetic processes at relatively low
aphid densities (Macedo et al. 2003). Soybean aphid is
also a vector of numerous plant viruses (Clark and
Perry 2002, Davis et al. 2005), which can further limit
soybean yield and seed quality.
Aphid population declines in annual cropping sys-
tems are attributed to variable host plant quality (e.g.,
physiological age and antibiosis), increased activities
of natural enemies, and weather extremes (van den
Berg et al. 1997, Fox et al. 2004, Karley et al. 2004, Li
et al. 2004). In controlled environments, soybean
aphid populations can double in 1.5 d (McCornack et
al. 2004), but these high intrinsic rates of increase are
only obtainable under ideal conditions where popu-
lation growth is not constrained by host quality, effects
of weather, or natural enemies. Soybean aphid biology
and the speciÞc conditions that trigger rapid increases
in population densities that are associated with yield
reductions are not well understood in North America
(Ragsdale et al. 2004). However, Þeld estimates of
soybean aphid population growth rates are less than
1
Corresponding author, e-mail: ragsd001@umn.edu.
2
U.S. Forest Service, North Central Research Station, 1561 Lindig
Ave., St. Paul, MN 55108.
3
Southwest Research & Outreach Center, University of Minnesota,
Lamberton, MN 56152.
4
Department of Entomology, Iowa State University, Ames, IA
50011.
5
Department of Entomology, Purdue University, 901 W. State St.,
West Lafayette, IN 47907.
6
Department of Entomology, Michigan State University, 243 Nat-
ural Science Bldg., East Lansing, MI 48824.
7
Department of Entomology, University of Nebraska-Lincoln,
Haskell Agricultural Laboratory, 57905 866 Rd., Concord, NE 68728.
8
University of Minnesota, Regional Extension Center-Moorhead,
715 11th St. N, Suite 107C Moorhead, MN 56560.
9
Department of Entomology, 536 Russell Labs, University of Wis-
consin-Madison, 1630 Linden Dr., Madison, WI 53706.
0022-0493/07/1258Ð1267$04.00/0 䉷 2007 Entomological Society of America
the theoretical intrinsic rate of increase (Costamagna
and Landis 2006). Therefore, basing an economic
threshold (ET) on population doubling times derived
from laboratory experiments that occurred in the ab-
sence of any environmental resistance will result in an
artiÞcially low economic threshold. Such an economic
threshold based on laboratory derived intrinsic rate of
increase has been calculated (Olson and Badibanga
2005a), resulting in a threshold of three aphids per
plant, which in their model had the highest economic
return. Such a threshold is not realistic, because it
assumes that the multiple sources of environmental
resistance would not prevent exponential growth of
soybean aphid populations.
The objective of this study was to quantify the
relationship between aphid densities and yield loss
under Þeld conditions in which biotic and abiotic
factors were allowed to inßuence soybean aphid den-
sities. These data were used to estimate the aphid
density at which control measures should be applied
to prevent yield losses. ETs and economic injury levels
(EIL) were developed based on current expected
yields, control costs, and market values for U.S. soy-
bean.
Materials and Methods
Field Plot Design. In 2003, 2004, and 2005, a com-
mon experimental protocol was used at sites located in
six states (Iowa, Michigan, Minnesota, Nebraska,
North Dakota, and Wisconsin), so that comparisons
could be made across locations and years (19 location-
years). At each location, a soybean variety was se-
lected that was adapted for that area, and it was
planted from mid- to late May (Table 1). Plots were
3.0 m in width (four rows) by 12.3 m in length with
a 76.2-cm (30-in.) row spacing. We used predeter-
mined, targeted aphid population densities based on
cumulative aphid-days (CAD) of 0, 2,000, 4,000, 8,000,
12,000, 16,000, and an untreated control (⫽maximum
CAD) as treatments. Cumulative aphid-days is a single
value that provides a measure of aphid abundance
over time, and it can be calculated weekly as sampling
occurs. We calculated CAD by using the procedures
outlined in HanaÞ et al. (1989).
Insecticide treatments varied among locations and
years and depended largely upon the natural level of
aphid infestation in any given location-year. Each tar-
get aphid density was replicated a minimum of four
times within each location-year, and treatments were
arranged in a randomized block design. With the ex-
ception of one location-year in Minnesota in 2003
where the study was located in a commercial produc-
tion Þeld, fallow ground of ⬇3 m surrounded each plot
to facilitate application of insecticide to individual
plots, minimize spray drift among plots, and encourage
uniform aphid colonization throughout the experi-
ment (DiFonzo et al. 1996, Hodgson et al. 2005). Soy-
bean aphids were allowed to naturally colonize the
Þeld except in Nebraska in 2004 where soybean aphids
were seeded into plots by using Þeld-collected aphids
from a nearby Þeld. In the Nebraska plots, an expand-
ing trifoliolate containing three to Þve aphids was
excised and placed on approximately one plant per 30
cm of row within each plot on 23 July 2004. The timing
of this artiÞcial infestation matched the general ap-
pearance and density of aphids in most Nebraska soy-
bean Þelds. In all location-years, a foliar insecticide,
lambda-cyhalothrin at 16.8 Ð28.0 g (AI)/ha (Warrior
with Zeon Technology, Syngenta Crop Protection,
Greensboro, NC), was applied to all plots in a given
treatment by using ground equipment once a target
aphid density in a treatment in terms of CAD was
reached (averaged across all blocks). In all cases, in-
secticides were applied within 2 d after aphid counts
were completed. If soybean aphid populations began
to increase after the initial insecticide application,
additional applications were applied to prevent aphid
populations from increasing.
Aphid Sampling and Soybean Yield. Nondestruc-
tive whole-plant samples were taken to enumerate the
total number of aphids per plant. To detect small
populations early in the season, up to 20 plants per plot
were inspected. As the season progressed and the
frequency of encountering plants with aphids in-
creased to 50%, 10 plants per plot were sampled. When
⬎80% of plants were aphid infested, Þve plants were
counted per plot at each sampling date. For analysis,
all data were converted to mean number of aphids per
plant per plot. Soybean growth stages (Fehr and Cavi-
ness 1977), whether vegetative or reproductive, were
noted each week.
Yield was estimated by harvesting the entire middle
two rows of each plot with a small-plot combine and
adjusting seed moisture to 13%. Linear regression
(PROC REG; SAS Institute 2001) was then used to
relate percentage yield reduction to CAD; slope and
intercept estimates were used in all EIL calculations.
Values Used in Calculation of an Economic Injury
Level. Cost estimates for insecticide and application
costs, market value, and expected yield were used to
calculate an EIL for soybean aphid. A gain threshold
(GT) expressed in percentage yield loss was calcu-
lated by estimating control costs (C) [$/ha] divided
by estimated market value (V) [$/ton] by using var-
ious yield potentials (Y) [tons/ha] (Pedigo et al.
1986), which is equivalent to
GT 共% yield loss兲 ⫽
C
V ⫻ Y
⫻ 100 [1]
Average retail price of representative insecticides reg-
istered for soybean aphid control and their associated
application costs were obtained from an informal
phone survey of multiple local elevators along with
published sources (Dobbins et al. 2004, WASS 2004,
Edwards and Smith 2005). Average soybean prices
from 2000 to 2005 were obtained from the National
Agriculture Statistical Services (NASS 2006). Finally,
soybean yield potentials used in the calculation of the
GT represent the range of long-term average soybean
yield throughout the North Central growing region
(NASS 2006).
August 2007 R
AGSDALE ET AL.: ECONOMIC THRESHOLD FOR SOYBEAN APHID 1259
Table 1. Soybean aphid population growth rates for the untreated controls in each of the 19 location-years
Location (city, state)
Planting
date
Brand; variety
Julian
start date
a
Julian
end date
b
Population
growth rate
c
(r) ⫾ SEM
Intercept
d
⫾ SEM
R
2
P value
Discrete daily
growth rate
(
)
Doubling
time(d)
e
Rosemount, MN 21 May 2003 NK; S19-V2(RR) 189 209 0.192 ⫾ 0.010 ⫺33.9 ⫾ 2.0 0.995 0.003 1.211 3.6
Rosemount, MN 3 June 2003 NK; S19-V2(RR) 189 209 0.229 ⫾ 0.024 ⫺40.1 ⫾ 4.7 0.979 0.010 1.257 3.0
Lamberton, MN, Þeld 1 10 May 2003 Thompson; 7227CR 200 234 0.198 ⫾ 0.016 ⫺38.1 ⫾ 3.5 0.974 ⬍0.001 1.219 3.5
Lamberton, MN, Þeld 2 16 May 2003 Cropplan; R1976 199 227 0.261 ⫾ 0.031 ⫺50.5 ⫾ 6.8 0.957 0.004 1.298 2.7
New Ulm, MN 5 May 2003 Stine; 1918-4 193 220 0.074 ⫾ 0.022 ⫺9.5 ⫾ 4.6 0.733 0.030 1.077 9.4
Prosper, ND 24 May 2003 Asgrow; AG0801(RR) 202 230 0.144 ⫾ 0.006 ⫺27.9 ⫾ 1.3 0.995 ⬍0.001 1.155 4.8
Rosemount, MN, Þeld 1 28 May 2004 Pioneer; 91B91(RR) 209 223 0.113 ⫾ 0.012 ⫺22.0 ⫾ 2.6 0.988 0.069 1.119 6.2
Rosemount, MN, Þeld 2 28 May 2004 Pioneer; 91B91(RR) 209 223 0.143 ⫾ 0.024 ⫺19.0 ⫾ 11.9 0.781 0.310 1.110 6.7
Rosemount, MN, Þeld 3 28 May 2004 NK; S19-R5(RR) 179 214 0.143 ⫾ 0.024 ⫺25.2 ⫾ 4.7 0.901 0.004 1.153 4.9
Concord, NE 29 May 2004 Asgrow; 2703 207 242 0.139 ⫾ 0.018 ⫺25.5 ⫾ 4.1 0.934 0.002 1.149 5.0
Chariton, IA 10 May 2005 Stine; 3532-4 209 249 0.071 ⫾ 0.022 ⫺11.9 ⫾ 5.0 0.783 0.046 1.073 9.8
24 May 2005 Stine; 3532-4 209 249 0.067 ⫾ 0.024 ⫺10.8 ⫾ 5.6 0.715 0.071 1.069 10.4
Ames, IA 23 May 2005 Prairie Brand; PB-2183RR 202 241 0.077 ⫾ 0.009 ⫺11.8 ⫾ 2.0 0.944 0.001 1.080 9.0
16 June 2005 Prairie Brand; PB-2183RR 202 241 0.085 ⫾ 0.014 ⫺14.1 ⫾ 3.0 0.906 0.003 1.089 8.2
Nashua, Iowa 5 May 2005 Crows; C 2133 R 206 243 0.053 ⫾ 0.011 ⫺6.5 ⫾ 2.5 0.887 0.017 1.055 13.1
21 May 2005 Crows; C 2133 R 206 243 0.052 ⫾ 0.014 ⫺6.3 ⫾ 3.3 0.810 0.038 1.053 13.4
East Lansing, MI 1 May 2005 Pioneer; 92B38(RR) 179 229 0.128 ⫾ 0.013 ⫺20.5 ⫾ 2.7 0.942 ⬍0.001 1.137 5.4
Rosemount, MN 24 May 2005 NK; S19-R5(RR) 192 214 0.161 ⫾ 0.016 ⫺28.4 ⫾ 3.3 0.980 0.010 1.175 4.3
Arlington, WI 18 May 2005 NK; S19-V2(RR) 164 194 0.129 ⫾ 0.015 ⫺20.4 ⫾ 2.6 0.950 0.001 1.137 5.4
Mean 197 228 0.127 1.138 6.8
SEM 3 4 0.014 0.016 0.8
a
Date when soybean plants in untreated control plots reached 80% infestation.
b
Date when soybean aphid populations peaked in untreated control plots.
c
The underlying model for population growth is N
t
⫽ N
0
e
rt
, which is equivalent to ln(N
t
) ⫽ ln(N
0
) ⫹ rt, where N
0
is the initial density, r is the population growth rate (i.e., slope from the linear regression),
and t is expressed in Julian days.
d
The intercept equals the ln(N
0
), where N
0
is the initial density.
e
Doubling time equals ln(2) divided by r.
1260 JOURNAL OF ECONOMIC ENTOMOLOGY Vol. 100, no. 4
