This July issue of Walking Your Fields newsletter contains articles about: soybean pests, fungicide applications and the role of water in corn development. Articles are written by DuPont Pioneer agronomists in North Dakota and northern Minnesota and are distributed on behalf of DuPont Pioneer account managers and Pioneer sales reps.

1. Soybean aphid is an insect pest that can cause signifi-
cant yield loss in soybeans. Scouting and proper man-
agement of this pest is critical to optimize profitability.
Early season scouting for soybean aphids usually begins
around the third of week June. Weekly scouting is a good
practice until soybeans reach about the R6, or full seed,
stage.
There has been a
large amount of
research conduct-
ed over the past
several years to
determine when
soybean aphid
populations reach
a level at which
they should be
controlled. This
research indi-
cates that soy-
bean aphid insec-
ticide treatments
should be applied
when there are
>250 aphids/plant
on at least 80
percent of plants
in a field. This economic threshold provides the least yield
risk and economic risk. This threshold refers to a field
average, and not field borders or hotspots. The econom-
ic threshold is reached well in advance (seven days or
more) of when cumulative aphid injury is expected to
cause yield loss, allowing time for an insecticide applica-
tion to be made before economic yield loss occurs.
Common recommendations are to scout five plants at five
locations for every 20 acres in a field. With a threshold of
250 aphids per plant, this would be a lengthy process.
Fortunately, there is a faster way. Researchers at the
University of Minnesota have developed a „speed scout-
ing‟ method that is much faster than conventional scout-
ing. With a little practice, you can scout as fast as you
can walk. A worksheet and details of this scouting meth-
od can be obtained at this website:
http://www.soybeans.umn.edu/pdfs/2007/aphid/speed_scouting_2007.pdf
Growers may be tempted to spray before the economic
threshold has been reached, or add an insecticide when
they apply herbicides for weed control even when few, if
any, aphids are present. This may be referred to as the
„insurance approach.‟ The insurance approach occasion-
ally works when it coincides with the economic threshold.
There is, however, much data demonstrating yield risk
with the insurance approach. U of MN researchers Ken
Ostlie, Dave Ragsdale, and Bruce Potter noted their con-
cerns regarding this approach for soybean aphid control.
The following is an excerpt from their 2009 writings:
Concerns about the Insurance Approach to Insecti-
cide Treatments for Soybean Aphids:
1) Cost. There is no data to suggest that very low aphid
populations reduce yield. Early applications are more
likely to be re-colonized and require re-treatment.
Claims of insecticide residual activity lasting a month
or longer have little factual basis.
2) Resistance. The more often soybean aphids are ex-
posed to insecticide the more quickly insecticide re-
sistant populations will develop.
3) Increasing populations of soybean aphid, or other
arthropod pests (e.g. spider mites) by removal of
beneficial species. Removal of beneficials (predators
and parasites) can have unexpected consequences.
Yes, this really does happen! Imagine how quickly
newly arrived aphids reproduce when you’ve already
removed the beneficials for them. When we do this
with cages that exclude predators, aphid populations
go from 10 to more than 1000 in a little more than a
week.
4) Compromises leading to poor insect and/or weed
control. Ideal nozzle, water volume and pressure se-
lection for insecticide and herbicide applications may
not be the same. Herbicide and insecticide timings
should be based on when to apply to the target pest
(weed or aphid) to be most successful.
You are responsible for managing your crop for a prof-
it. There is nothing illegal about applying an insecticide
labeled for soybean when aphid populations are below
threshold. However, insecticide applications do have
consequences in the environment. We wish only to
point out that there are potential short and long term
risks when insecticide applications are made without
regard to pest populations.
WALKING YOUR FIELDS® newsletter is brought to you by your local account manager for DuPont. It is sent to customers several times throughout the growing
season, courtesy of your Pioneer sales professional. The DuPont Oval Logo is a registered trademark of DuPont. PIONEER® brand products are provided subject to
the terms and conditions of purchase which are part of the labeling and purchase documents. ®, TM, SM Trademarks and service marks of Pioneer. © 2013 PHII.
Soybean Aphids
Photo: Derek Crompton, DuPont Pioneer
WALKING
YOUR FIELDS
®
www.pioneer.com
July 24, 2013 - Issue 4

2. COMMON QUESTIONS ABOUT SOYBEAN
APHIDS AND THEIR CONTROL
What effect does temperature have on soybean
aphids? Optimum temperatures for soybean aphid
reproduction are in the low 80s. The rate of reproduc-
tion decreases as temperature increases, and is near
zero at 95o
F. However, when the temperature drops
back down into the 70s at night, soybean aphids will
actively reproduce.
What effect does rainfall have on soybean aphid pop-
ulations? Rainfall intensity, wind and plant size all
impact aphid response to rainfall. High intensity rain-
fall with high wind can reduce aphid populations the
most. Moist weather conditions may also increase the
fungal pathogens that infect the aphids. However,
some aphids are likely to survive any rainfall event
and can quickly re-infest plants. Rainfall may knock
the population back, but don‟t count on it. Keep
scouting!
Can soybean aphids re-infest the same field after it
has been sprayed? Yes. Research shows that in-
secticides typically protect plants 14 days or less.
The insecticide degrades over time and any new
growth is unprotected. Aphids can migrate back into
the field. Aphid scouting should commence about 10
days after insecticide application on plants that are
not yet at R6.
For additional information on soybean aphids, visit: http://
www.soybeans.umn.edu/crop/insects/aphid/aphid.htm
Soybean cyst nematode (SCN) can cause significant
yield loss in soybeans and is a pest that is spreading in
northwest MN and eastern ND. Planting SCN-resistant
soybean varieties that include the Peking and PI88788
source of resistance, along with rotating to a non-host
crop are two of the best ways to manage SCN infesta-
tions. University researchers have advised that overuse
of any single source of SCN resistance can lead to SCN
race shifts over
time. Therefore,
it is important to
switch between
varieties and
always scout
fields.
To scout for
SCN, dig up soy-
bean roots and
gently wash the
soil away. Look
for the small light
colored lemon
shaped cysts,
as shown in the
photo.
DuPont Pioneer offers soybean varieties with both
the Peking and the PI88788 source of resistance.
Below are crop and soybean resistant variety rotation
recommendations from the Plant Health Initiative funded
by the Soybean Checkoff:
 Rotate with non-host crops to reduce SCN numbers.
 Rotate with resistant soybean varieties to reduce yield
loss due to SCN.
 Rotate the resistant varieties you use: don‟t use the
same one twice in a row.
 Rotate with SCN susceptible soybean varieties only if
SCN numbers are low.
Water is essential to corn growth, especially at tasseling;
when the plants uptake of water increases dramatically.
From tassel through the dent stage, corn demands an
average of a ¼ inch of water per day. Inability to receive
this amount of water can lead to decreased yields.
Root growth can help compensate for the increased wa-
ter need, as the expansion of the root system reaches
deeper into the soil profile to help the plants extract more
soil moisture. However, air temperatures also greatly in-
fluence water needs. Temperatures in the 90s during
corn growth will double the corn water requirement when
compared to temperatures in the 60s. Water stress im-
pacts corn yield the most during pollination, followed by
grain fill, and vegetative growth stages.
Soybean Cyst Nematodes
Photo: DuPont Pioneer
Female cysts of SCN on plant roots.
Role of Water in Corn Development
Table 1. Estimated Corn Evapotranspiration and Yield
Loss per Stress Day During Various Growth Stages
Growth Stage
Evapo-
transpiration*
(inches/day)
% yield loss per
day of stress
(min-ave-max)
Seedling to 4 leaf 0.06 ---
4 leaf to 8 leaf 0.10 ---
8 leaf to 12 leaf 0.18 ---
12 leaf to 16 leaf 0.21 2.1 - 3.0 - 3.7
16 leaf to tasseling 0.33 2.5 - 3.2 - 4.0
Pollination (R1) 0.33 3.0 - 6.8 - 8.0
Blister (R2) 0.33 3.0 - 4.2 - 6.0
Milk (R3) 0.26 3.0 - 4.2 - 5.8
Dough (R4) 0.26 3.0 - 4.0 - 5.0
Dent (R5) 0.26 2.5 - 3.0 - 4.0
Maturity (R6) 0.23 0.0
Adapted from Rhoads and Bennett (1990) and Shaw (1988). From: What
Happens Within The Corn Plant When Drought Occurs? Joe Lauer, Uni-
versity of Wisconsin-Extension, 2003.
*Evapotranspiration is the total of the water used by the corn plant
and water lost to evaporation.
>>

3. Irrigation, when available, is an obvious way to prevent
moisture stress. Checking your fields using probing meth-
ods is important as irrigation must be started early
enough to fully benefit the crop. Most irrigation systems
cannot keep up with crop water demands during the later
critical growth periods (VT to R3). The first priority for irri-
gation should be a 3-week period starting just before tas-
sel (VT) and ending just after silking. Corn is less suscep-
tible to water stress during later grain-development stag-
es (R3). Soil water levels should be maintained to allow
the crop to reach maturity (R6).
Understanding Water Usage in Corn
Water is acquired, used, and lost through evapotranspira-
tion (ET). In this process, water is removed directly from
the soil surface to the atmosphere by evaporation and
through the plant by transpiration. Plant transpiration is
the evaporation of water from leaf and other plant surfac-
es. For corn, evaporation often accounts for 20 to 30 per-
cent and transpiration 70 to 80 percent of total ET over
the length of a growing season.
The amount of water retained for the plant is affected by
the soil texture and organic matter. Soil serves as a res-
ervoir to the plant, even though not all water in the soil is
available to the plant. Water held by the soil between field
capacity and perma-
nent wilting point is
called “plant-available
water”, and varies by
soil texture. Fine
sandy soils have only
about 0.7 to 1.0 inch
of plant available wa-
ter per ft. of soil.
Loams and silt loams
have the highest plant
available water at
about 2 inches per
foot of soil.
Several areas received crop damage due to the recent
hail storms. Yield losses vary based on crop stage and
severity of the damage. Many growers are wondering if a
foliar fungicide should be applied to protect the injured
plant tissue from potential diseases which could cause
additional yield losses later in the growing season.
Fungicides control certain fungi, but provide no control of
bacteria. Many diseases favored by wounding or plant
damage are not controlled with foliar fungicides, such as
the bacterial disease Goss‟s Wilt, and the fungal diseases
that cause common smut and stalk rots in corn. Likewise,
fungicides do not control bacterial diseases in soybeans
such as bacterial blight.
Dr. Carl Bradley, Extension Plant Pathologist at Universi-
ty of Illinois, conducted a study in 2007 near Champaign,
IL. In this replicated study, a weed-eater was used to sim-
ulate hail damage. Treatments included “hail-damaged”
and “non-hail damaged corn” that was either sprayed or
not sprayed with a fungicide. Fungicide applications did
not statistically increase yield when applied on tasseling
corn that was “hail-damaged” the previous day. University
of WI researchers also observed no significant yield ben-
efit from applying fungicides to hail damaged corn or soy-
beans. For the full presentation with details, download:
http://www.soils.wisc.edu/extension/wcmc/2010/ppt/
Conley_hail.pdf
Fungicide applications on hail damaged plants should
only be recommended if there are significant fungal dis-
eases present. Fungicides are effective tools for protect-
ing yield potential of plants if significant disease levels are
present but do not improve the yield potential of crops.
One way to track corn pollination success is to carefully
remove the husk cover from the ear, being careful not to
pull on the silks. Silks that remain attached are evidence
of ovules (kernels) that have not been fertilized. If the
silks fall away, the ovules have been fertilized.
The most critical
period of corn
development is
the two weeks
before and after
silking. During this
period the corn
plant requires its
maximum amount
of water at about
one inch of water
every three days
(0.33 inches of evapotranspiration per day).
Foliar Fungicide Applications on
Damaged Crops
Check Corn Pollination Progress
Image 1. Evapotranspiration is the sum
of evaporation from the land‟s surface +
transpiration from the plant. Image:
SalinityManagementGuide.org
Figure 1. General relationship between soil moisture and tex-
ture. Ohio Agronomy Guide, 14th edition, Bulletin 472-05

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