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.

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Soybean Aphids

Photo: Derek Crompton, DuPont Pioneer

WALKING YOUR FIELDS

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www.pioneer.com

July 24, 2013 - Issue 4

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 95

oF. 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.

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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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