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10/25/2010 - 10/31/2010

When to Sample for Nematodes on Corn - It Depends

By Greg Tylka, Department of Plant Pathology

A recurring question often asked concerning plant-parasitic nematodes that feed on corn is, “When should samples be collected to check for these pests?” The answer depends on the details of each specific field situation. In most cases, fall sampling to test for nematodes that feed on corn is not recommended.

Most nematode species present during early to mid season
The reason that fall sampling generally is not recommended is because to determine if nematodes are causing or have caused damage to a corn crop, the species and population densities (numbers) of the nematodes in a sample are compared to damage thresholds. But the numbers of most nematode species that feed on corn tend to decline as the corn crop matures and dies. So low numbers from samples collected in the fall may be because the nematode numbers were not very high (and not damaging) during the growing season or may be the result of numbers declining from some higher, possibly damaging, level earlier in the season. There is no way to tell exactly what occurred during the growing season based on low nematode numbers in samples collected in the fall.

To determine if plant-parasitic nematodes are causing damage to corn, samples should be collected early to mid season, when nematode numbers are generally greater. Iowa State University recommends collecting samples whenever symptoms of damage, such as stunting, yellowing of foliage, mid-day wilting, lack of fine roots, swollen roots, and/or dead areas on roots are observed.

There are a few instances when fall sampling is warranted or even preferred, namely for root-lesion and lance nematodes and for needle and sting nematodes, as described below.

Root-lesion and lance nematodes
These two nematode species are endoparasites that enter corn roots and feed and reproduce almost completely within the roots throughout the growing season (see figure). Early or mid-season soil sampling is acceptable for the root-lesion and lance nematodes. But fall sampling for these nematodes also is acceptable because nematode numbers accumulate in root tissue throughout the season. Root-lesion and lance nematodes need to be extracted from both soil and root fragments in order to accurately determine the population densities of these nematodes.

Sting and needle nematodes
These two nematode species are among the largest plant-parasitic nematodes, and their distribution is limited to soils with 70 percent or greater sand content. These nematodes reportedly migrate deep into the soil profile during the middle of the growing season and thus, they may be missed in soil samples collected mid season, even if soil cores were collected to a depth of 12 inches. The damage threshold for these nematode species is very low (one worm per 100 cm3 or about a half-cup of soil) and their numbers do not get very high. So it is important to collect samples when there is the greatest likelihood of detecting low numbers of these nematodes. If nematode damage to corn is suspected in fields with high sand content (over 70 percent), spring or fall soil sampling for needle and sting nematodes is warranted.

Guidelines for collecting soil and root samples to check for plant-parasitic nematodes that feed on corn were discussed in an ICM News article July 19, 2010. More information about the different species of nematodes that can feed on corn was published in ICM News August 3, 2009.

Endoparasitic root-lesion nematode (arrow) inside of corn root tissue.



Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes.

SmartStax: Multi-trait Corn Offered by Dow and Monsanto

By Aaron Gassmann and Erin Hodgson, Department of Entomology

Beginning in 2009, Dow AgroSciences and Monsanto have entered into an agreement to offer SmartStax, which combines the Bt traits found in Herculex XTRA and VT Triple Pro. This product will include several Bt traits, with multiple toxins providing control of some key pests and with a broader overall spectrum of pest control. Additionally, there will be a reduction in the refuge requirement with the product. Growers should carefully consider how the pest-control benefits of this product balance against seed costs when deciding what to plant in 2011.     

How does SmartStax differ from other products?
SmartStax combines the Bt traits found in VT Triple Pro and Herculex XTRA. VT Triple Pro contains Cry3Bb1 which targets corn rootworm species along with Cry1A.105 and Cry2Ab2 for control of Lepidoptera including European corn borer and corn earworm. Herculex XTRA contains Cry34/35Ab1 for control of corn rootworm and Cry1F for control of Lepidoptera including European corn borer and western bean cutworm. Additionally, SmartStax has tolerance of the herbicides glyphosate and glufosinate.

Implications for pest control
Historically, the primary corn pests of concern in Iowa are the European corn borer and corn rootworm. Both of these pests are controlled effectively in most cases by either VT Triple Pro or Herculex XTRA. SmartStax should provide control that is as good as or better than the parent products. Secondary pests of corn in Iowa currently controlled by Bt corn include corn earworm, western bean cutworm, black cutworm and armyworms. High adaption of Bt corn over the last decade is likely responsible for the drastic reduction in European corn borer populations, making it less of a problem in corn. Conversely, some secondary pests may become more problematic over the coming years; for example, the western bean cutworm has only recently expanded its range to include Iowa. Smartstax offers some advantages for control of secondary pests compared with either VT Triple Pro or Herculex XTRA. 
Changes in refuge requirement
SmartStax requires a smaller refuge than either Herculex XTRA or VT Triple Pro. In Iowa, a five percent block refuge is required for all pests targeted by Smartstax. This includes below ground pests (western and northern corn rootworm) and above ground pests such as European corn borer and corn earworm. The refuge must be within or adjacent to the SmartStax field and cannot contain any Bt traits for pest control. 

Things to consider
Advantages of SmartStax include a reduced refuge requirement and broader control of secondary pests. While the use of multiple traits should increase the durability of SmartStax, this benefit may be counterbalanced by the reduced refuge requirement, which should decrease durability. The extent to which SmartStax will increase profits on the farm depends on pest pressure, pest management strategies and seed costs. For example, corn rootworm can be effectively controlled by crop rotation. In such cases, a stacked event such as SmartStax or a single-trait rootworm event such as VT Triple Pro should confer little additional benefit of increasing yield by reducing pest injury. 


Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at or phone (515) 294-2847. Aaron Gassmann is an assistant professor of entomology with research and teaching responsibilities in insect pest management. He can be contacted by email at or (515) 294-7623.

Additional Disaster Payments for 2009 Iowa Soybeans

By Chad Hart, Department of Economics

USDA has announced additional disaster payments for the 2009 crop year via the Crop Assistance Program (CAP). Payments will be made to producers of rice, upland cotton, sweet potatoes and soybeans in counties that received Secretarial disaster designations in 2009 for excessive moisture or related conditions. In Iowa, 30 counties qualify for the payments, see the list of counties and map below.

In order for producers to qualify for payments, they must have suffered a five percent crop loss in 2009 from excessive moisture or related conditions. The five percent crop loss requirement is based on a comparison of the producer’s 2009 actual yield to the higher of the producer’s crop insurance APH yield or the county expected yield as determined by the Farm Service Agency (FSA) State Committee.  The payment is based on a flat dollar per acre amount, $15.62 for soybeans. If the total payments under the CAP exceed $550 million, then the payment rate will be prorated to cap payments at $550 million. A qualified producer will receive the payment on all 2009 planted acres for the eligible crops on land that is physically located in one of the Secretarially declared disaster counties. 

Producers will initially receive 75 percent of their expected payment, with the remaining amount being paid when sign-up is complete and the final payment rates are determined. There is a payment limit of $100,000 per producer in this program. And the payments will be considered as revenue under the Supplemental Revenue Assistance Payments (SURE) program for 2009.

The CAP payments are being administered by the FSA. To sign-up, visit your local FSA office. Sign-up for the payments began Oct. 25, 2010 and continues through Thursday, Dec. 9, 2010. Producers will self-certify the crop losses, but should have documentation to support the crop loss claim. Acreage will be certified from FSA acreage reports. CAP is being funded from a standing USDA program that allows the Secretary of Agriculture to reestablish the purchasing power of agricultural producers.


Chad Hart is the Iowa State University extension grain marketing economist. He can be reached at (515) 294-9911 or

Enjoy the Fall Weather –Take a Walk and Collect Some Soil Samples

By Greg Tylka, Department of Plant Pathology

Once harvest is completed, a very productive way to enjoy the fall weather is to collect soil samples for soybean cyst nematode (SCN).
In the 1990s and much of the past decade, fall soil sampling for SCN was strongly recommended as a way to scout fields for the presence of this pest. If fields have not yet been tested for SCN, soil samples should be collected for this purpose. But many fields infested with SCN in Iowa likely have already been discovered.

Another reason to collect soil samples for SCN in the fall is to determine SCN egg population densities (numbers). These numbers will be useful for comparison when soil samples are collected again sometime in the future. Growers and agronomists are advised to take soil samples every six to eight years to assess SCN population densities as a check that management efforts are adequately controlling the nematode. Doing this is important because many SCN populations in Iowa and throughout the Midwest are developing increased ability to reproduce on the most common type or source of SCN resistance, called PI 88788. The key to profitable long-term soybean production in SCN-infested fields is to prevent SCN population densities from increasing.

Comparing results of soil samples collected six to eight years apart requires good record keeping and also consistent soil sample collection methods. Accurate and detailed notes of when and how samples were collected are needed so the same methods can be used in future years. Details should include the specific areas of fields that are sampled, the number of cores that are collected and their depth, the specific sampling date, whether samples are collected before or after a soybean or other crop, and which laboratory processes the samples.

Following are some general soil sampling guidelines for this purpose.

• The more soil cores collected and the smaller the area sampled, the more accurate the results will be.

• Soil cores should be from the upper eight inches of soil.

• If corn or some other nonhost crop was last grown in the field, it doesn’t matter if soil cores are collected in the previous crop’s row.

• It is better to collect soil cores after the previous corn (or other nonhost crop) rows have been destroyed by tillage.

• If soybeans were last grown in the field, collect soil cores from under the old crop rows.

• If sampling conventionally (not grid sampling), collect 15 to 20 soil cores in a zigzag pattern from no more than 20 acres. The 20-acre parcels of the field do not need to be square or rectangular; samples can be collected from zones according to the agronomic features of the field (see figures).

• If grid sampling: collect one or two extra soil cores from every grid cell sample and combine these extra cores from the number of cells that represent approximately 20 acres.

Many private soil laboratories can process soil samples to determine SCN egg population densities. Samples also can be sent to the Iowa State University Plant and Insect Diagnostic Clinic, Room 327 Bessey Hall, Iowa State University, Ames, Iowa 50011-1020.  The current fee for SCN analysis at the ISU clinic is $15 per sample for samples from Iowa. Samples sent to the ISU clinic should be accompanied by a completed Plant Nematode Sample Submission Form.

More information about the biology, scouting, and management of SCN can be found at

Collect 20 or more soil cores from areas in the field no larger than 20 acres.


Sampling areas can be designated according to agronomic features of the field.


Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes.

This article was published originally on 11/1/2010 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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