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7/28/2008 - 8/3/2008

Now is the Time to Sample for Corn Nematodes

By Greg Tylka, Department of Plant Pathology
Interest in plant-parasitic nematodes as pests of corn in Iowa, and much of the Midwest, has increased dramatically in the past few years. Damage from these microscopic worms, which feed from within the roots and on the outside of roots, can cause overall stunting of the plant and yellowing of foliage. Ears may be stunted and poorly filled due to damage by this pest. Feeding by some nematode species also causes swelling of corn roots. None of these symptoms are unique and useful in identifying fields in which such damage is occurring. 

To determine if nematodes are responsible for damage being observed to corn, a root and soil sample must be collected mid season to determine the nematode species present and their population densities (numbers). Sampling mid season, when numbers are greatest, is necessary because corn nematode population densities increase through the growing season and the different nematode species vary in the number needed to cause damage.

NOW is prime time to be checking corn fields for the presence of corn nematodes.

To test for corn nematodes, collect 20 or more 12-inch-deep soil cores from plants showing symptoms that might be due to corn nematode feeding damage. Also collect two or three root balls from plants that are showing symptoms.

Collect soil cores from root zone of corn. sampling for corn nematodes step one


Collect 12-inch-deep soil cores. sampling for corn nematodes step 2


Collect 2 or 3 root systems in addition to soil cores.sampling for corn nematodes step 3


Mix the soil cores well, then place soil and roots in a moisture-proof bag and submit for processing as soon as possible.  In the hot summer months, avoid shipping the samples near the end of the work week which can leave samples exposed to high heat in uncooled delivery trucks over the weekend. Such heat will damage the nematodes within the sample and can affect the sample results.

Samples for corn nematode diagnosis, a complete nematode count, can be sent to:

ISU Plant and Insect Diagnostic Clinic,
327 Bessey Hall,
Iowa State University,
Ames, IA 50011.

Samples sent to ISU should be accompanied by a completed Plant Nematode Sample Submission Form  and a check for the $30 per sample processing fee.  For more information about corn nematodes, read the ISU Extension publication titled “Nematodes That Attack Corn in Iowa.”


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

Watch Soybean Aphid Numbers - NOW

Jon Tollefson, Department of Entomology

Last week the conditions in Minnesota were very good for soybean aphid reproduction. With those conditions, the populations increased rapidly in many locations throughout Minnesota. We have been expecting 2008 soybean aphid populations to be lower as they were in 2004 and 2006, but the 2008 weather has not been exactly “typical.”

The increase in aphid numbers in states north of us serves as a reminder that this is the time you must be scouting soybeans to avoid an outbreak of aphids in any of your soybean fields.

For some helpful reminders, the treatment threshold for soybean aphids should remain at an average of 250 aphids per plant, even with increased value of soybeans as discussed in an Integrated Crop Management article released today.

The scouting technique to monitor aphid numbers was described in an Integrated Crop Management article in July of 2007.  There is a faster, binomial method to make an aphid management decision in soybeans. The “Speed Scouting” technique is described in the article as well; additional information is available from the University of Minnesota.  A free quick field guide to using this method can be ordered from Iowa State University Extension. 


Jon Tollefson is a professor of entomology with extension and research responsibilities.

Reviewing Decision Thresholds for Pest Insect Control

Jon Tollefson, Matt O’Neal, and Marlin Rice, Department of Entomology

Three tools need to be understood in order to make decisions regarding the control of insects found on crops: damage boundary, economic injury level, and the economic threshold. The existing values for these three variables are now being questioned with the increase value of Iowa’s row crops and the cost of controlling the pests. This article reviews the use of these tools in regards to soybean aphid control.

The damage boundary is the number of a pest insect that must be present before their injury can be measured as yield loss. There is no reason to spend money and effort to control insects that are present in numbers fewer than the damage boundary because there will not be any observable return in protected yield and there can be detrimental effects on the natural enemies.

The economic injury level (EIL) is the number of insects that need to be present for the value of the lost yield to equal the costs of control.

The economic threshold (ET) is a decision point. It is the number of pest insects that need to be present for a control to be applied and keep the increasing pest population from reaching the EIL.

soybean aphids on bean leaf

Soybean aphids on a bean leaf, Jon Tollefson.

In the case of the soybean aphid attacking soybeans, the economic injury level (EIL) has been an average of 654 plus or minus 95 aphids per plant. This calculation indicates that when there are 654 aphids per plant, the yield loss they cause will equal the insecticide and management costs to control them. This calculation was based on a $8.65 to $21 range of control costs, with 30 to 60 bushels per acre yield and market values of $5.50 to $6.50 per bushel.

Now that the market value for soybeans has risen, a lowered EIL can be calculated. David Ragsdale, the lead author on the paper describing the existing EIL for soybean aphids, calculated a new EIL for soybeans selling at $15 per bushel, with $8 per acre control cost, and an anticipated yield of 50 bushels per acre. With these values the EIL is lowered to 452 aphids per plant.

Although a lower EIL has been calculated, we are NOT recommending a lower ET. Here is why:

  • The current 250 ET is below the damage boundary. In the field research from which the original EIL and ET were calculated, there were no yield losses observed for populations that peaked at 250 aphids per plant. Therefore a threshold of 250 aphids per plant is below the damage boundary.
  • Lower ETs do not provide greater yield protection. In follow up studies conducted in Iowa, there has not been significant yield protection when insecticides were applied at populations at or below 250 aphids per plant. In 2007 and 2005, soybeans that received an insecticide applied at the 250 ET had higher yields than plots that did not receive an insecticide. Plots that received an insecticide at much lower thresholds, greater than 10 per plant, required multiple applications but where indistinguishable from those that received an insecticide at the 250 ET. In 2006, when aphid populations peaked at less than 250 per plant, we did not see a yield difference between treated and untreated plots; details of the report are available.

Soybean aphid populations do not always exceed the economic threshold (ET). During 2004 and 2006 on-farm and research farm studies of soybean aphids in Iowa, soybean aphid populations rarely exceeded 250 aphids per plant. When these fields with lower populations were sprayed for research purposes, no consistent yield protection was observed. This is an experience shared by many entomologists and researchers across the Midwest.

Even though we can now find soybean aphids in many of the fields, there is no guarantee that they will reach levels that will cause yield loss. Each grower must determine the risk for their own fields as year to year and location to location variation can occur in soybean aphid populations. For maximizing the value of an insecticide application we continue to recommend scouting and the use of the 250 ET.

The consequences of preventative insecticide applications can be severe. If growers apply insecticide as a preventative treatment (i.e. with a tank-mix of herbicide or fungicide) when they are not needed, the risk for insecticide resistance increases. Persistent use of any insecticide can lead to resistance as observed in other crops where aphids have been a pest.

A second consequence of applying an insecticide before it is needed is pest resurgence.  All of the insecticides labeled for use in soybeans are “weapons of mass destruction” to insects. They will kill the aphids AND the predators than can be found in soybeans even before the aphid arrives. By using insecticides in a preventative manner, growers can inadvertently make a bad situation worse by removing these beneficial insects and producing greater populations of aphids, and possibly other insect pests, than would have been present had they not have sprayed at all.

In summary, we continue to recommend that growers scout and employ the ET average of 250 aphids per plant. To date we have no evidence that lower thresholds or preventative approaches, where the insecticide is applied regardless of the aphid density, will result in greater yield protection than this recommendation. However, we are constantly re-evaluating this recommendation and will report research findings as they become available.



Jon Tollefson is a professor of entomology with extension and research responsibilities. Marlin E. Rice is a professor of entomology with extension and research responsibilities. Matt O’Neal is a professor of entomology with extension and research responsibilities.

Two Soybean Aphid Field Resources Now Available

By Rich Pope, Department of Plant Pathology

Knowing how to manage and "speed scout" soybean aphids, just got easier, thanks to two new pocket-sized  publications authored by Iowa State University Extension entomologists Marlin Rice and Matt O'Neal.

The companion soybean aphid publications are right off the press and available from ISU Extension. One is a spiral-bound "Soybean Aphid Management field guide for 2008," extension publication number CSI 11. The 43-page pocket guide is focused on management of soybean aphid in Iowa. The guide is a handy reference to the history, identification, development and management strategies for this relatively new insect pest for Iowa.

Image of Soybean Aphid management field guide         Image of the Speed Scouting--Soybean Aphid field card

                 CSI--11                                                           CSI--15

The second publication, "Speed Scouting—Soybean Aphids” is a double-sided card useful for field-by-field evaluation of soybean aphid populations. It uses the speed scouting technique that was recently developed by entomologists at the University of Minnesota.

Supplies of these references, as well as the publication Soybean Disease and Pest Management field guide (CSI 10) are available for no charge from the Iowa State University Extension Distribution Center. Look for the following:

  • CSI 10   Soybean Disease and Pest Management field guide
  • CSI 11  Soybean Aphid Management field guide
  • CSI 15  Speed Scouting—Soybean Aphids field card

All three publications were produced and funded for no-cost distribution with funding provided by the Iowa Soybean Association.


Rich Pope is an extension specialist with the Corn and Soybean Initiative.

Preventative vs. Curative Fungicides

By Daren Mueller and Alison Robertson, Department of Plant Pathology

We have received several questions about selection of fungicides, especially with regards to deciding between “curative” and “preventative” fungicides. People want to know if they should be applying a strobilurin fungicide, triazole fungicide, or a premix of both. 

To fully understand how a fungicide reduces disease development, we need to understand the disease cycle of a foliar pathogen. Foliar pathogens are polycyclic, which means that many disease cycles can occur in a single season. A disease cycle consists of the following steps: infection, colonization, symptoms and spore production. The period from infection until symptoms are first seen is known as latent infection.

In southeast Iowa, we usually start to see gray leaf spot (GLS) lesions on the lower leaves of the corn plant starting in mid-July. These lesions will produce spores that will infect leaves higher up the plant, cause lesions and produce more spores. Under favorable conditions, the first symptoms of GLS occur approximately 12 days after infection. So in a single growing season there may be three or four disease cycles of GLS. Fungicides reduce disease development by breaking the disease cycle. 

Strobilurin and triazole fungicides are both considered “locally systemic”, meaning they are absorbed into plant tissue and do not remain on the outer plant surfaces exposed to the elements. While both fungicide groups are systemic, they break the disease cycle at different points and thus differ in their role in protection of plants from infection. 

Preventative activity occurs when a fungicide is present on or in the plant before the pathogen arrives or begins to develop. The fungicide acts as a protective barrier and prevents infection from occurring. This is also referred to as a protective activity.

Curative or early-infection activity occurs when the active fungicide ingredient is present within plant tissue and stops early growth of the pathogen (colonization) in the plant tissues. This type of fungicide is usually most effective 24 to 72 hours after infection occurs, depending on the fungicide. Most fungicides that prevent early-infection also have preventative activity and thus are most effective when applied before infection occurs.

Therefore, it is important to remember that “curative” fungicides will NOT cure a plant from a disease. They are effective if applied prior to infection or in the first 72 hours after infection, but they are not effective against more advanced latent infections. 

Furthermore, preventative fungicides can still be used if the disease is present at low levels, since they will prevent new infections from occurring on the remaining leaf tissue with no infection.

Putting it into practical terms 

  • In July, most of the corn and soybean fields in Iowa do not have disease pressure at levels that will cause yield losses, although this seems to be when most people consider applying fungicides.
  • Typically, disease levels do not increase until August. So, when choosing fungicides, the number one factor should be choosing a product that is most effective against diseases that are likely to occur in your field. 
  • If disease is present at low levels, protecting the remaining green tissue should suffice for protecting yield.
  • Remember it is only the ear leaf and leaves above the ear leaf that contribute to grain fill – these are the leaves that need to be protected. It is also okay to have a few spots on these leaves before a fungicide is applied, since it is unlikely that these lesions will impact yield. 

For tentative guidelines for estimating yield loss for GLS, see pages 3 and 4 of Kentucky Pest News.   


Alison Robertson is an assistant professor of plant pathology with research and extension responsibilities in field crop diseases. Daren Mueller is an extension specialist with responsibilities in the Corn and Soybean Initiative.

This article was published originally on 8/4/2008 The information contained within the article may or may not be up to date depending on when you are accessing the information.

Links to this material are strongly encouraged. This article may be republished without further permission if it is published as written and includes credit to the author, Integrated Crop Management News and Iowa State University Extension. Prior permission from the author is required if this article is republished in any other manner.