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6/1/2009 - 6/7/2009

Early Signs of Corn Stress

By Roger Elmore and Lori Abendroth, Department of Agronomy

Iowa weather conditions this spring resulted in wide ranges in corn plant color - from green to yellow-green to purple and some with stripes. The color variability in most of our fields is not a cause for alarm as the plants simply reflect the 2009 environment to date. Above- and below-ground growth is slowed with low temperatures, despite the plentiful moisture and sunshine we have had. A few days of sun and warmer temperatures will change plant colors quickly.

Planting in Iowa this year went far better than in 2008. Ninety-nine percent of our corn is planted as of May 31 and 90 percent emerged. This equals the five year average (based on USDA-NASS data). Corn development currently ranges from the second to eighth leaf growth stages. 

Although the crop appears as if it is off to a good start, there are reports of less-than-ideal plant colors. Symptoms like this occur occasionally and are often affected by tillage system, hybrid, and other management variables - see past reports of this.   

story county compaction

Striped corn leaves due to environmental stress. Story Co. IA, 4 June 2009, R.W. Elmore. 


2009 Early- season Observations
Growing degree day unit accumulation (GDD)  is close to average across the state. A few warm days will make up the small deficit quickly. Although we have experienced near normal GDD accumulation, low temperatures May 16 and 17 resulted in radiational cooling damage to corn seedlings (see CropWatch Blog, May 21, 2009). In addition, the days surrounding that event were abnormally cool with slow GDD accumulation (Figure 1).

Soil temperatures from May 14 through the 17 also declined across the state. Figure 2 shows this for Ames and Lewis. These temperatures are measured at four inches below bare soil. 

Fields with residue cover will likely exhibit cooler soil temperatures than those in Figure 2. Licht and Al-Kaisi (2005) documented soil temperature at two-inches deep in no-till fields were 2 to 2.5⁰F less than either strip tillage or chisel plow systems in Ames. The temperature differences caused poorer emergence rates in no-till compared to the two other tillage systems although grain yields were similar among all three tillage systems. There were no differences in soil temperatures at their Nashua site. This year, the majority of reports on yellow corn are from no-till or reduced-till fields. 

Cool soil temperatures slow root and shoot growth as well as decrease soil N mineralization. Short-term environmental stress likely caused the color differences in corn this year. These symptoms are usually temporary; seedlings should regain a healthy green color as weather conditions improve without affecting yield.


Figure 1. Growing degree day accumulations, May 12th through May 18th lagged behind normal across the state (Figure by Rich Pope).figure 1


Figure 2. Soil temperatures, at 4 inches under bare soil, fell below normal in mid-May, 2009 at Ames and Lewis, IA. figure 2



Reference:  Licht, Mark A., and Mahdi Al-Kaisi. 2005. Strip-tillage effect on seedbed soil temperature and other soil physical properties. Soil & Tillage Research. 80:233-249.


Roger Elmore is a professor of agronomy with research and extension responsibilities in corn production. Lori Abendroth is an agronomy specialist with research and extension responsibilities in corn production. Elmore can be contacted by email at or (515) 294-6655; Abendroth can be contacted by email at or (515) 294-5692.

Growers, Check Corn Stands NOW

By Jon Tollefson and Erin Hodgson, Department of Entomology

This is a very good time to walk corn fields and check the plant stand. Estimating your plant stand will help you know what plant population actually emerged compared to what you thought you planted. And it will help you scout for early-season insects and disease.

There are several insects that might be decreasing your plant stand right now. We have had reports of dingy cutworms, black cutworms, wireworms, and armyworms. You can still take action against the black cutworms and armyworms. Even though it is too late to treat for wireworms now, finding them in corn fields can help you be prepared for them next year.

Cutting of corn by black cutworms was in progress by the end of last week and should now be happening in central and northern Iowa, as reported in the ICM article Predicted black cutworm cutting dates in corn. This insect can be stopped with foliar insecticide sprays. A recent ICM Newsletter article, Dynamic BCW Action Threshold, provides calculation aids for the treatment thresholds that should be used.

To scout and control armyworms, read the ICM News article on armyworms. The recent stalk bore article will help you prepare to watch for stalk borer if they begin to move into your field.

Update on wireworm presence in Iowa
An unexpected event with the wireworm in Iowa and surrounding states is that they have caused stand loss in fields of corn where the seed had been treated with Poncho 250. If you have planted seed with a Poncho treatment, don’t ignore it. If there is stand loss from wireworms in those fields you might consider going to the higher rate (Poncho 1250) next year or to treat the fields with a granular insecticide instead.

cut corn



Jon Tollefson is a professor of entomology with responsibilities in research and extension. He can be reached at (515) 294-8044 or 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.

Size Restrictions for Postemergence Herbicides in Corn

by Bob Hartzler, Department of Agronomy

While most farmers and applicators have good intentions to apply herbicides in a timely fashion, it isn’t uncommon for equipment limitations or weather to result in delayed applications. Nearly all postemergence herbicides have restrictions on how late in the season they can be delayed. 

The most common reason for the restriction is the reduction in crop tolerance to the herbicide as corn size increases. For some products the application timing is restricted to prevent herbicide residues from exceeding tolerance levels in the grain. Regardless of the reason, it is important to follow the restriction.

Timely applications not only avoid problems with label restrictions, but they also provide more effective control due to treating weeds at a more susceptible stage and at a time when the crop canopy does not interfere with coverage, and  also are more effective at protecting yields from early-season competition than late applications.

corn size restrictions


Bob Hartzler is a professor of weed science with extension, teaching and research responsibilities. He can be contacted by email at or phone (515) 294-1164.

Stalk Borers Forecasted to Begin Movement Into Corn

by Erin Hodgson, Department of Entomology and Rich Pope, Integrated Pest Management Program 

Stalk borer is native to Iowa and has only one generation per year. Adult stalk borers are grayish-brown moths with white spots along the forewing  with a 1-inch wingspan. Adults emerge in early August and lay eggs in grasses and broadleaf weeds until October. Larvae hatch in late April and early May, or when 500 degree days (base 41°F) have accumulated.

Small larvae are cream-colored with a dark brown or purple band around the body; lengthwise stripes may also be present along the abdomen. Young larvae burrow and feed in grass and weed stems until they outgrow the stem. These older larvae move to larger plants around 1,400 degree days, or earlier if the grass is cut or killed with herbicides. Fully-grown larvae drop to the soil to pupate, emerging in August to complete the life cycle.

Stalk borers have a wide host range, with larvae feeding on over 175 different plant species. In the spring, young larvae are commonly found on brome grass and giant ragweed. Eventually, older larvae move to corn and occasionally soybean. 

Stalk borer feeding in corn can cause wilting of upper leaves or may make irregular holes in newly unrolled leaves. Stalk borer larvae, as the common name suggests, typically burrow into the stalk at soil level and tunnel upward. Infested corn plants will be stunted and not produce viable ears. In some cases, young corn plants die.

Iowa State University research has found Bt corn (YieldGard® corn borer hybrid) will suppress but not completely control stalk borers. Therefore, non-Bt corn is more likely to be damaged, especially fields adjacent to grassy areas, waterways, ditches and fencerows.

Start looking for migrating larvae when 10 percent movement is predicted. This will vary by location and year since the prediction is based on temperature. The 2009 forecast for 10 percent stalk borer movement in Iowa is shown here.

projected 10% migration date for stalk borer

The larvae are not highly mobile, and typically only move into the first four to six rows of corn. Look for new leaves with irregular feeding holes or for small larvae resting inside the corn whorls. Larvae will excrete a considerable amount of frass pellets in the whorl or at the entry hole in the stalk. Exposed larvae can be killed with a foliar insecticide treatment, but tunneling larvae are not susceptible. Young corn is particularly vulnerable to severe damage, but plants are unlikely to be killed once reaching V7 (seven true leaves).

Regular weed management within and around corn fields is crucial for reducing stalk borer populations. Stalk borers can cause damage throughout a field if grasses and broadleaf weeds are not controlled in a no-till system. Just killing weeds in a highly infested area will force larvae to feed on corn - this practice could significantly reduce a stand.

To prevent stand loss, scout and determine the percent of infested plants. The use of an economic threshold (Table 1), first developed by ISU entomologist Dr. Larry Pedigo, will help determine justifiable insecticide treatments based on market value and plant stage. Young plants have a lower threshold because they are more easily killed by stalk borer larvae.

If an insecticide is warranted, an insecticide can be tank-mixed with a fast burndown herbicide, or applied seven days after a slow burndown herbicide. A number of insecticides are available for stalk borer control (Table 2). Border treatment should be considered if infestations are localized. Insecticides must be well-timed so that products are reaching exposed larvae before they burrow into the stalk. Make sure to read the label and follow directions, especially if tank-mixing with a herbicide, for optimal stalk borer control.

Table 1. Economic thresholds for stalk borer in corn.corn borer thresholds


Table 2. Insecticides labeled for stalk borer control in corn.

table of Insecticides for stalk borer 2009



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. Rich Pope is an extension program specialist with responsibilities in integrated pest management

Degree days - Crops in Good Condition Despite Cool Temperatures

by Rich Pope, Corn and Soybean Initiative

The 2009 growing season finished off May a bit cool, with degree day accumulations for only one of the nine crop reporting districts, west central, besting its long-term average. May was marginally cooler in the east, although that is not a problem, and crops are off to a great start statewide. 

Degree day map for May 31

As we projected, black cutworm larvae were large enough to damage corn in southern Iowa by May 25 - with light black cutworm feeding on corn noted last week in southeast and south central Iowa.  These infestations have been light and sporadic, but they warrant monitoring.

Weedy fields are a concern statewide. Timely and appropriate weed management, especially in no-till fields, can be economically significant.


Rich Pope is a program specialist with responsibilities with Integrated Pest Management. Pope can be contacted by email at or by calling (515)294-5899.

Cancellation of Furadan (carbofuran) for Crops Use

By Jon Tollefson and Erin Hodgson, Department of Entomology

Due to toxicity and potential environmental hazard, the Environmental Protection Agency (EPA) has announced its intention to revoke all tolerances for carbofuran. Carbofuran is the active ingredient of Furadan™ 4F, most commonly used in Iowa as a liquid formulation applied to corn.

The Agency’s announcement is in the Federal Registry and states that the final rule revoking all tolerances for carbofuran will be effective on August 13, 2009.

The announcement of the EPA’s intended actions was followed a public announcement from Iowa Secretary of Agriculture Northey to increase Iowans’ awareness of the cancelation.

This process began several years ago. A Risk and Benefits Committee reported to the EPA that carbofuran was an unduly hazardous chemical and there were alternative treatments that could be used. In 2006 FMC Corporation, the makers of Furadan™, requested an opportunity to appear before the Committee. Tollefson was an academic expert on Furadan use in field crops and a Professor from Purdue testified concerning Furadan use in horticulture crops. There were letters from Secretaries of Agriculture supporting the use of Furadan in crops, data from field experiments, and several growers testified.

Upon considering the information presented, the EPA maintained its intention to not renew the registration of carbofuran. FMC appealed the decision, but the appeal was not upheld. The process has now come to the point where the final cancellation of tolerances is occurring. It is likely that FMC will challenge the EPA’s conclusions by requesting an administrative hearing, but these rebuttals have not altered the Agency’s direction to this point.

Now how does this affect the Iowa growers? There are several points to consider:

• This year you will be able to purchase and use Furadan on field crops according to the label.

• Don’t purchase and stockpile labeled Furadan. Any crops receiving a Furadan application after January 1, 2010 cannot be legally sold.

• Historically, there has not been very much liquid Furadan used in Iowa. However, this has changed somewhat with the development of genetically engineered corn that is resistant to corn rootworms; the requirement of a refuge planting to preserve effectiveness of the Bt varieties; the development of neonicotinoid seed treatments; and planters that have central-fill seed hoppers. These changes have resulted in Bt and refuge fields being planted with no soil insecticide applied for corn rootworm control and a liquid application being made to the refuge after the corn has emerged. Furadan has been a product that fits these needs. Now an alternative product will be needed.

If alternative treatments for protecting corn from corn rootworm larval feeding are needed, what might be available? If the area planted is to be a corn rootworm refuge, there would be two options: one option would be to add granular applicator units to the planter and the second would be to buy seed that has been treated with the rootworm rate of a neonicotinoid insecticide (e.g., Poncho™, Cruiser™). If a post-emergence liquid treatment is desired, the options would include chemicals such as Lorsban™ 4E and Capture™ 2EC. In general the planting-time granular insecticides provide the best, most consistent control. There is not as much research information on post-emergence liquid applications.


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

June 1 Iowa Crop and Weather Report

By Doug Cooper, Extension Communications Specialist

The June 1 Iowa crop and weather report includes interviews with Iowa State University Extension climatologist Elwynn Taylor, integrated pest management specialist Rich Pope, and soybean agronomist Palle Pedersen.

Pedersen reports soybean planting is near complete. He says the time is right to go to the fields and count stands to determine if replanting may be necessary.

Taylor talks of El Niño’s return by early August, which could benefit soybeans more than corn.

Weeds are the topic for integrated pest management specialist Pope. The weeds that are starting to be a problem in some fields should be managed early in the season, as weed control can be tougher later in the growing season.

Hear the complete crop and weather report with Doug Cooper

This article was published originally on 6/8/2009 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.