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7/29/2013 - 8/4/2013

Time to Assess Corn Rootworm Activity in Iowa

By Erin Hodgson and Aaron Gassmann, Department of Entomology

Corn rootworm egg hatch was delayed this year because of cool spring temperatures, and adults are beginning to emerge throughout Iowa. Now is a good time to assess root injury caused by larvae and also monitor for adult activity in corn.



Evaluate root injury to better understand the efficacy of your management program. Monitoring over several years will help establish a historical record of how larvae respond to management tactics (e.g., crop rotation, Bt corn, soil insecticides, etc.). One common outcome of severe larval feeding is lodging of corn plants. However, it is important to confirm that feeding from corn rootworm was the cause of lodging and that it did not results from other factors such as strong winds.

The most common scale for rating root injury is the 0-3 scale developed at Iowa State University:

  • 0 = no injury
  • 1 = one complete node (approximately 10 roots) is pruned to within 1.5 inches of the stalk
  • 2 = two complete nodes (approximately 20 roots) are pruned to within 1.5 inches of the stalk
  • 3 = three complete nodes (approximately 30 roots) are pruned to within 1.5 inches of the stalk

The Iowa State University root injury scale is linear and directly related to plant lodging and yield loss. Root injury that exceeds 0.25 is likely causing economic loss. For Bt hybrids, any injury ratings that exceed 1.0 would be considered unexpected. Resistance to Cry3Bb1 has been confirmed in Iowa since 2011 and is suspected for mCry3a. Consider diversifying corn rootworm management to delay resistance and improve larval management. Examples to consider include crop rotation, rotation of Bt traits, and rotations to soil-applied insecticide without a Bt trait.



Corn rootworm will feed on leaves and cause some scarring; however, this does not cause economic loss. Adults can cause yield loss if emergence occurs when corn is silking, and therefore, this is a critical time period to scout fields. Adults are strongly attracted to silks and will mass on plants to feed and mate (Photo 1). Adults that trim silks during pollen shed will interfere with optimal pollination.

Photo 1. Western corn rootworms are strongly attracted to corn silks and can interfere with pollination. Photo by John Obermeyer, Purdue University Extension.


Weather plays an important role in determining how plants respond to silk feeding. Plants do not tolerate as much feeding during pollination in hot and dry weather. For example, under ideal moisture conditions, plants could tolerate 15 beetles per plant, but that number is reduced to just five per plant under drought stress (Photo 2).

Photo 2. Rolling corn leaves is an indication of moisture stress. If plants are droughty during silking, fewer beetles are needed to cause economic loss. Photo by Virgil Schmitt, ISU Extension and Outreach.


Late-planted fields or late-flowering hybrids are generally attractive to adult corn rootworm. Silks will still be developing in these fields when older fields have brown or drying silks. Adults may migrate and aggregate in this later-maturing fields.

It is always a smart idea to scout for insects in corn fields during pollination. But with adult corn rootworm becoming active during silking this year, we highly recommend keeping an eye on fields in order to protect yield. A foliar insecticide may be warranted if there are five or more beetles per plant, silks have been clipped to less than one-half inch of the ear tip, and pollination is not complete. Also take into consideration other insects that may be feeding on the silks at the same time (e.g., Japanese beetle).

If you find injury to Bt corn of more than one node, this may be due to western corn rootworm that have developed Bt resistance, and resistance should be suspected in these cases. The Department of Entomology at Iowa State University is working to understand the extent of resistance within the state and to develop management recommendations for Bt-resistant populations of western corn rootworm.  Please contact Aaron Gassmann or Erin Hodgson to report fields with suspected resistance.  


Erin Hodgson is an associate professor of entomology with extension and research responsibilities; contact 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 reached at 515-294-7623 or

ISU Extension and Outreach Offers Iowa Drainage School

By Kapil Arora and Brent Pringnitz, ISU Extension and Outreach

Agricultural drainage is becoming increasingly important due to the critical role it plays for Iowa's emerging bio-economy. Drainage systems that are properly designed and operating are essential to achieving excellent agricultural production capability.

The Iowa Drainage School is being offered to address these issues on Aug. 20-22, 2013, at the Borlaug Learning Center on the Northeast Research and Demonstration Farm near Nashua, Iowa.

"People looking to install a new drainage system or retrofit an existing system will want to attend this school," said Matt Helmers, Iowa State University Extension and Outreach agricultural engineer. "The workshop will focus on drainage design, economics of drainage, water management and legal issues related to drainage."

The intent of the Iowa Drainage School is to provide training about agricultural drainage concepts; planning and laying out drainage systems including surveying a profile, calculating tile line sizes and spacing using actual field data; and making connections and setting up drainage control structures. Regulatory considerations and fixing common drainage system issues will also be discussed.

Drainage contractors, landowners, professional engineers and consultants, NRCS professionals, county administrators and others who are involved in making drainage design decisions are invited to attend.

The three-day school features a combination of hands-on training, lecture and discussion, and problem solving using examples. By attending this school, participants will be able to plan and layout subsurface drainage systems and work out project costs.

Registration fees for this three-day school are $325 per person if registered by midnight, Aug. 7. Late registration is $375 and must be received by midnight, Aug. 14. Class size is limited to 40 participants and pre-registration is required. Registration fees include meals indicated on the agenda, refreshments and notebook.

Additional information, a detailed agenda, and online registration are available at

Kapil Arora is an agricultural engineering specialist. He can be reached at (515) 382-6551or Brent Pringnitz is an ANR Program Services coordinator. He can be reached at (515) 294-6429 or

Short Corn and Variable Growth – 2013

By Roger Elmore, Mahdi Al-Kaisi and Elwynn Taylor, Department of Agronomy

“Knee high by the fourth of July!” Some of us grew up with that old saying which either foreshadowed disaster or forecast bumper yields. If corn was knee high by the fourth, most thought it would mature before the first fall frost. That was good news! As we look back, the statement perhaps was valid back when the first target corn planting date was mid-May.


Conditions and responses change

Now farmers begin to plant in mid- to late- April and most wrap up planting by mid-May. These changes were driven by many factors including reduced tillage systems, better herbicides, more cold-tolerant hybrids, improved seed treatments, improved planter systems, larger farms, climate change ….and we’ve probably missed others.  And now, we’re used to seeing corn “…as high as an elephant’s eye, An’ it looks like its climbin’ clear up to the sky” (Rodgers and Hammerstein, 1943, musical, Oklahoma) by the fourth of July. But that didn’t happen this year. 

Many agronomists report not only shorter corn plants than normal this year - at least in some parts of Iowa – but also more variability across what usually appear as uniform fields.  Why?


Late planting dates

Remember when 2013 first rolled in, the impact and lingering effects of the 2012 drought still haunted us. Several articles in this newsletter reflected our concerns.  By April 28 only 2 percent of Iowa’s corn lay in seed furrows, soaking up water, and bracing for up to several inches of snow. Then, after the snow, wet conditions stymied planting. By May 12th, only 15 percent of Iowa’s corn had been planted ( USDA data). A week later though, another 56 percent was planted! We planted about 1.5 million acres per field work day the week of May 13th – probably the most we’ve ever accomplished in one ‘field work day’ (Figure 1).

Figure 1. Corn planting progress in Iowa, 2004 and 2008 to 2013. Data compiled from USDA. The “blue bar” in the figure represents half (50 percent) of the acres planted. In 2010 (yellow) the crop was half planted April 18th; in 2013 the half-planted date was about May 15th. The very high yield year of 2004 was 50 percent planted by April 30th. Full-size image


We finally reached 99 percent of Iowa’s corn planted on July 1st. That ranks 2013 alongside 2008 in recent history; two of the slowest and most drawn out planting seasons of modern times.  Incidentally, 2010 reflects the earliest we’ve ever planted corn in Iowa, and 2004 and 2009 reflect years with the highest state-wide average yields recorded.

But later planting dates usually result in taller corn than earlier planting dates since plants develop during periods with long days. Plant height increases because of elongated internodes during such days.  So, what happened this year?


Soil temperature effect on corn height

We know that small differences in soil temperature and moisture affect both above- and below-ground corn growth.  Root depth follows the downward progression of temperature increases.  Generally, roots do not grow in soils much colder than 50⁰F.

Most reports dealing with corn height and development and correlations with soil temperatures are found in residue management research.  More residue cover increases soil moisture content and decreases soil temperatures.  Associated with the reduced soil temperatures is a decrease in plant height and decreased dry matter at specific calendar dates during vegetative development.  However, corn grown with the same residue amounts had similar heights and dry matter when compared at the same vegetative development stages.  Since seedling emergence is correlated with temperature, cold soils result in slow emergence.

Illinois researchers compared corn growth and development by altering soil temperatures (±9 ⁰F from ambient soil temperatures) with an underground heating system utilized through V5 - 5 collared leaves. In addition to delaying development in terms of calendar days, grain yield declined with the coldest soil temperatures. With these coldest soils, lower canopy leaves (leaves 1 to 13) had greater leaf area than upper canopy leaves (leaves 15-21). The opposite was true with the warm soil treatment.  Upper canopy leaves contribute more to yield than lower canopy leaves.

All leaf development initiates while the growing point is still underground - before V6; soil temperatures up to V6 thus affect early crop development.   Noticeable stalk elongation begins at around V5.  Root development and leaf initiation have priority before that time.  Cell expansion occurs near the internode bases. This process of cell expansion is influenced not only by the amount of light and day length – the latter discussed above in relation to late planting – but also by temperature.  Early-season cold temperatures increase internode rigidity and limit cell expansion and internode elongation.


2013 soil temperatures

Soil temperatures this year fell below normal during the five weeks when most of Iowa’s corn was planted and considerably below what we experienced in 2012 (Figure 2).  We speculate that abnormally cool 2013 soil temperatures affected corn internode elongation and thus resulted in shorter corn plants, especially those planted from April through mid- to late- May. 

Figure 2. Soil temperatures in 2013 compared to those of 2012 and the 10-year average at Gilbert Iowa, Central Iowa. Data compiled from MESONET. Iowa soil temperature was 5 to 8⁰F colder than usual in the late April to May 5 period. Between May 6th and June 10th soils were again several degrees colder than usual. Full-size image


In addition, within field variations in plant heights seen in fields this year may be due to uneven residue distribution across the field. Differing residue levels alters both soil temperatures and soil moisture affecting corn growth and development.  Other factors may affect within-field plant height variations as well.

Plant height itself is not necessarily a good indicator of corn yields if light interception is near complete at silking.  As mentioned above though, if upper canopy leaf areas are reduced by the cool temperatures this spring, grain yield will be reduced. However, yield potential in 2013 is likely compromised by the delayed planting and possibly cool and wet soil conditions experienced early in the growing season. An early frost will be detrimental.

Cool, wet weather this spring reduced corn plant heights and increased variability of plant heights across fields.  Shorter plants are not necessarily lower yielding. Whether corn is knee-high or elephant-eye high by the 4th of July is less important than whether the crop intercepts near maximum light at silking and the timing of the first major fall frost.



Abendroth, L.J. R.W. Elmore, M.J. Boyer, S.K. Marlay. 2011. Corn growth and development. PMR 1009. Iowa State University Extension and Outreach, Ames, Iowa.

Blanco-Canqui, Humberto,  R. Lal, W. M. Post, and L. B. Owens. 2006. Changes in Long-Term No-Till Corn Growth and Yield under Different Rates of Stover Mulch. Agron. J. 98:1128–1136.

Bolero, G. A.; D.G. Bullock, and S.E. Hollinger. 1996. Soil temperature and planting date effects on corn yield, leaf area and plant development.  Agronomy Journal, Madison, v. 88, n. 3, p. 385-390.

Fortin, M.-C.  and F.J. Pierce. 1990. Developmental and Growth Effects of Crop Residues on Corn. Agron. J. 82:710-715.

Hicks, Dale R. 2004. Corn comments: Uneven plant height.  Minnesota Crop News. 27 July 2004.

Kaspar, T.C. and W.L. Bland. 1992. Soil temperature and root growth. Soil Sci. 154:290-299.

Kravchenko, Anatoliy G.,  and Kurt D. Thelen. 2007.Effect of Winter Wheat Crop Residue on No-Till Corn Growth and Development.   Agron. J. 99:549–555.

Licht, M. and M. Al-Kaisi. 2005. Strip-Tillage Effect on Seedbed Soil Temperature and Other Soil Physical Properties. Soil Tillage Res. J. 80:233-249.

Nielsen, R.L. 2001. Short corn at tasseling.  Purdue Univ. 


The authors are professors in the Department of Agronmy. Roger Elmore, corn agronomy, can be reached at 515-294-6655 or e-mail Mahdi Al-Kaisi, soil management, can be reached at 515-294-8304 or e-mail Elwynn Taylor, ag meterology, can be reached at 515-294-1923 or e-mail

This article was published originally on 8/5/2013 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.