Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


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June 29, 2007

June 29, 2007




Cupped Soybean Leaves


I have been getting a number of calls related to soybeans that have cupped up or have malformed leaves that look like dicamba or growth regulator herbicide injury. When this problem occurs, the possibility of spray drift needs to be investigated. If an herbicide has drifted from a nearby corn field or roadside, the symptoms should be greatest near the source of the drift and gradually diminish with a greater distance from the source. Also the symptoms should be less near anything that would have obstructed the drift, such as tall grass or trees. If a drift pattern is not evident, then it is unlikely to be the reason for the symptoms.

 If the symptoms appear after a field is sprayed and there is a sprayer pattern to the symptoms, the possibility of sprayer contamination with dicamba (Banvel, Clarity, Northstar, Distinct) or another growth regulator herbicide should be investigated. However, we do occasionally see these symptoms in the absence of a growth regulator herbicide. Occasionally additives, such as ammonium sulfate (AMS), 28% Nitrogen solution, or surfactants can cause these symptoms. I have also seen symptoms not show up until 2 or more weeks after the field is sprayed. If dicamba was the source of the problem, symptoms should show up within a day or two.

In some cases the entire field showed the problem before anything was sprayed on the field or in neighboring fields. Sometimes the symptoms are uniform across the entire field and sometimes certain parts of the field are worse than others, but there is no drift pattern. I see this most often when the soybeans go through a growth spurt when temperatures are high after some cool weather or after a heavy rainfall. Apparently under these conditions, the balance of naturally occurring hormones in the plant is disrupted, resulting in symptoms characteristic of growth regulator herbicide damage. Usually the soybeans recover from this condition with little to no effect on the final yield. See the July 19, 1999 ICM Newsletter for more information (


Asian Soybean Rust


Soybean rust has been detected on sentinel plots in central Louisiana, which is the furthest north so far this year.  This is near the area which was the source of the inoculum for the northward movement of the disease last September, so we need to keep an eye on the south. See for the latest info.


Soybean Aphids


Soybean aphids are starting to appear in some soybean fields north of I80 in very low numbers. At these low levels the beneficial insects can keep the problem in check. Unnecessary insecticide applications can contribute to aphid outbreaks by killing off the beneficial insects. The economic threshold for soybean aphids is 250 aphids per plant.





Japanese Beetles


In addition to the 3.3” of rain in my rain gauge near Shueyville in northern Johnson County last Saturday, I found 4 Japanese beetles. It’s beginning to look like we may have high numbers of beetles in the areas that have had problems in past years.

Huge numbers of Japanese beetles have been reported in Illinois this year. It was reported that a total of 71,984 Japanese beetles were caught in a single trap in a 24-hour period in Massac county southern Illinois on June 20-21. The beetles have mainly been a problem near urban areas. The beetles will feed on soybeans and corn (as well as hundreds of other species of plants), but the damage to soybeans usually isn’t sufficient to pay for an insecticide treatment. A general threshold for soybeans is to consider an insecticide if there is 20% leaf defoliation during the reproductive stages. Most people tend to greatly over estimate percent defoliation. The pictures in the following ICM article can help in estimating leaf defoliation:


The beetles can cause a substantial yield loss in corn, since they often feed on the silks, so corn fields need to be watched closely in the next few weeks in the areas where Japanese beetles are being observed, such as near Cedar Rapids.  An insecticide should be considered if the beetles are keeping the silks clipped to within 0.5” of the ear. In past years, much of the silk clipping was done by the beetles after pollination was complete. It seems like the beetles are emerging earlier than in the past, which means any silk clipping is more likely to affect pollination. The beetles will continue to emerge for several weeks and can live for 30-45 days, so they can continue to cause problems into August. For more information including pictures of the beetles, see the August 19, 2002 ICM Newsletter at




Midwest Strip Till Conference – July 31, 2007

9 a.m. to 3:00 p.m.



Organized by Research and Extension of Iowa State University, the University of Minnesota, the University of Wisconsin, and Hawkeye Community College. Manufacturers will demonstrate equipment for strip-tillage and associated operations, including auto-guidance systems and fertilizer injectors.  Researchers, farmers, and industry representatives will present the latest information on strip-tillage related topics, including equipment selection, fertility management, and guidance technology. Participants will review information booths all day, and lunch is available on site.  This program is free and open to the public. Five Certified Crop Advisor CEUs (4.5 SW & 0.5 NM) will be available for a nominal fee.  Expo details are at:


Soybean Aphid and Bean Leaf Beetle Management Tour – August 8


Management techniques for the soybean aphid and bean leaf beetle will be highlighted at a tour on the Iowa Learning farm site on the Rob Stout farm south of West Chester on Wednesday, Aug. 8. Since first being discovered in the Midwest in 2000, soybean aphids have tended to be more of a concern in odd numbered years, so this may be more of a pest this year than last. No-till soybean plots that were planted with and without the seed treatment “Cruiser” are the focus of research conducted on this Iowa Learning Farm site. Seed applied insecticides can provide good early season bean leaf beetle control and also provide some control of soybean aphids, especially when planting is delayed as it was this spring. Also discussed at the tour will be value added crop opportunities, including “low lin” soybeans. A rain simulator will also be demonstrated at the site. A free meal, courtesy of QUALISOY ( will be available at 6:30 p.m. followed by the tour. The Iowa Learning Farm project is a unique partnership of agencies, farm and conservation groups, the general public and Iowa State University. Iowa Learning Farm project staff work to increase the adoption of residue management and conservation practices that are expected to improve water quality.



If you have any questions, please feel free to contact the Iowa State University Extension Office.

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Last Update: June 29, 2007
Contact: Jim Fawcett

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