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East-Central and Southeast Iowa Crop Information


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July 17, 2008

July 17, 2008




Japanese Beetles


Japanese Beetles were reported on ornamentals in Scott County some time ago, and their population exploded in some fields within the last few days.  Reports of high populations in agronomic fields have come in from eastern Jackson, eastern Clinton, Scott, and northeastern Muscatine County.  The adults have a metallic green head and pronotum (neck region) and reddish bronze wing covers with a row of white hair tufts along the abdomen. There is a single generation in the Midwest, with beetles living for 30 – 45 days. Beetle feeding tends to occur from late June to early September, with the heaviest defoliation in July and August.

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In soybean, the economic threshold for the beetles is the general leaf defoliation threshold used for other pests. During the reproductive stages of soybeans, the threshold for considering an insecticide treatment is if greater than 20% of the leaf area is gone due to feeding. However, the high price of soybean may lower the threshold slightly.  But remember that most people tend to overestimate defoliation. If it looks like 20% defoliation, most likely it’s only 10%. The pictures in the following article can help in estimating percent defoliation: 


In corn, Japanese Beetle adults feed on leaves, tassels, silks, and pollen and are capable of interfering with pollination   With later-than-usual pollination this year, silk clipping may be more of a threat than usual.  If silks are being clipped to within 0.5 inch of the end of the ear and pollination has not yet occurred, consider and insecticidal treatment.


For more information and insecticides labeled for Japanese beetles see the July 28, 2003 ICM Newsletter at


Foliar Fertilization?


The higher grain prices are increasing interest in trying to boost yields with foliar fertilizer.


Antonio Mallarino, ISU Extension Soil Fertility Specialist, has extensively tested foliar fertilization of soybeans recently. Although he occasionally obtained yield boosts of 5 bushels per acre or more, the average yield increase he obtained with foliar fertilization was less than 1 bushel per acre. For more information, see Antonio’s article in the ICM News at


Less work has been done on foliar fertilization of corn.  Two items worth reviewing are from John Sawyer, ISU Extension Soil Fertility Specialist, and others, at and from Robert Dobbs and others, Mississippi State University, at




Soybean Aphid


I have not yet personally observed any soybean aphid activity in any of the counties I cover.  However, a very credible scout reported finding a few small colonies in one soybean field in central Muscatine County and sent me some pictures.  So soybean aphids may be, and most likely are, in some other fields in the counties I cover. 


Soybean aphids tend to not be a problem in even-numbered years, and, so far, it looks like 2008 will continue the pattern.  If the pattern does continue, there may still be a few fields with economic injury, so looking for this pest during routine scouting is advisable.  The economic threshold is 250 aphids per plant with 80% of the plants being infested and with populations increasing.  Spraying too early can actually make the problem worse by killing the beneficial insects that are helping to keep the population in check and may increase the likelihood that the field will need to be sprayed later.  Scouting techniques and management information can be found in SP 247, Soybean Aphids in Iowa – 2007, which can downloaded from





If you have any questions, please feel free to contact the Iowa State University Extension Office.
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Last Update: July 17, 2008
Contact: Virgil Schmitt

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