Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


August 4, 2005

August 4, 2005



The cooler temperatures will provide some relief for the crops, but unfortunately those who needed the rain the most got little rainfall. Unless a major weather system moves through the area soon to break the drought pattern, itís likely that the driest areas will continue to be dry. Although corn pollinated well in many fields, some kernel abortion has occurred causing tips of ears to be barren. Northern corn rootworm beetle populations have also been very high in some fields and silk clipping occurred prior to pollination further reducing yield potential. Continued dry weather will take its toll on the soybean crop as well. For those wanting to estimate corn yields, its just a matter of counting kernels:

Pull several ears of corn at random and count the number of rows of kernels and the number of kernels per row. Ignore kernels less than half size at the tip of the ear. Also measure off 1/1000 acre along one row in several places in the field and count the number of ears:

1/1000 acre =††††††† 17 ft 5" in 30" rows

14 ft 6" in 36"

13 ft 9" in 38" rows
(For 20" rows, count 2 rows 13' 1" long)


Number of kernels per 1/1000 acre = kernels per row X rows per ear X ears per 1/1000 acre.

Divide this number by about 90 to get bu/A (since there are about 90,000 kernels per bushel). If you think kernel size and weight will be smaller than normal, divide by a larger number (100+).



Two Spotted Spidermites


Spider Mites can be found in many fields in the area now, especially in the eastern part of the area I cover. Spraying has been common in Jones County and some spraying has occurred in Linn and Johnson Counties. The cooler temperatures should help somewhat to reduce the problems with the pest. An insecticide is recommended if spider mite damage is occurring in the field, mites are present, and hot, dry conditions are expected to continue. Spider mites are more common in areas where the soybeans are under stress. If damage is localized along the edge of the field, and mites can not be found in the rest of the field, it may not pay to spray the entire field. If mite damage is localized but mites can be found in low numbers in the rest of the field, spraying the entire field may pay off, especially if dry conditions persist. Most have been spraying Lorsban for spider mites, because of its better effectiveness on soybean aphids which are also present in most fields. For more information see the July 22, 2002 ICM Newsletter at and the July 18, 2005 article at


Soybean Aphids


Soybean aphid numbers have increased above the economic threshold in some fields and many fields have been sprayed in the past several days in Jones County. Some spraying has also occurred in Linn and Benton Counties. Aphid numbers usually peak in mid-late July and then have a second peak in August. Numbers have declined in some fields in the past week even when they are increasing in nearby fields. Every field is different, so fields need to be monitored closely for at least a couple of more weeks. Usually aphid numbers drop off rapidly sometime after mid-August. Treatment is recommended if aphid numbers are increasing and surpass 250 aphids/plant. The cooler temperatures will decrease moisture stress to the soybeans, but unfortunately may result in greater soybean aphid reproduction. Under the right conditions, populations can double in 2-3 days, so 250/plant can increase to 1000/plant in less than a week. For more information on soybean aphids see and the July 11, 2005  ICM Newsletter at

Asian Soybean Rust


There have been more finds in the far southern U.S. Almost all of the finds have been on sentinel plots, research plots, or on kudzu. For the latest see


Other Soybean Foliar Diseases


For some reason, downy mildew, a disease favored by cool, damp conditions, has been common this year in many soybean fields. Frogeye leafspot is also showing up in some fields. Photos of common soybean diseases can be found at:

Soybean Cyst Nematode


With all of the attention on aphids and rust, don't forget the soybean pest in Iowa that still causes the most yield lost state-wide.  Damage from the pest is more evident in dry years. The July 25 ICM Newsletter at provides photos and scouting information to help deal with this pest.




Spider Mites


Spider mites are also being found in some corn fields. An article in the July 22, 2002 ICM Newsletter shows a table to calculate economic thresholds for spider mites in corn based on work done in Nebraska and Texas. If a market value of $300 per acres is assumed for the corn and a treatment cost of $10/A is assumed, a treatment may pay if more than 20% of the leaves have 10% or more of their area covered with mites. Insecticides labeled for mites in corn include dimethoate and Capture. See the article at



Potato Leafhopper


Potato Leafhopper numbers continue to be high in many fields, so scouting of hay fields continues to be an important activity.  For information on managing potato leafhopper, see pages 107 - 110 of the June 21, 1999 Iowa State University Integrated Crop Management Newsletter or . A couple of sources for sweep nets are and



If you have any questions, please feel free to contact the Iowa State University Extension Office.
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Last Update: August 4, 2005
Contact: Jim Fawcett

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