Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


July 27, 2006

July 31, 2006


Rotation Resistant Corn Rootworm

Emergence traps in corn planted on soybean ground are confirming the presence of corn rootworms that are resistant to the practice of crop rotation in several fields in SE Iowa. The majority of the problem is due to northern corn rootworms that have extended diapause (eggs hatch out 2 years after being laid), although some cases of the variant of the western rootworm that lays its eggs in soybean fields have also been found.  Growers in the counties I cover want to monitor soybean fields for western corn rootworm beetles by using the Pherocon AM yellow sticky traps. Usually when western corn rootworm beetles (the striped ones) are found in soybean fields it indicates that they are laying eggs, which could cause a problem in next year’s corn crop. Northern corn rootworm beetles (the solid yellow/green ones) are commonly found in soybeans. It is not believed that they lay eggs in soybeans, although it is not known for certain, and additional research is being conducted to investigate this. One source of the Pherocon AM traps is here:  For more information see pages 209 – 211 of the July 24, 2006 ICM Newsletter or


Selecting Soybean Insecticides

Several insects are catching peoples’ attention.  If you encounter populations requiring spraying, remember:

1.     Synthetic pyrethroids kill many beneficials but have little effect on spider mites.  If a field has a low level of spider mites and a synthetic pyrethroid is used for another insect, the destruction of the beneficials can cause spider mites to flair up.  So, especially in some of the drier areas, be sure to inspect the field closely for evidence of spider mites before treating with a synthetic pyrethroid.  If spider mites are present, consider selecting chlorpyrifos (Lorsban or a generic).

2.     Dimethoate products have excellent activity on spider mites and many other insects (including beneficials), but are weak on soybean aphids; soybean aphids can, therefore flair up after using a dimethoate product.  Before using a dimethoate product, inspect fields closely for soybean aphids.  If soybean aphids are present, consider selecting chlorpyrifos (Lorsban or a generic).

3.     Chlorpyrifos products are generally considered to have shorter residual activity than do synthetic pyrethroids.  However, in Illinois in 2003, in areas where the weather was very hot (above 95 degrees), the experience was that chlorpyrifos had a longer residual life.  Dimethoate has a very short residual life, so consider it to be a knock-down only.

4.     Be sure to watch Pre-Harvest Intervals (PHIs).  Note that Warrior’s PHI has changed from 45 days to 30 days.  I am note aware of any others that have changed since last year, but check the label.

Spider Mites

Spider mites have been found in soybean fields in northern Des Moines County.  Be especially alert for this pest in the areas that have missed most of the rains. For more information see pages 148 – 148 of the July 22, 2002 ICM Newsletter at

Japanese Beetles

The economic threshold for Japanese beetles (as well as other defoliating insects such as bean leaf beetles and grasshoppers) is 20% defoliation during pod fill. What most people call 50% defoliation is actually closer to 20%, so it is very rare for soybeans to exceed the 20% threshold this time of year. For more information and insecticides labeled for Japanese beetles see pages 131 – 132 of the July 28, 2003 ICM Newsletter at

Bean Leaf Beetles


Sweep nets are now picking up a combination of gray (newly emerged) and colored (older) bean leaf beetles.  While it is impossible to be certain that the gray beetles are not late emerging first generation beetles, most likely they are early emerging second generation beetles.  Likewise, it is impossible to be certain if colored beetles are old first generation beetles or second generation beetles that have already gained their color.  So how do you interpret what you find in your sweep net?  This is a weakness of the system.  According to Marlin Rice, Extension Entomologist, because we cannot be certain about the age of the beetles in the sweep net at this time, it is best to assume that they all are second generation beetles.  Threshold information can be found on pages 155 – 157 of the August 21, 2000 ICM Newsletter or:


Soybean Aphids


Soybean aphids started showing up a couple of weeks ago in the area, but are still present at very low levels, usually less than 10 aphids per plant. The economic threshold is 250 aphids per plant. Low levels of aphids are being reported throughout the Midwest, so this is not likely to be a bad aphid year, although scouting should continue. Spraying aphids when aphid numbers are so low could actually make the problem worse by killing off the beneficials which can help to keep the aphid numbers down.

White Mold

White mold is starting to show up again in areas that had the problem two years ago, especially if rainfall has been normal to above normal. There is nothing to do at this time for white mold. Cobra and/or fungicide applications at beginning flowering, around July 1, can reduce infections, but by the time the disease symptoms show up, it’s too late. For more information, see page 193 of the July 3, 2006 ICM Newsletter or .

Other Soybean Diseases

Most soybean fields have had low levels of disease this year, although it is common to find brown spot on the lowest leaves. Bacterial blight and downy mildew can be found at low levels in many fields on the upper leaves, and sudden death syndrome is starting to show up in some fields. Frogeye leaf spot is at low levels in many fields now. Many fields show some soybeans with virus-like disease symptoms (distorted leaves), likely caused by bean pod mottle virus and/or soybean mosaic.


Potato Leafhoppers

High numbers of potato leafhoppers can be found in some hay fields. For more information on managing potato leafhopper, see pages 107 – 110 of the June 21, 1999 Iowa State University Integrated Crop Management Newsletter or  Remember, waiting to see hopperburn is waiting too long as substantial losses have already occurred by that time.

Cowpea Aphids

While we are awaiting DNA confirmation, we are quite sure that cowpea aphids were discovered in alfalfa a couple of weeks ago near where Cedar, Clinton, and Scott Counties join and also in Muscatine County. There are unconfirmed reports that this pest was found in the Maquoketa area in 2005.  This is the first report of this pest problem in SE Iowa. It has caused problems in western and NE Iowa for several years now. If you are finding black aphids in alfalfa, they are almost certainly cowpea aphids. Although there are not yet specific economic thresholds for this pest, some use the same threshold as for the blue alfalfa aphid, which is 12/stem on newly cut alfalfa and 60/stem on alfalfa 12” tall and taller. See pages 116 – 117 of the July 14, 2003 ICM Newsletter or for more information.

Late Summer Seedings

The first three weeks of August is the best time to attempt late summer seedings of forages in the central third of the state. They can be successful up to September 1 in the southern part of the state. They are most successful if there is adequate soil moisture, so if the dry areas remain dry, it may be best to wait until next spring. For some tips on late summer forage seeding, see pages 187 – 188 of the June 26, 2006 ICM Newsletter or





Agronomy Field Day at the Northwestern Illinois Agricultural Research and Demonstration Center (Monmouth, IL) – August 22, 2006, 8:00 a.m.  See for details.


Forage Field Day of the Eastern Iowa Hay Producers Association – August 28, 2006.  Details are being finalized.  Watch for details.


Farm Progress Show at Amana – August 29 – 31, 2006.  Stop at the Iowa State University display.  See for show information.


Fall Field Day at the Southeast Research and Demonstration Farm (Crawfordsville, IA) – September 13, 2006, 1:30 p.m.  See for details as they are announced.


If you have any questions, please feel free to contact the Iowa State University Extension Office.
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Last Update: July 31, 2006
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