Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information

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July 27, 2006

July 27, 2006

CORN

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 in the far eastern part of the area.  Growers may want to monitor soybean fields in the eastern part of the area 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: http://www.trece.com/phercat.html#am5.  For more information see the last ICM Newsletter at http://www.ipm.iastate.edu/ipm/icm/2006/7-24/resistantcrw.html.

SOYBEANS

Spider Mites

The rains and high humidity have likely helped to keep this pest in check. Continue scouting, especially in the areas that have missed the rains. For more information see the last ICM Newsletter at http://www.ipm.iastate.edu/ipm/icm/2006/7-10/spidermites.html.

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 the July 28, 2003 ICM Newsletter at http://www.ipm.iastate.edu/ipm/icm/2003/7-28-2003/japanesebeetle.html.

Bean Leaf Beetles

 

It seldom pays to spray the first generation bean leaf beetles which are present in most soybean fields now.  Most economic damage occurs when the second generation beetles emerge in mid August, mainly because of the pod feeding that they do. The general threshold for reproductive stage soybeans is 4 to 5 per sweep in row beans and 3 to 4 per sweep in drilled beans.  More detailed threshold information can be found at:

http://www.ipm.iastate.edu/ipm/icm/1999/8-9-1999/blbeffects.html.

 

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 at least through the first week of August. 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.

Soybean Rust

The first find of soybean rust on soybeans in Louisiana was reported yesterday. This is the fourth southern state reporting rust on soybeans. Most of the finds have been in kudzu. We can thank the dry conditions in the south for reducing the threat of soybean rust this year. More info can be found at http://www.plantpath.iastate.edu/soybeanrust/node/165.

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, its too late. See the July 3, 2006 ICM Newsletter for more information at http://www.ipm.iastate.edu/ipm/icm/2006/7-3/whitemold.html. .

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 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. I am also seeing frogeye leafspot at low levels in many fields now. Most fields that I have been monitoring show some soybeans with virus-like disease symptoms, likely caused by bean pod mottle virus and/or soybean mosaic.

HAY

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 http://www.extension.iastate.edu/Pages/eccrops/potatoleafhopper.html.  Remember, waiting to see hopperburn is waiting too long as substantial losses have already occurred by that time.

Cowpea Aphids

Virgil Schmitt reported finding cowpea aphids in alfalfa a couple of weeks ago. 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 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 the July 12, 2003 ICM Newsletter for more information at http://www.ipm.iastate.edu/ipm/icm/2003/7-14-2003/cowpea.html.

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 the June 26, 2006 ICM Newsletter at http://www.ipm.iastate.edu/ipm/icm/2006/6-26/forage.html.

 

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

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Last Update: July 29, 2006
Contact: Jim Fawcett fawcett@iastate.edu


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