July 27, 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
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.
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:
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
The first find of
soybean rust on soybeans in
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.
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.
reported finding cowpea aphids in alfalfa a couple of weeks ago. This is the
first report of this pest problem in
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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