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
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).
Chlorpyrifos products are generally considered to
have shorter residual activity than do synthetic pyrethroids. However, in
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 have
been found in soybean fields in northern
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 http://www.ipm.iastate.edu/ipm/icm/2003/7-28-2003/japanesebeetle.html.
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:
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
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 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 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.
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.
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,
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 http://www.ipm.iastate.edu/ipm/icm/2006/6-26/forage.html.
FOR YOUR CALENDAR
Agronomy Field Day at the Northwestern
Illinois Agricultural Research and
Forage Field Day of the
Farm Progress Show at Amana – August 29
– 31, 2006. Stop at the
Fall Field Day at the Southeast
Research and Demonstration Farm (
If you have any questions, please feel free to contact the Iowa State University Extension Office.
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