August 22, 2008
Hopefully we are close to the end of the soybean aphid season, but I don’t think we’d better use the calendar as a guide as to when to stop spraying. It appears that soybean aphid populations are continuing to increase in many fields. The later planted fields and fields where a “preventive insecticide” (killing the beneficial insects) was included in the last herbicide application appear to be the fields most commonly over the threshold for Soybean Aphid. The economic threshold is 250 aphids per plant with 80% of the plants being infested and with populations increasing.
Once the soybeans reach growth stage R 5.5, an insecticide application is not needed. Stage R5.5 is halfway between R5 (seed is 1/8” in pods from one of the upper four nodes) and R6 (full seed size). Early planted fields are reaching this stage now, but most fields planted in mid-May or later have a few days to a week+ to go. Soybean aphid populations are declining in some fields, and if populations are declining or if the soybean aphids are preparing to leave the field, an insecticide application is not needed. So, take note of the presence of winged aphids and alatoid nymphs (with wing pads), high predator activity, and/or diseased aphids as these are all signs that the population is in decline or will leave the field shortly. Scout these same fields again within a few days to note if populations are increasing or decreasing.
One alternative for scouting is to use the “speed scouting” method developed at the University of Minnesota. You only have to be able to count to 40 to use this method, but need to take a spreadsheet to the field to take notes and make a decision. The following site describes the method and provides a link to download the spreadsheet. http://www.soybeans.umn.edu/crop/insects/aphid/aphid_sampling.htm
Scouting techniques and management information can be found in SP 247, Soybean Aphids in Iowa – 2007, which can downloaded from http://www.extension.iastate.edu/Pages/eccrops/transfer/07SBA.pdf.
I have been receiving many inquiries about when to expect feeding from Japanese Beetles to cease. There is a single generation in the Midwest, with beetles living for 30 – 45 days. Beetle feeding tends to occur from late June to early September, with the heaviest defoliation in July and August. With the generally cooler-than-normal weather, I would expect the beetles to live closer to 45 days than to 30 days. The population of beetles in many soybean fields “exploded” about July 12 – 14, so adding 45 days to that would suggest feeding will continue for most of the rest of August.
In soybean, the economic threshold for the beetles is the general leaf defoliation threshold used for other pests. During the reproductive stages of soybeans, the threshold for considering an insecticide treatment is if greater than 20% of the leaf area is gone due to feeding. However, the high price of soybean may lower the threshold slightly. But remember that most people tend to overestimate defoliation. If it looks like 20% defoliation, most likely it’s only 10%. The pictures in the following article can help in estimating percent defoliation: http://www.ipm.iastate.edu/ipm/icm/2002/7-29-2002/soydefoliation.html.
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.
Soybean Sudden Death Syndrome
Soybean Sudden Death Syndrome (SDS) is common again this year, but does not appear to be quite as widespread as last year. You can’t really identify SDS from your pickup window because Brown Stem Rot (BSR) can cause leaf symptoms identical to SDS. You need to split some stems lengthwise to know which disease is present. See pages 70 – 72 of the March 26, 2007 Integrated Crop Management Newsletter or http://www.ipm.iastate.edu/ipm/icm/2007/3-26/bsr_vs_sds.html for identification and management of SDS and BSR.
If the field has not been tested for Soybean Cyst Nematode (SCN), the presence of SDS in the field should prompt a soil test for SCN as SCN is usually present if SDS is present. The sample submission form and instructions for taking the sample are in PD-32 “Plant Nematode Sample Submission Form” which is available at Iowa State University Extension offices or can be downloaded from http://www.extension.iastate.edu/Publications/PD32.pdf.
FOR YOUR CALENDAR
Fall Field Day
SE IA Research & Demonstration Farm – Crawfordsville
Wednesday, September 10 1:00 p.m.
Tour stops will include: current crop concerns, farm bill update and marketing in volatile times, harvest, drying, and storage concerns with the 2008 crop, and alternatives to glyphosate for soybean weed control.
If you have any questions, please feel free to contact the Iowa State University Extension Office.
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