August 10, 2008
 

Iowa State University Extension Information for Southeast Iowa
By Jim Fawcett, ISU Extension Field Agronomist
4265 Oak Crest Hill Rd. SE
Iowa City, IA 52246
319-337-2145

 

SOYBEANS
 
Soybean Aphid Update - Many Fields Over Economic Threshold

 

Many fields are over the economic threshold of 250 aphids per plant (&  80% of plants infested) and spraying has been going strong for over a week north of I-80.  Some fields have now also reached the threshold in the south. It is obvious that this will be a worse year for aphids than what we experienced two years ago. Some have seen aphid numbers decline recently in fields that haven't reached the threshold, so fields need to be scouted to determine whether an insecticide treatment is justified. The hot weather may be doing some good. Some studies have shown that soybean aphid's rate of reproduction slows with temperatures above 86F and stops at 95F.  Heavy rains may reduce aphid numbers somewhat, but it is not likely that rains will solve the problem. Many are using a combination of about a 1/3 rate of Lorsban plus 2/3-3/4 rate of a synthetic pyrethroid. The Lorsban gives a quick kill and can help to prevent spider mite outbreaks and the pyrethroid gives longer residual. August is a critical month for soybeans so its important to protect those good potential yields out there, especially with the price of beans today. Brian Lang found substantial yield increases by spraying soybeans that were over the aphid threshold in 2005 even though the soybeans didn't show stress and were growing under good conditions. The average yield increase on 8 soybean varieties at the NE Iowa Research farm in 2005 was about 9 bu/A with an economic return of about $31/A. Many are seeing the small white aphids again this August. These are soybean aphids and are also causing damage, so should not be ignored, although they likely do not cause as much damage as the more healthy greenish aphids. Don't confuse the shed white skins with the living aphids. Aphid numbers usually drop dramatically at the end of August when winged aphids return to buckthorn to lay their eggs, but we have at least another 2 weeks that fields need to be scouted for this pest.

 

It is best to count all aphids on a few plants to get a feel for what 100 and 250 look like and then estimate from that point on. Another method of scouting developed at the University of Minnesota that can help to speed up the process can be found at http://www.soybeans.umn.edu/crop/insects/aphid/aphid_sampling.htm.

 

Some insecticide trial results on aphids can be found at http://www.ipm.iastate.edu/ipm/icm/2006/1-23/insecticide.html. See the latest Iowa aphid information at http://www.ipm.iastate.edu/ipm/icm/2007/7-16/soybeanaphid.html

 

Asian Soybean Rust

 

X.B. Yang now predicts that there is a greater than 50% chance that Iowa will see its first Asian soybean rust this year, but the good news is that if it arrives it will be late enough in the season that the impact will be minimal. Visit the USDA Soybean Rust website for more updates. http://www.sbrusa.net/

 

Other Soybean Diseases

 

The high humidity has been good for white mold development and problems are now showing up in some fields. Sudden death is also apparent in some fields.

FOR YOUR CALENDAR
 

White Mold, Managing Rotation Resistant Rootworms & Fungicides on Corn Discussed at Pest Management Tour on August 27 - 6:00 p.m.

Benton County

Corn and soybean disease management and managing rotation resistant rootworms will be featured at the Dean Folkman farm just east of Newhall (just west of Linn Coop). X.B. Yang will discuss the results of a white mold fungicide trial on the farm and give a soybean rust update. Early observations about fungicides sprayed on corn this year will be discussed as well. Patti Prasifka, ISU Entomologist, will discuss the results of the rotation resistant rootworm monitoring that has been done in the area this year as well as management options for the problem. BASF and Linn Coop. will be providing a free meal and 6:00 p.m. followed by the tour.

 

Fall Field Day SE Iowa Research Farm  - Crawfordsville

September 6 - 1:30 p.m.

Topics will include grain storage issues, grain marketing, low linolenic soybeans and other alternative crop opportunities, and managing extended diapause northern corn rootworms.