Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


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May 17, 2011




Black Cutworm


Black cutworms have started feeding in southern Iowa, essentially south of Highway 92. Based on temperatures since the first significant flights of Black Cutworm moths into Iowa, and using National Weather Service forecasts through May 22, cutting in eastern Iowa, if it occurs, should have begun along Highway 34 on Wednesday, May 18 and should begin along Interstate 80 on Friday, May 20, along Highway 30 on Saturday, May 21, and along Highway 20 on Monday, May 23. It is always good to start scouting a few days before the predicted onset of cutting.


Of course, any changes in predicted temperatures may alter the onset of any cutting. You can monitor black cutworm growing degree days for east-central and southeast Iowa at   A more comprehensive overview of the 2011 black cutworm situation is posted at


Also, remember that, with high priced corn, the damage threshold for treatment is lower. An excellent tool is the dynamic black cutworm action threshold, which is an Excel spreadsheet. A description of this tool and a link to download it are at





Brian Lang, my counterpart to the north, wrote a nice item about armyworms, which follows. It is appropriate for the counties I cover, too.


We had some Armyworm problems last year in mid-May to mid-June. As you walk emerged corn fields consider watching for Armyworm, especially if it’s corn following winter rye or in a spring herbicide killed CRP field. Winter rye is an excellent "bug magnet" to attract armyworm moths for egg laying. Background:  Moths usually arrive in April and lay eggs in lush grass vegetation. Larvae often hide during the day and feed at night or on overcast days. They tend to be most active early morning and late evening. They tend to feed on leaf edges first. Heavy infestations on young corn (V7 or less) can consume the entire plant. On older plants, they often do not consume the tougher midrib. Feeding usually starts on the lower leaves and progresses up the plant.  Many larvae may feed on the same plant. As long as they do not consume the main growing point, corn can recover. Treatment:  Spraying is justified if when 25% of the corn plants are damaged and the insect is still present. Many insecticides are labeled for control, and Armyworm is easily killed with insecticides. Armyworms larger than 1.5 inches are basically done feeding and will soon be pupating. Additional information and photos are at”





Plant Populations


Even though optimum seeding rates have been increasing every year for corn, recommended seeding rates for soybeans have been going in the other direction. Recent work by Palle Pedersen has shown that the optimum final stand for soybeans is 100,000 plants per acre. Optimum seeding rates will vary depending on the seeding method used and germination of the seed, but it will seldom pay to seed at higher than 125,000-140,000 seeds per acre. Because of soybean’s ability to compensate for lower stands by branching out and producing more pods/plant and more seeds/pod, yields do not decrease much until populations get below 75,000 plants/acre. See Palle’s fact sheet “Optimum Plant Population in Iowa” at for more information.


Seed Treatments?


As we get later into May with warmer soil temperatures, it becomes less likely that fungicide seed treatments for soybeans will pay off. Insecticide seed treatments do a nice job of controlling overwintering bean leaf beetles, but they are a greater problem with the early planted soybeans which are already in the ground. The cold winter should also have reduced our bean leaf beetle populations. Insecticide seed treatments are not very effective for controlling soybean aphids, because the treatment does not last long enough for killing aphids in August.


Seedling Diseases Wanted (Believe it or not!)


Dr. Alison Robertson is doing research, as part of a national study, on soybean seedling diseases / damping off and is looking for fields with severe damping off issues. She will come to the field shortly after receiving notification and collect some samples. If you have or find a field with damping off problems and would be willing to have it be part of this research, please contact Alison at or (515) 294-6708 or you may contact me.





Alfalfa Weevil


I have not encountered nor heard of any significant feeding of alfalfa weevils in the counties I cover.   However, it is the better part of valor to continue to watch alfalfa fields for signs of feeding. For details on scouting for and managing alfalfa weevils, see the Integrated Crop News article at and Alfalfa weevil activity is based on Growing Degree Days Base 48. Growing Degree Days Base 48 for Burlington, Davenport, Cedar Rapids, and Dubuque are posted at







SPRING FIELD DAY (afternoon) &


JUNE 15, 2011

Details are posted at




Doug Nolte and the Iowa Learning Farm (Iowa State University), hosts

1021 Highway 6, West Liberty, IA

JUNE 22, 2011, 10:30 a.m. – Noon, followed by lunch

The farm is just in Muscatine County near the Johnson County line. Program details will be forthcoming.




JUNE 28, 2011, 10 a.m. – 3 p.m., DYERSVILLE, IA

JUNE 29, 2011, 10 a.m. to 3 p.m., WELTON, IA

These workshops will focus on manure management issues. Details will be forthcoming about June 1.





JUNE 29, 2011, 1 – 4:30 p.m.

Details are posted at





JULY 19, 2011

Details will be posted soon at








If you have any questions, please feel free to contact the Iowa State University Extension Office.
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Last Update: May 17, 2011
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