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5/25/2009 - 5/31/2009

Week of May 26 Crop and Weather Report

Doug Cooper, Iowa State University Extension communications specialist, interviewed climatologist Elwynn Taylor, integrated pest management specialist Rich Pope, and corn agronomist Roger Elmore Tuesday, May 26 for the weekly Iowa crop and weather report.

Elmore reports heavy rain has created a bit of a dilemma for farmers trying to determine whether or not to replant corn. He says replanting at the end of May will normally reduce production by about 30 percent compared to late April early May plantings.

The rain should end as May comes to an end, according to Taylor’s interview. And once fields are suitable for fieldwork most of the spring chores should be completed.

Pope talks about early movement of black cutworm showing up in black cutworm traps. Now is the time to begin scouting for this pest that migrates into Iowa from southern locations.

Dynamic Black Cutworm Action Threshold

By Erin Hodgson and Jon Tollefson, Department of Entomology

Two things must be balanced when deciding whether to apply pesticide: the cost of control and the value of the commodity that will be saved. Action thresholds are commonly used in field crop entomology, and are defined as the pest density at which chemical controls should be applied. Action thresholds are generally meant to be dynamic based on crop market value and ever-changing control costs.

An action threshold has been established for black cutworm larval damage in corn.  Last year, 2008, corn was much more valuable and the thresholds were lowered. This year corn prices are somewhat lower and it would be appropriate to raise the threshold accordingly, i.e., don’t spend money on control unless it is profitable. This shows that it would be appropriate to routinely adjust the control action threshold as the expected yield and price per bushel of corn changes and the cost of control increases or decreases.

To facilitate calculating the changes in the black cutworm action threshold, an Excel spreadsheet with a dynamic threshold has been created. The calculations use the density of corn (plants per acre), expected yield (bushels per acre), and the anticipated value of a bushel of corn to determine the value of a single plant.

Then by dividing the cost of control by the value of a single plant, the number of plants that can be lost per acre to break even will be determined. Finally, by dividing the number of plants that can be lost to equal the cost of control by the actual stand count and multiplying by 100, the percentage of the plants that can be lost to break even is identified.

To make these calculations easier, follow this link to a downloadable spreadsheet template. There is an explanation with two examples to demonstrate the spreadsheet tools. The third column, Ex.3, contains a formula. If the user downloads the Excel spreadsheet and inserts their values for plant population, expected yield, anticipated market value, and the cost of control - an action threshold, in percent stand lost, will be calculated by the spreadsheet.

Please use anticipated yield estimates that are appropriate for the stand that has been planted and is established. By saving the Excel spreadsheet to the user’s personal computer, it can be used repeatedly as the value of corn and cost of control change.



Erin Hodgson is an assistant professor of entomology with extension and research responsibilities. She can be contacted by email at or phone (515) 294-2847. Jon Tollefson is a professor of entomology with responsibilities in research and extension. He can be reached at (515) 294-8044 or

SCN and Corn Nematode Workshops Set for July

By Greg Tylka, Department of Plant Pathology

SCN Workshop
The soybean cyst nematode (SCN) is widely considered the most damaging soybean pest in the Midwest. Effective long-term management of SCN requires thorough knowledge of SCN biology and use of field scouting, resistant soybean varieties, and nonhost crops in an integrated management approach. An all-day workshop focused on SCN will be held on Wednesday, July 29, 2009, in Ames. Topics of discussion at the workshop will include:
• basic biology of SCN
• how the nematode modifies living soybean cells to support itself
• proper techniques for in-season scouting and soil sampling for SCN
• integrated management of SCN to maintain profitable soybean production
• how performance of SCN-resistant soybean varieties varies among SCN-infested fields

There will be opportunities for participants to observe SCN-infected plants and various SCN life stages under microscopes. Also, there will be hands-on demonstrations of the procedures used to extract SCN cysts from soil and to extract SCN eggs from cysts. Workshop instructors will be Palle Pedersen, ISU extension soybean agronomist, and ISU nematologists Thomas Baum and Greg Tylka. Participants can earn 7.0 Certified Crop Adviser pest management CEUs.

Corn Nematode Workshop
There is increased interest in plant-parasitic nematodes as pests of corn. On Thursday, July 30, 2009, there will be an all-day workshop in Ames focused on the basics of the biology and management of corn nematodes. Specific topics of discussion in the workshop will include:
• review of the nematode species that can damage corn
• corn nematode life cycles and feeding habits
• symptoms of nematode damage on corn
• how to sample to determine if nematodes are damaging corn
• current and future management options for nematodes on corn

The workshop also will include demonstrations of procedures used to extract plant-parasitic nematodes from soil and corn root tissue. The workshop will be taught by ISU nematologist Greg Tylka and participants can earn 7.0 Certified Crop Adviser pest management CEUs.

Participants in both workshops will receive printed course notes, other print publications, and computer training modules on CDs. The registration fee for each workshop is $150, and there is a 10 percent discount for individuals registering for both workshops. The nematode workshop flyers contain more information and registration form.

scn females on roots

Soybean cyst nematode (SCN) females on roots.



stunt nematode

Stunt nematode feeding on corn root.



Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes. Tylka can be contacted at or by calling (515) 294-3021.

Degree Days - From a Brisk Week to a Balmy One

by Rich Pope, Corn and Soybean Initiative

Although the week of May 10 was seasonally cold; we made up most of that week's lost heat during the week of May 17.

 Accumulated degree days in Iowa from May 1 through May 25, 2009  

Corn planting is nearing completion across Iowa, and soybean planting is wrapping up in some regions as well.  

Corn fields should be monitored for black cutworm now, based on pheromone trap capture data.


Rich Pope is a program specialist with responsibilities with Integrated Pest Management. Pope can be contacted by email at or by calling 515-294-5899.

This article was published originally on 6/1/2009 The information contained within the article may or may not be up to date depending on when you are accessing the information.

Links to this material are strongly encouraged. This article may be republished without further permission if it is published as written and includes credit to the author, Integrated Crop Management News and Iowa State University Extension. Prior permission from the author is required if this article is republished in any other manner.