Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


Printer Friendly Version

July 10, 2012



Although most of Iowa is in better shape than much of the rest of the Midwest, the lack of rain and heat are taking a toll on the yield potential of the corn crop. The 100+ degree heat was likely especially hard on corn that was tasseling and silking last week. A rough rule of thumb is that corn loses about 3% of its yield potential for every 12 hours corn leaves are rolled during tassseling and silking. Dr. Shaw, ISU climatologist, has research that showed that severely drought-stressed corn during pollination had yield reductions ranging from 3 to 13% per day. For a discussion on how stress affects pollination see Roger Elmore’s article at You can see how the rest of the country is faring on the drought monitor website at .


It will take about 10 to 14 days after silking to assess whether pollination was successful.  Here’s a publication from the University of Nebraska that discusses and illustrates (1) how to assess pollination success, (2)  harvesting drought-stressed corn for grain, and (3)  harvesting drought-stressed corn as forage with considerations on nitrates, grazing, green-chop and silage production.    This publication was written in 2008, so its comments on pricing silage, stover, etc., may not be valid in 2012.




Japanese Beetles & Spider Mites


 Japanese beetle populations appear to be greater than in past years, and many soybean fields have already been sprayed. One thing to keep in mind is that the damage usually looks worse than it actually is. The threshold for spraying Japanese beetles and other defoliating insects is 20% defoliation during flowering and 30% defoliation prior to flowering. This is a picture showing about 20-25% defoliation that Marlin Rice took a few years ago:


Soybean defoliation



Most people have a hard time waiting until they see this much damage before pulling the trigger, and many would likely call this 50%, rather than 20-25%. The longer you can hold off spraying, the more likely you can get by with one application and perhaps also control other insects later in the season. Most insecticides have good knockdown of the beetles but poor residual.


If a field does need to be sprayed, I would recommend using an insecticide or insecticide combination that also controls spider mites, because spider mites are showing up in the area and this is ideal weather to get spider mite population explosions. Most synthetic pyrethoids do not control spider mites, so can actually cause a spider mite problem by killing off the predators of the mites.  Hero (zeta-cypermethrin + bifenthrin) and Cobalt Advanced (chlorpyrifos + lambda-cyhalothrin) are a couple of insecticides that have both spider mites and Japanese beetles on the label. Lorsban (and generic chlorpyrifos) and dimethoate are the most widely used insecticides for spider mites. Unfortunately neither have Japanese beetles on the label, although they will likely kill some of the beetles as well. For more information on spider mites see





There have been numerous reports of postemergence herbicide applications not working well this year, especially on waterhemp. There are some populations of waterhemp in Iowa that are now resistant to glyphosate, although I think that in most cases the poor results with glyphosate are likely at least partly related to the weather conditions. Hopefully with the “cooler” weather present now any herbicide applications will be more effective, but with the moisture stress and the taller weeds to control, it is likely that we will just have to tolerate some weedy fields. The herbicides being used on non-GMO fields, such as Flexstar, are also not working as well. Cobra, Phoenix, and Cadet are other PPO-inhibitor herbicides that can be effective on waterhemp, but don’t expect miracles with any herbicide sprayed on 2 foot tall weeds.






Corn Rootworm Larvae


There have been a few reports of heavy root feeding by rootworm larvae.  If any of these situation are with Bt-rootworm trait corn, ISU Entomologist Aaron Gassman would be interested.  Please drop me an email with your contact information and I’ll pass it on to Aaron for possible inclusion in his rootworm resistance research.


Corn Rootworm Adults & Japanese Beetles


There have also been reports of high numbers of rootworm beetles in some fields. Treat if beetles are clipping silks back to <1/2 inch and pollination is not yet complete.  With the high numbers of Japanese beetles this year, we may also have more of a problem with them as well. Most of the Japanese beetle damage is done on the outside dozen or so rows of corn as the beetles move from the soybeans into the corn. Here is a recent ICM News article with a few more comments about this:





Planned Foliar Fungicide Programs?


Disease activity continues to be quite minor with the warm dry weather, so checking corn fields for the presence of disease before spraying is especially important this year. If you aren’t seeing the presence of disease lesions on leaves 3 leaves below the ear leaf and higher on the plant at this time, it is less likely the fungicide will result in enough of a yield increase to pay for the treatment. Last year’s trials with timing and disease ratings can be found at  Hundreds of corn trials over the years show little profitability from fungicides when low disease levels exist:





July 17, 2012, Field Extension Education Laboratory (FEEL) near Boone, IA


Two half-day sessions (morning session repeated in the afternoon) (no cost to participants) will be conducted, focusing on:

- Nozzle selection/use with demonstration on spray table

- Balancing efficacy and drift

- Environmental factors, adjuvants and limitations, field demonstration, etc.

The morning session is FULL, but there is still room in the afternoon. More information and registration will soon be available at



Managing Corn Residue with Different Tillage Systems and N Applications

July 18 10:00 a.m. - noon


Ben & John Olson Farm – about 5 miles NE of Van Horne on 21 Ave. Dr.

Free Lunch & 2 hours of CCA credit (S&W)


For more information, see



If you have any questions, please feel free to contact the Iowa State University Extension Office.

maroon ball East Central and Southeast Iowa Crops Home Page maroon ball ISU Extension and Outreach maroon ball ISU maroon ball ISU Extension Agronomy maroon ball ISU Agronomy
maroon ball Calendar maroon ball Search maroon ball Jobs maroon ball Feedback maroon ball Internet Resources maroon ball State Extension Sites in Other States maroon ball Local Extension Offices in Other States maroon ball ISU Extension and Outreach Policies

Last Update: July 10, 2012
Contact: Jim Fawcett

Nondiscrimination Statement and Information Disclosures