Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


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June 6, 2012



White Grubs


True white grubs are present in some corn, soybean, and even alfalfa fields.  There are no rescue treatments available.  Initially, the true white grub injury to corn was a surprise because work done by Marlin Rice, former ISU Extension Entomologist, in 2004 and 2005 indicated that insecticide seed treatments, even at the low rate, provide nearly 99% protection against true white grubs.  See  However, drilling down more deeply into the data, the actual kill rate varied from about 29% to about 94%.  Work done by Jon Tollefson and Jim Oleson, both now retired from Iowa State University, in 2003 - 2005 did show low levels of injury to plants, but not enough to be of concern.  So apparently this past winter, extremely high populations of true white grubs survived the mild winter and overwhelmed the insecticide seed treatments.


So what about white grub management in the future?  There are annual white grubs and true white grubs.  Annual white grubs seldom, if ever, cause economic losses while true white grubs, with their three-year life cycle, can cause significant economic losses, generally only in corn.  Rice outlined a pre-planting scouting protocol along with information to help differentiate between annual and true white grubs in; note the article is from 2002 and mentions some insecticides that are no longer available and also does not mention the insecticide seed treatments as they were not in use at that time.



Corn Rootworms


Corn rootworms are now hatching and careful inspection of corn roots and the related root ball will disclose them in many fields.  See for more information.  Remember that neither rootworm insecticides nor Bt CRW kill close to 100% of the rootworms, so finding some should not be a surprise.  The goal of the rootworm insecticides and Bt CRW is to protect roots.  If you would like to assess root protection later this season, you may want to use for guidance.



European Corn Borers


European corn borer (ECB) moths are around, but my informal windshield survey (splats on the windshield per mile) suggests that numbers are fairly low.  I have been told of instances where an inexpensive insecticide has been included in post-emergence herbicide applications to kill ECB moths.  It is important to remember that moths do not cause injury and also that they tend to hang out in grassy areas, like ditches, fence lines, and waterways, and then fly into the corn to lay eggs.  In addition, corn plants create a substance called dimboa (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one) that provides protection against the larvae (borers) until the plants are 17 – 22 inches tall and most corn being sprayed is shorter than that.  So even with a low cost insecticide and relatively expensive corn, it is unlikely that such an insecticide application will be cost-effective.  It is much more prudent to scout non-Bt ECB corn for evidence of borer activity, and if the field exceeds the threshold, use an insecticide at that time.  Scouting information and a threshold calculator are at



Potassium Deficiency Symptoms


We are again seeing corn fields with areas in the fields that are stunted with the lower leaves yellowing and browning along the margins. This is a symptom of potassium (K) deficiency, but is often due to poor root function rather than a shortage of K in the soil. Usually the fields look fine until the corn reaches the rapid-growth phase, then corn stops growing and the lower leaves turn yellow. Good corn can be right next to extremely stunted corn with no apparent reason for it. The corn in these areas remain stunted and the lower leaves remain yellow, but they usually yield better than expected.  There can be large differences among hybrids in showing this K deficiency phenomenon.

Anything that restricts root growth during the initiation of the nodal root system can lead to the problem. The nodal roots emerge within 0.75 – 1.0 inch of the soil surface. If there is something in the surface inch or so that the roots don't "like" they don't function properly.  Shallow planting and/or soil settling or eroding after planting aggravates the problem. This year, dry soils, compaction, planter slots that open back up, erosion, settling of fluffy soil, and shallow planting, either individually or in some combination, are the most common culprits.   It is showing up in no-till fields and in fields with all degrees of tillage.  In general, there is nothing that can be done when the problem appears.  In most cases, a nice slow rain is the best answer.  However, if the nodal root system is at or very near the soil surface, a cultivation throwing soil up around the plants may help by stimulating root growth, as described in the May 24, 2012 edition.


If the soil hasn't been tested recently, soil samples should be taken to make sure it is not a true K deficiency problem. Soils that are low or marginal in K are more likely to show the problem.  An excellent discussion of this problem in both corn and soybean is on pages 123 – 124 of the June 20, 2005 Integrated Crop Management (ICM) Newsletter or at





Two-spotted Spider Mites

I have received reports of two-spotted spider mites in soybean in the Highway 34 area.  Usually rain promotes a fungal disease that destroys spider mite populations; hopefully the rain of last week end has that effect, but rainfall events of short duration followed by low humidity often do not.  For details on scouting for and managing this pest go to or pages 148 - 149 of the July 22, 2002 Integrated Crop Management Newsletter.  If treatment for this pest is necessary, organophosphates (Dimethoate 400 or Lorsban 4E) are the most effective products.  Dimethoate 400 will have less residual activity.  If re-treatment is necessary, most, if not all, of the chlorpyrifos product (Lorsban and “generics”) labels state to not use chlorpyrifos twice in a row.  However, chlorpyrifos and dimethoate are both Group 1B insecticides, so the benefit of rotating between those products is questionable.  The Group 3A insecticides (pyrethroids) generally have little effect on spider mites and can actually cause spider mite populations to flare by killing predatory insects.  The exception is products that contain bifenthrin, such as Bifenture EC, Brigade 2EC, Discipline 2EC, Fanfare 2EC, Hero, Sniper, and Tundra EC; these pyrethroids appear to have good activity against two-spotted spider mites when applied at the rate stated on the label for this pest.





SPRING FIELD DAY and 25th Anniversary Celebration (afternoon) &


JUNE 21, 2012

Details are posted at




JUNE 28, 2012, 1 – 4:30 p.m.

Details are posted at



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.

More information and registration is at


If you have any questions, please feel free to contact the Iowa State University Extension Office.
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Last Update: June 6, 2012
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