Northwest Iowa Crop Update Newsletter by Todd Vagts ISU Extension Crops Specialist Counties Served:  Carroll, Calhoun, Crawford, Ida, Monona, Pocahontas and Sac.
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Volume 2, Number 17

Northwest IA Crop Update, July 15, 2002
(Word Document)

In this issue 
DD50 accumulation and Corn development
Corn lodging assessment
Scout for WBC egg masses now
1st generation bean leaf beetle peak emergence
Corn Rootworm beetles emerging
Grasshoppers still in soybeans and corn
Soybean leaf defoliation chart

Introduction
Last week’s storms brought much needed moisture to most of west-central and north-west Iowa, but at the same time caused significant damage to area corn fields.  Lodging looked bad immediately after the storm, but recent recovery (goose-necking) appears to be good.  Reduced sunlight interception, small root systems and harvest will be challenges the crop and producers have yet to overcome.  The area soybean crop appears to be doing very well as it progresses into full flower.  Western bean cutworm moth flight has hit high gear over the last several days, scouting of area fields should be done now.  Corn rootworm beetle, bean leaf beetle and grasshopper populations continue to increase in area fields as well.

Weather update
Last week’s storms brought much needed moisture to area corn and soybean fields with rainfall accumulations from 1.0 to 2.5 inches.  Mild temperatures last week and (predicted) mile temps towards the end of this week will help with pollination and crop development.  More rainfall will be needed shortly as the recent rainfall will be quickly used up and soil moisture reserves are near zero.

Storm damage
Lodging of area corn fields was widespread last week as very high winds and heavy rain swept through west-central Iowa on Wednesday morning.  From my observations, the greatest lodging of corn occurred in an area between Schleswig, Kiron, Westside and Breda.  Over the last few days the corn has gone through a remarkable recovery.  The base of the plant has rerooted and the upper 2/3’s of the plant has goose-necked upwards.  The immediate concern was pollination, but I believe in most fields the plant has gooseneck enough to lift the ears off the ground and the silk will be able to catch pollen grains.  Future concerns will be light interception, water and nutrient availability and harvest.  For more detailed information on lodging concerns in corn, refer to the following web site http://www.extension.iastate.edu/carroll/crops/storm-damage-02.htm

Growing Degree Day Accumulation and Crop Development
For the first time in several weeks, Northwest IA accumulated below average degree-days for the week at 149 Degree-days (base 50).  The area is forecasted to pick up 183 degree-days this week (Figure 1).  NW Iowa is on track with last year’s degree-day accumulation and is well above 11-yr average trend line for this time of the year.

Corn Development has reached VT (tassel) and R1 (pollination) in most fields.  The VT stage is initiated when the last branch of the tassel is completely visible and the silks have not yet emerged. VT begins approximately 2-3 days before silk emergence, during which time the corn plant will almost attain its full height and pollen shed begins. The time between VT and R1 can fluctuate considerably depending on the hybrid and environmental conditions. Under field conditions, pollen shed (also termed pollen drop) usually occurs in the late mornings and early evenings. R1 begins when any silks are visible outside the husks. Pollination occurs when these new moist silks catch the falling pollen grains. A captured pollen grain takes about 24 hours to grow down the silk to the ovule where fertilization occurs and the ovule becomes a kernel.  Generally 2-3 days are required for all silks on a single ear to be exposed and pollinated.  The shank and husks attain full size between the R1 and R2 stages.

Soybean Development has reached the R2 stage in many fields. R2 is the classification for full flower.  At R2 root development has moved across the space of a 40-inch row and root development is tapping the lower soil depths.  The soybean plant has accumulated approximately 25% of its final biomass weight and has attained 50% of its final node number.  This is a period of rapid growth and dry matter accumulation, which will continue through R6 (Full Seed).   The soybean crop is demanding from 0.2 to 0.3 inches of water per day from R2 through R5.  The greatest impact of water stress will not occur until Pod Fill.  Nutrient uptake at R2 has been minimal by the soybean plant; approximately 31%, 30% and 34% of the total Nitrogen, Phosphorus, and Potassium respectively have been accumulated in the plant at this time.  Greatest nutrient demand occurs during pod elongation and fill (R3 – R7). 

Pest Management
Western Bean Cutworm moth catch has been increasing in all locations in northwest Iowa.  Field scouting should be initiated when western bean cutworm moths are first noticed (NebGuide G98-1359-A). 

You can monitor current degree-day accumulations and trap catch numbers at the following web page:  http://extension.iastate.edu/carroll/crops/wbc-2002.htm

A picture of an egg mass can be seen at this web address
http://www.ent.iastate.edu/imagegal/lepidoptera/wbcutworm/western_bean_cutworm_eggs.html

For more detailed information on the WBC, refer to the Nebraska publication G98-1359-A. http://www.ianr.unl.edu/pubs/insects/g1359.htm and an ISU article at http://www.ipm.iastate.edu/ipm/icm/2002/7-8-2002/wbeancutworm.html

Corn rootworm beetle emergence has begun across the area.  ISU corn rootworm specialist Dr. Jon Tollefson stated that rootworm infestation levels have been at high levels this year so expect lots of beetles to emerge.  Fields that have been at risk should be scouted for silk clipping once beetle emergence begins.  More information on rootworm beetle scouting and control can be obtained in the most recent ISU IPM newsletter http://www.ipm.iastate.edu/ipm/icm/2002/7-15-2002/rw.html

Bean Leaf Beetle degree-day accumulation has surpassed the 1212 mark for soybeans emerging through the end of May.  Peak 1^st generation emergence will occur around 1212 GDD’s following soybean emergence. Begin sampling fields 1 week after peak emergence.  If the sample is below threshold, continue scouting for two weeks.  If 1st generation beetles are above threshold, scout again mid August to confirm beetles are present, then spray.  Refer to the following web page for population thresholds and updated degree-days http://extension.iastate.edu/carroll/crops/blb-2002-1st-gen.htm

Insect defoliation levels of soybean leaflets. Soybean defoliation is notoriously difficult to estimate and is almost always overestimated because the injury is so dramatic and often not all parts of the canopy are considered. Source:  July 10, 2001 NE Crop Watch Article.

Grasshoppers continue to ravage soybean field edges and may be causing significant defoliation.  Dr. Rice suggests to consider treatment if grasshoppers are present and defoliation reaches 40 percent in the preblooming stages or 20 percent in the pod-forming and pod-filling stages. Reductions in yield can occur during any crop stage and pod-forming and pod-filling stages are at greater risk than other plant stages. A 40 percent leaf loss during any vegetative stage will result in only a 3-7 percent yield reduction. Defoliation of 20 percent during the pod-forming and pod-filling stages will result in similar yield reductions (ISU IPM 6/28/1999).  More information on hopper control can be obtained from ISU http://www.ipm.iastate.edu/ipm/icm/2002/7-1-2002/westhoppers.html and Nebraska http://www.ianr.unl.edu/pubs/insects/nf328.htm

(Word Document)