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6/10/2013 - 6/16/2013

Corn Rootworm Egg Hatch Underway in 2013

By Erin Hodgson and Adam Sisson, Department of Entomology

 

A few areas of Iowa are approaching 50 percent corn rootworm egg hatch now (Figure 1), including the Muscatine area. Many other regions will be reaching 700 degree days within 7-14 days, depending on future temperatures. Cooler spring temperatures in 2013 have slowed down development, especially compared to rootworm hatch last year.

Figure 1. Predicted corn rootworm egg hatch in Iowa as of June 13, 2013. Expect 50 percent hatch between 684-767 degree days. Map data courtesy of Iowa Environmental Mesonet, ISU Department of Agronomy. Click here for current degree day accumulation for corn rootworm.

 

Corn rootworm egg hatch in Iowa typically occurs from late May to the middle of June, with an average hatching date around June 6. Development is driven by soil temperature, which is measured by degree days. Research suggests about 50 percent of egg hatch occurs between 684-767 accumulated degree days (base 52°F, soil). Shortly after each egg hatch, young larvae will begin feeding on root hairs and inside roots. At they develop, larvae (Photo 1) will begin feeding on root tips. A severe infestation can destroy nodes 4-6, which interferes with water/nutrient uptake and makes the plant unstable (Photo 2).

Photo 1. Corn rootworm larvae are slender and white with a dark head and tail section. Photo by Scott Bauer, USDA agricultural Research Service, Bugwood.org.


 

Photo 2. Severe corn rootworm larvae feeding can cause plants to lodge. Photo by Gary Munkvold, Iowa State University.

 

Roger Elmore, ISU Extension corn agronomist, reported the weather has delayed corn planting in 2013. But with the predicted egg hatch starting in late May and early June, the larvae should have sufficient root tissue to feed on because corn will be germinated. Saturated soils during egg hatch will diminish overall corn rootworm pressure, and the high adoption of Bt corn should decrease populations in most fields. However, every field should be scouted for corn rootworm damage regardless of the seed selection (i.e., corn rootworm populations are the highest priority for insepction. Assess corn rootworm root injury and adjust management strategies if the average injury is above 0.5 on a 0-3 rating scale. Aaron Gassmann, ISU corn entomologist, has a webpage for additional corn rootworm management information, including an interactive node-injury scale demonstration and efficacy evaluations.

 

Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact her at ewh@iastate.edu or (515) 294-2847. Adam Sisson is an Integrated Pest Management extension specialist. Sisson can be reached at ajsisson@iastate.edu or (515) 294-5899.

Nitrogen Loss - Spring 2013

By John Sawyer, Department of Agronomy

Dry soils across Iowa from deficit precipitation following the drought in 2012 have changed to excess wetness this spring. Unfortunately, precipitation to replenish subsoil moisture has been much more than needed. As I wrote in an ICM News article earlier this spring, one downside to the excess precipitation has been movement of carryover nitrate deeper into the profile and to tile lines, resulting in high nitrate-N concentrations in surface waters. This means there is less nitrate-N carryover to be utilized by 2013 crops, although as noted in the previous article, not all of the carryover N moved below the three-foot depth.

As in past wet springs, excess wetness can lead to loss of N applied for the current corn crop and result in greater than normal N response. I wrote several ICM News articles in the spring of 2008 and 2011 covering several aspects about N transformations in soil and N loss potential, and estimating the need for additional N application. Links to those articles are listed here.

www.extension.iastate.edu/CropNews/2013/0604Sawyer.htm
www.extension.iastate.edu/CropNews/2011/0613sawyer4.htm
www.extension.iastate.edu/CropNews/2011/0613sawyer3.htm
www.extension.iastate.edu/CropNews/2011/0613sawyer2.htm
www.extension.iastate.edu/CropNews/2011/0613sawyer1.htm

A web-based program to estimate sidedress N application for corn, called Adapt-N, is currently being promoted across the Corn Belt. It was developed at Cornell University in New York and is a combination of models that simulate soil N processes, influence of precipitation and corn N uptake. It has also recently been promoted as a way to estimate N losses from fall applied N. My evaluation the last three years indicates the program consistently and significantly underestimates needed N application (compared to economic optimal N rates at response trials). This brings into question if the program has been adequately calibrated and tested with Midwest conditions. If you try the program this spring, be cautious and consider use on only a limited trial basis.

The spring this year has been cold. Nitrification and denitrification are slow in cold soils, which will decrease potential for loss of applied N. But cold temperatures will not slow leaching of nitrate present in the soil. With slow conversion of ammonium to nitrate, there should be less loss of applied N – especially for recently applied ammonium fertilizers, such as anhydrous ammonia. Nitrification will also be less for applications such as late fall applied anhydrous ammonia and inclusion of a nitrification inhibitor. Early fall applied N, such as liquid manure, would have significant conversion to nitrate last fall. The nitrate component of UAN solution (28 or 32 percent N) is immediately subject to loss, and the nitrification rate of urea and ammonium in UAN will be more rapid than anhydrous ammonia. Therefore, loss potential is greater with such applications.

An aspect of this spring that is somewhat different from past wet springs is the late corn planting and slow growth. From my observations in north central Iowa, the corn crop at this time is the smallest I remember in the 2000s. Corn N demand has been very low so far, and for late planted fields, season-long demand may be less due to lower productivity.

When warm, wet soils provide a good environment for microbial mineralization, the conversion of organic N to ammonium. Ammonium will accumulate under anaerobic conditions. While corn may die in ponded field areas, if replanted, the accumulated ammonium will supply crop-available N and perhaps an amount adequate to meet crop needs.

For corn fields with intended sidedress or split/sidedress N application, major loss conditions this spring have been avoided. For fields that had fall or early spring N applied, a visual way to check if corn will respond to additional N is to apply sidedress N fertilizer strips as soon as possible across several fields and watch the corn response. This can give a visual clue to potential for more N need and provide reference areas for mid-vegetative stage crop greenness measurement or canopy sensing.


John Sawyer is a professor of agronomy with research and extension responsibilities in soil fertility and nutrient management. He can be contacted at jsawyer@iastate.edu.



This article was published originally on 6/17/2013 The information contained within the article may or may not be up to date depending on when you are accessing the information.


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