FARMINGTON, Minn. — Bruce Potter, University of Minnesota Integrated Pest Management Specialist reports that the past week brought rain and black cutworm (BCW) moths to many trap locations in Minnesota, both unwelcome. This is as large and widespread early-season influx of moths as we have seen for several years.
There appear to be several migration events that we can use for a bio-fix (event to use as a starting point) for cutworm development. The first was the nights of April 29-30 but other systems look like they brought moths May 1 – 4. If you had thunderstorms pass through your area the past week, there is a chance it brought black cutworms with the passing of these weather systems.
Using the April 30th date and historical weather data (Midwest Regional Climate Center U2U corn growing degree days), cutworm larvae should be large enough to cut corn (4th instar). This takes approximately 312 degree-days base 50°F. This ranges from May 28 in the SW corner of Minnesota for an April 30 – May 1 flight to June 1 for a May 4th flight in Central Minnesota. Small corn is more susceptible to cutting and the later corn planting this year will lead to more corn acres in a susceptible stage.
What is the best source of nitrogen to top-dress if you have to apply all the Nitrogen after corn planting? Dr. Fabian Fernandez, University of Minnesota Nutrient Management Specialist recommends that any form of readily available N can be used. You can inject either UAN or anhydrous ammonia between rows to reduce plant injury. There is no advantage to trying to apply N close to the row since roots will grow into the row centers by the 4th leaf stage. It is also possible to apply N in every other row instead of every row without negatively impacting yield because every row will have N applied at one side or the other. You can also apply urea with a urease inhibitor (NBPT), UAN dribble on the surface. If you need to, you can apply UAN in a broadcast application when the crop is small to reduce the chance of leaf injury.
Should I consider a nitrification inhibitor? It is important to understand that there are two types of inhibitors. Nitrification inhibitors delay the process of converting N in the ammonium form to nitrate. This is significant, as the nitrate form is subject to loss via denitrification (gassing off into the atmosphere), or leaching into groundwater. As the season progresses, the likelihood of loss decreases because the amount of time between application and plant use is shorter. This makes the instances of needing an inhibitor less likely. Additionally, as the soil warms the inhibitor will not keep its effect as long in the soil as compared to a fall or early spring application. The bottom line is there is nothing wrong environmentally with using a nitrification inhibitor at this time, but the likelihood of it paying for itself decreases by the day.
The other type of inhibitors are urease inhibitors. These are intended to prevent urea from volatilizing and disappearing into the air. The risk of volatilization happening is much greater when urea is top-dressed without incorporation. Use a urease inhibitor in this situation, unless you are certain that there will be a significant rainfall (at least 0.2 in.) in the 48 hours following an application. More information can be found within the University of Minnesota Crop E News at https://z.umn.edu/5-n-faqs.
— David Nicolai, University of Minnesota Regional Extension Educator – Crops
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