Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


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May 19, 2008



Time to Scout for Cutworms


With the first emerged corn in the area also comes the risk of cutworm losses. A network of pheromone traps across Iowa indicated that a significant flight of black cutworm moths occurred the weekend of April 18. This does not necessarily indicate that we will have a problem with cutworms, but does tell us when to start scouting for corn cutting. Based on degree days accumulated, cutting could have begun as early as May 17 in SE Iowa and May 18 in EC Iowa. There was also a later flight, so cutting could go on for an extended period of time. Most years black cutworms are not a problem, but it’s prudent to take some time and monitor fields until they reach V5 (5 leaves fully emerged from the whorl). Remember that Yieldgard hybrids are not resistant to black cutworms. Although Herculex hybrids do show good tolerance, you can still get enough cutting that an insecticide treatment is sometimes needed. See Rich Pope’s article in the ICM News for more information on scouting for cutworms including information on insecticides that can be used as rescue treatments at

New Thresholds for Black Cutworms

            With $6 corn many producers are wondering if the standard threshold of 2-3% cutting for small cutworms and 5% for larger worms is still valid. With a little math it’s not hard to figure out that it might be time to re-visit those thresholds. Based on today’s economics, Jon Tollefson and Marlin Rice have lowered the threshold to 1-2% of the plants cut when the cutworms are less than ¾” long and 3% of the plants cut when the worms are greater than an inch long. The larger cutworms are nearly ready to pupate, thus have a higher threshold. If every plant cut resulted in a lost plant, it would be much easier to come up with exact economic thresholds, but many cut plants do re-grow if the cutting occurs near the soil surface. To see more on the new thresholds as well as pictures of the black and dingy cutworms see Jon & Marlin’s article in the ICM News at



            Stalk borers love giant ragweed as well as corn. If fields with large populations of ragweed are being planted to corn, there is an increased chance of having stalk borer problems. When the ragweed is killed by herbicides or tillage, the stalk borers will move from the ragweed to the corn as it emerges. One time when it is possible to kill stalk borers with insecticides is when they are moving from one plant to another. Once they are inside the corn plants they are protected from the insecticide. For any fields that have ragweed or grass weed problems, consider including an insecticide with the herbicide burndown or make an insecticide treatment as soon as the corn is emerging on tilled fields. For more information and a picture of stalk borers see Marlin Rice’s article in the May 16, 2005 ICM Newsletter at





            Now that some areas have finally gone a week without much rain, some are wondering if the drought has started. Some good news on that front is according to Elwynn Taylor, the La Nina is now fading, indicating a reduced risk for drought. According to Roger Elmore, the 3 seasons in the past 30 years that we had the most similar start to the season as this year, the corn yields were above trendline each year. We may wind up with a decent crop yet.








            Certified Crop Advisors can obtain 5 hours of credit (including 2 hours of soil and water) by attending a special session in the morning followed by the afternoon tour at the ISU SE Iowa Research & Demonstration Farm near Crawfordsville on June 26. This will include a tour in the morning featuring the soil drainage research on the farm.  More details will be posted soon.



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

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Last Update: May 19, 2008
Contact: Jim Fawcett

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