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Black cutworms rising

Agriculture.com Staff 05/19/2008 @ 9:02am

Black cutworms were seen on their way to the western and central Corn Belt en masse a month ago, and now is the time to see if your fields are infected and do something about it, specialists say.

Over the weekend of April 18, pheromone traps around the state showed that a "significant flight of black cutworm adults (moths)" made it to the state, meaning the pest's cutting damage to emerging corn seedlings could be underway already in southern parts of Iowa.

According to current projections and pheromone trap numbers, cutworm damage will likely begin this week, making it a key time to scout for the pest, according to Iowa State University (ISU) entomologists Marlin Rice, Rich Pope and Jon Tollefson. From southeast to northwest in the state of Iowa, for example, the first cutworm cuttings are expected between May 17 to May 23.

But, the entomologists point out this doesn't necessarily mean your fields will fall victim to black cutworm. That's what makes scouting at this stage so crucial, they add.

Pheromone traps do not predict the amount of cutting in a field nor the counties where cutting will occur. Each year, one of our concerns is that radio advertisements may predict a cutworm 'outbreak' in your county just because moths were trapped there in April," according to a report from the ISU entomologists. "Neither the traps nor our predictions based on the trap catches can predict the amount of cutworm injury in a field. Therefore, scout and be diligent."

Because an economic threshold must be breached before justifying insecticide treatments for black cutworms, specific steps must be taken when scouting. And, the ISU entomologists point out, it's important to confirm whether the pest you're seeing in the field is a black or dingy cutworm, as the latter doesn't cause economic cutting damage, only feeding on corn leaves (READ MORE).

"Look for cutworm injury on corn leaves. Dingy cutworms also feed on young corn leaves but rarely cut corn. If leaf feeding is detected, try to find the cutworms to determine whether they are black or dingy. Very large cutworms found during the earliest black cutworm cutting dates are often dingy cutworms because dingy cutworms overwinter in Iowa as partially grown larvae. Also, fields with winter annual weeds are more likely to have cutworms than clean fields, and soybean stubble is more attractive to the moths than corn stubble," according to Rice, Pope and Tollefson.

"If you find leaf feeding and only black cutworms, then mark off 100 plants in a row with stakes or flags, and scout these same plants for cutting over a period of several days at several locations across the field. Then you can monitor the cutworm activity and determine whether they are cutting plants and the percent cut plants."

Also, know when to not only start, but stop scouting to get a clear picture of the severity of cutworm populations and feeding. "Scout fields several days before the first cutting date projection. By doing so, you may be able to find "hot spots" based upon leaf feeding, thereby getting a head start on management decisions. Stop scouting when the field is sprayed or when plants have five fully developed leaves (stage V5)," the ISU entomologists say. "Cutworms have difficulty in cutting plants in the V5 stage because of the larger stalk diameter, although occasionally they chew into the side of the stalk and kill a larger plant."

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