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Corn Illustrated: Some GMO traits are no longer as effective against this pest, according to entomologists.

Tom J Bechman 1, Editor, Indiana Prairie Farmer

August 17, 2021

3 Min Read
ears of corn showing western bean cutworm feeding damage
DEVASTATING PEST: In areas where western bean cutworm attacks, it is a mean pest because it bores through ears in multiple places, opening them up to diseases and other forms of damage. Tom J. Bechman

Certain insect pests may not affect an entire state, or they don’t show up every year. But just like there is no such thing as minor surgery if you are the patient, there is no such thing as a minor pest if you are the one whose field is dinged by it. If western bean cutworm invades your cornfield once, you will never forget it.

Why? Because the pest chews multiple holes, not just one, in corn ears, moving right through the husks. The feeding opens up ears to other disease pathogens. Given the right weather conditions, it can also lead to sprouting and rotting right on the ear. Western bean cutworm can turn an attractive cornfield into what looks like a war zone in short order. The aftereffects of feeding — including disease, sprouting and general decay — may be the best reason to mark an infested field for early harvest.

Prevention and control

For several years, farmers where western bean cutworm appears most often were keeping it at bay with Bt traits that protect against aboveground feeding damage by insects. However, Purdue University entomologists John Obermeyer and Christian Krupke, reporting in the July 28 edition of the Purdue Pest&Crop newsletter, say not all traits are as effective against western bean cutworm as they once were.

“Control of this pest with Bt corn traits is difficult, as it has evolved resistance to one of the toxins present in most traited corn, Cry1F, the endotoxin for aboveground feeding caterpillars,” they write. “This includes SmartStax varieties and Optimum AcreMax hybrids, among others. In terms of Bt hybrids, only those expressing the Vip protein will offer control of this pest — this is something to keep in mind when ordering seed for 2022.” 

If field scouting and well-timed insecticide applications are your only options for western bean cutworm management, those options do work well, the entomologists note. However, you need to scout and make timely insecticide applications before the insects are in a position where they are difficult to control.

The Purdue University Corn & Soybean Field Guide suggests examining 20 plants in five areas of the field and looking for egg masses on the top side of uppermost leaves, or larval feeding in the whorl or on pollen within leaf axils. Eggs can be unevenly distributed within a field, so you need to check different areas, entomologists note. If 5% of plants have an egg mass or a larva in the whorl, treatment is recommended. The larva you’re looking for has two distinct dark rectangles immediately behind an orange-colored head. Once larvae are inside the ears, they’re protected from an insecticide.

At that point, you’re probably down to scheduling the field for early harvest and checking with your seed rep for hybrids with a GMO trait that is still effective against the pest.

You can keep up with more information about trapping reports and suggestions about western bean cutworm and other insects in the Purdue Pest&Crop newsletter.

About the Author(s)

Tom J Bechman 1

Editor, Indiana Prairie Farmer

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