Western Bean Cutworm Growing Pest In Corn, Dry Beans

Peak moth flight time is usually around the third week of July.

Published on: Mar 6, 2013

A corn and dry bean pest that has been mostly confined to parts of Kansas, Colorado and southwestern Nebraska has been expanding east over the past few years. "Western bean cutworm has been sporadic," said Wayne Ohnesorg, University of Nebraska Extension educator, who spoke at the UNL Crop Production Clinic held in Norfolk recently.

They've been identified for more than a century, with the first genus described in 1887. "It's been quiet for a few years. Last year was a moderate year for it," Ohnesorg said. While the normal WBC range has been the western Great Plains, is has spread as far east as Pennsylvania over the past decade.

Larval feeding damages crop yields and quality. Direct feeding losses in corn can be made worse by fungal infections that are associated with larval feeding and waste products. WBC produces one generation per year. "Peak moth flight is usually around the third week of July," Ohnesorg said.

EAR FEEDING: In heavy infestations
EAR FEEDING: In heavy infestations

Because of the unusual growing season in 2012, it came earlier than normal. Moths are typically about ¾ inch long and light brown, with dark brown wings and a wing span of about 1-½ inches. After mating, eggs are laid on field corn and other corn crops, or dry beans. They hatch after a week, with larvae then moving to protected feeding sites like the whorl of the corn plant. They can eventually feed on the tassel, silks and the ear. They can even infest adjacent plants down the same row.

Depending on the extent of infestation, corn yields can be diminished by three to 14 bushels per acre. In years of heavy infestation, two or more larvae could be experienced on a single ear of corn.

Scouting for WBC should be started after the first moths have been observed. Generally, if 4 to 8% of field corn plants have egg masses or small larvae, an insecticide application might be warranted. Some Bt corn varieties will work against WBC. According to Ohnesorg, the 1Ab protein has little if any impact, but 1F, Cry and pyramidal genes can have some positive results in controlling larvae. But fields planted to these varieties should still be scouted to insure that WBC is being controlled enough to prevent yield or quality losses.

WBC MOTH: Moths take flight normally around the third week of July. (University of Nebraska-Lincoln photos)
WBC MOTH: Moths take flight normally around the third week of July. (University of Nebraska-Lincoln photos)

A free WBC speed scouting mobile app was developed by UNL and University of Minnesota entomologists and produced by Educational Media at UNL last summer to help farmers detect threshold quantities in their fields. It is available through the iTunes store and can be downloaded to iPads, iPhones and iPod Touch. A WBC speed scouting spreadsheet will run on any device capable of running a Microsoft Excel file.

If you'd like to learn more about WBC or the new speed scouting app, contact Ohnesorg at 402-370-4044 or email wohnesorg2@unl.edu.