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Variegated Cutworms Ravage Soybeans

May 25, 2001
Contributing to the Lepidoptera madness are variegated cutworms in soybeans. In fact, I suspect that some of the problems in soybeans that have been blamed on armyworms were, in actuality, caused by variegated cutworms. The larvae of the two species are similar in appearance. Kevin Black and Howard Brown, both with Growmark in Bloomington, have visited fields in central Illinois that have been seriously damaged by variegated cutworm larvae. Kevin was in a 65-acre field of soybeans in McLean County that had been completely destroyed. Damage in several other fields has been significant, requiring replanting.

Variegated cutworm larva. (Photo courtesy of Marlin E. Rice, Iowa State University.)

The general color of a variegated cutworm larva varies from dark brown to light gray. Although the color of a larva varies considerably, a narrow line of pale yellow dots along the middle of the back is almost always present. However, these dots or spots may not be evident on some larvae. In the final instar (sixth), there is usually a black W-shaped mark on the dorsum of the eighth abdominal segment, followed by a conspicuous yellow or orange area. Also, there is usually a narrow, orange-brown spiracular stripe, which also occurs on armyworm larvae. Fully grown larvae are 1-1/2 inches long.

Like armyworm and black cutworm adults, variegated cutworm adults migrate from southern regions into the Midwest every year. Females deposit eggs in pastures; fencerow grasses; low, densely growing weeds; and debris, often in fields that have not been tilled. Larvae have a very wide host range, including, but not limited to, alfalfa, clover, corn, cotton, soybean, sunflower, tobacco, wheat, fruit trees, garden vegetables, and ornamental flowers. Variegated cutworms complete two to four generations per year in North America north of Mexico.

Variegated cutworms are so-called climbing cutworms, usually climbing host plants and chewing on various plant parts. In the affected soybean fields, the cutworms chewed the plants to the ground rather than cut the plants off. This behavior is more similar to the type of injury caused by armyworms than by black cutworms.

Kevin Black reported that pupation was occurring in some areas, although he could find various sizes of larvae in some fields. The onset of pupation suggests that control measures probably are not warranted. However, if you find midsize variegated cutworm larvae feeding on soybean leaves, consider applying *Ambush at 3.2 to 6.4 oz per acre; *Asana XL at 5.8 to 9.6 oz per acre; *Lorsban 4E at 1/2 to 1 pt per acre; *Pounce 3.2EC at 2 to 4 oz per acre; or *Warrior at 3.2 to 3.84 oz per acre. Use of products preceded by an asterisk is restricted to certified applicators.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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