HPIPM:Western Bean Cutworm FC

Author: Frank B. Peairs[1]

Identification (and life cycle/seasonal history)

Western bean cutworm, Striacosta albicosta (Smith), larvae are dark brown with faint diamond-shaped markings on their backs. Fully developed larvae are about 38 millimeters (1.5 inches) in length and have three short dark stripes running lengthwise on the first segment behind the head. The body of the moth is about 20 millimeters (0.8 inch) long and brown in color. They have a wingspread of about 40 millimeters (1.6 inches) and are marked with creamy white stripes on the front leading edge. The light colored hind wings are not distinctly marked.

Western bean cutworms complete a single generation each year. Fully-grown larvae (pre-pupae) overwinter in the soil. In May and early June they change to pupae. The moths emerge between mid-July and early August. They are active at night and are attracted to lights. Eggs are laid shortly after the moths emerge. The eggs are deposited in clusters of four to 200 on the top surface of upper leaves. When first laid the eggs are white with a thin red ring around the top. As they age, they change to brown, then immediately prior to hatching they are purple to black in color. The eggs hatch in five to seven days. The majority of the western bean cutworms feed until mid-September. When mature, they enter the soil and change to the pre-pupal stage to overwinter.

Plant Response and Damage

Western bean cutworm was originally considered to be a pest of dry beans but now is recognized as a serious pest of field corn. Following hatch, young western bean cutworms move to one of two places on the corn plant, depending on the stage of development of the corn. If corn has not tasseled, larvae feed on pollen in the developing tassel. If corn has tasseled, larvae feed on silk in the ear: this type of silk feeding may cause pollination to be poor. Once the ear has formed, larvae feed on developing kernels. Destruction of the kernels may reduce corn yields by as much as 30 to 40 percent.

Management Approaches

Resistant Varieties

Control is expected with only those Bt corn hybrids containing the Vip3A toxin (Agrisure Viptera, Agrisure Duracade 5222). See Managing Corn Pests with Bt Corn, [2], for details on the use of Bt corn hybrids to control corn pests. See [3] for an excellent summary of currently available traits.

Chemical Control

Western bean cutworm eggs are laid on upper leaf surfaces on the upper part of the plant. Fields should be scouted closely, as once the larvae move into the ear, good control will be difficult to obtain. Chemical control should prove economical if eight percent or more of the plants have egg masses or small larvae in the tassels, and the crop is at least 95 percent tasseled. If tasseling is much less than this, the percentage of infested plants should be raised as fewer larvae are likely to reach the ears. Many of the insecticides registered for western bean cutworm control have been associated with spider mite outbreaks, so fields should be monitored for mites after a treatment is made.

Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.