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Corn Pests: Western Bean Cutworm

Western bean cutworm is a sporadic pest of corn in South Dakota. Like many of the other moth pests, injury to corn is due to caterpillar feeding. Western bean cutworm caterpillars feed on corn tissues during mid-summer and tunnel into ears feeding on developing kernels.

Identification

Adult western bean cutworm are small (3/4 inch), gray-brown moths. The moths have a characteristic cream colored stripe that runs along the outer margin of the forewing, a cream colored spot halfway along the front wings, and a comma-shaped mark found two-thirds of the way down on the front wings.

Western bean cutworm caterpillars have black head capsules and dark bodies immediately after hatching. The caterpillars become more distinctive midway through development, with the appearance of two black rectangles behind their orange head and a smooth body lacking bumps. Western bean cutworm eggs appear white immediately after being laid by female moths. Eggs soon turn tan and pink as they develop, and will turn purple 1 to 2 days before hatching.

Life Cycle
Western bean cutworm has one generation per year. Adult moths begin to appear around mid-June, with their populations peaking in mid to late July. Moths are nocturnal and do not injure corn plants. Females lay eggs on the upper surfaces of corn leaves during July and August, and prefer corn in the late whorl stage for egg laying. Eggs hatch roughly one week after being laid.

Newly hatched caterpillars will move up corn plants to feed on pollen if corn is in pretassel or tasseling. When corn has finished shedding pollen, caterpillars enter the ears either through the ear tip or by tunneling through the side of the ear and begin to feed on developing kernels. Multiple caterpillars can occupy a single corn ear. In late summer or early fall, the caterpillars drop from the corn plants into the soil to construct soil chambers for overwintering. Pupation is completed the following spring and adults emerge from the soil during the next summer.

Injury to Corn

Western bean cutworm caterpillars can be observed feeding on leaf tissue, silks, and tassels but this type of feeding rarely cause significant injury to corn, unless populations are very large. Caterpillars feeding directly on the kernels are the major source of yield losses associated with this pest. Fields that average one caterpillar per plant can suffer losses of 3.7 bushels/acre. Caterpillar feeding can also reduce grain quality as tunnels leading through the husk and into the ear can introduce fungi and mold.

Scouting and Economic Thresholds

Scouting for western bean cutworm involves a combination of trapping adults and searching corn plants for eggs masses. Adults can be trapped using black light or pheromone traps. Pheromone traps are more precise as black light traps are attractive to numerous other insects. Trapping should begin during pretassel corn as these are the most attractive to female moths for egg laying.

After adults are initially detected, start searching cornfields for egg masses. Females lay their eggs in a patchy pattern throughout cornfields, therefore search 20 consecutive plants in at least 5 different locations within a field. Information on economic thresholds is limited, but management is recommended if more than 5 – 8% of scouted corn plants have eggs masses or are infested with western bean cutworm caterpillars.

Management
Management of western bean cutworm can be difficult, especially once caterpillars have entered the ears. Conventional insecticides are effective for management of western bean cutworm (see the South Dakota Pest Management Guide: Corn). Broad-spectrum insecticides can also kill natural enemies of western bean cutworm. Bt-corn hybrids that express either the Cry1F or Vip3A toxins are also labeled for management of western bean cutworm, and should prevent large populations of caterpillars from becoming established.

Source: Adam J. Varenhorst, SDSU 

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