Western Bean Cutworm
Western bean cutworm is a pest of dry beans and corn that can be found in the Western U.S, the Corn Belt, and Great Lakes region. This pest produces one generation each year and has five stages in its lifecycle: egg, larva, prepupa, pupa, and adult moth. Western bean cutworm moths lay 5 to 200 egg masses in field corn, sweet corn, popcorn, or dry beans.
Moth emergence is generally around July, when moths lay eggs on the upper side of corn leaves and the bottom side of dry beans. 5 to 7 days later, larvae hatch and begin to eat on plant parts, depending on the host’s development stage. Larvae move to the whorl in pre-tassel corn and feast on the flag leaf and developing pollen in the tassel. If corn pollination has begun, the larvae feed on the silks. On dry beans, western bean cutworm larvae feed at night on the flowers, pods, and developing seeds. Around the fifth instar stage, larvae drop to the soil to overwinter in the prepupa stage.
Western Bean Cutworm Identification
Moths are ¾ inch in size and are a light brown color with a cream colored stripe on the front edge of the forewings. Below this stripe is a white spot, followed by a crescent-shaped spot toward the outer edge of the wing.
Eggs are pearly white and then turn dark purple shortly before hatching. Young larvae are dark brown with diamond markings on their back. Larvae become a lighter tan color as they progress in development. Three dark brown stripes running lengthwise behind the head can distinguish western bean cutworm from corn earworm larvae.
Management of Western Bean Cutworm
Management and treatment of western bean cutworm is difficult. Bt corn varieties are available for control of this pest, but not all varieties have the trait to control western bean cutworm (for example Cry1F or VIP3A). Scout fields before tasseling-- research has recommended using chemical control if 5 to 8 percent of plants in the field have egg masses. Treatments should be made when 95 percent of the tassels have emerged so that plants are treated before the larvae enter the ear and silk tip.
Dry beans are difficult to scout for western bean cutworm, and therefore pheromone traps are often used to monitor adult activity in late June to get an estimate of infestation. Moths captured in the traps should be removed and recorded approximately every three days or as needed. At the peak of moth trapping, if the number caught is below 700, then risk of damage is low. Between 700 and 1,000 moths per trap is moderate risk and above 1,000 moths is high risk of damage. Treatments should be made 10 to 21 days after peak moth flight.
The threshold for western bean cutworm may need to be adjusted depending on the crop’s value and cost of treatment.