Sweet Corn Pests - Cutworms
Full-grown cutworm larva.
Western Bean Cutworm (Striacosta albicosta)
Adult: Brown bodied moths, about ¾ inches (19 mm) long with a wingspan of 1.5 inches (38 mm) and marked with creamy white stripes on the leading edge of the forewings. Adjacent to the stripes, towards the center of the body, and in the middle of the wing lengthwise, is a circular white and tan spot. A crescent shaped mark is also located between the spot and the tip of the wing. The hind wings are light colored with no distinct markings.
Egg: Dome-shaped, and pinhead-sized, white with a thin, red ring around the top when newly hatched. Eggs change color with age from white to brown, and then finally turn a dark purple just before hatching.
Larva: Brown with faint crosshatching on their backs when newly hatched. As larvae mature they lighten to a gray-pinkish color and are about 1.5 inches (38 mm) long with three short dark stripes on the first segment behind the head.
Pupa: Dark brown, oval shape.
The western bean cutworm is a late-season pest of corn. Adult moths emerge mid-summer and mate shortly afterwards. Females lay eggs in July and August on a variety of non-cultivated and cultivated host plants including sweet corn. Females are attracted to fields with corn that is in late whorl or tasseling stage. They lay eggs in masses primarily on the upper surface of leaves. Egg masses contain an average of 50 eggs, but can range from 5 to 200 eggs per mass. Eggs mature in about a week. Newly hatched larvae feed on their egg shells before moving to other protected feeding sites. Larvae feed on corn plants for about 30 days. When feeding and development is complete, fully mature larvae drop to the ground and burrow 3 to 9 inches beneath the soil. Once in the soil, larvae construct earthen overwintering chambers with their salivary gland secretions. These larvae remain in a dormant state throughout the winter. As temperatures rise the following spring and early summer, larvae pupate and complete development into adults. Western bean cutworms have a single generation each year.
Larvae feed on leaf tissue, fallen anthers/pollen, and silks on their way to the ear where most of the feeding is concentrated. Larvae enter the ear through the tip or by chewing through the husk and feeding directly on developing kernels. Damaged kernels are more prone to molds and mycotoxin infection. Injury from larval feeding can result in lower quality and reduced yield. Larvae from a single egg mass can invade nearby plants within a 6 to 10 ft circle, causing patchy infestations throughout the field. Several larvae may also feed on a single ear of corn, especially during high infestations.
Image 2, Adam Sisson, Iowa State University, Bugwood.org