Western bean cutworm

Appearance and Life History. The Western Bean Cutworm is a native of North America that has recently expanded its range eastward from the Great Plains region. Following its initial discovery in southeastern Iowa in 2004, it was found the following year in western Illinois and in northwestern Indiana in 2006. Although primarily known a pest of field corn in many areas, its broad host range also includes legumes and it can be a pest of dryland beans (but not soybeans), where they are grown in abundance. It is not a true “cutworm” in that it feeds on the reproductive parts of the plants, and not by cutting stems as many cutworms do. The western bean cutworm is a late-season pest and its feeding can cause both yield loss and degrade quality of grain.
One generation occurs each year. Moths are primarily grayish-brown in color with a wing-span of approximately 1-1/2". The primary identifying characteristics are a whitish stripe at the front of the forewing with two cream-colored, outlined shapes immediately behind. Other identifying marks are a circular spot approximately halfway along the length of the forewing and a kidney-shaped mark along the same line, approximately 2/3 of the way to the wingtip.
Moth emergence begins in early July and usually peaks in the middle of the month, when male moths can be monitored using pheromone traps. Female moths will mate and lay eggs during July and August and will oviposit on a variety of cultivated and wild plants, although dryland beans and field corn are the most commonly chosen oviposition sites. Cornfields in the late whorl stage are preferred by female moths, who are seeking to lay eggs on corn that is near (but not past) pollination. Eggs will be laid on upper surfaces of leaves, often on leaves near the whorl that have not unfolded completely and vertical in orientation. Eggs are laid in masses of 20-200, but usually average around 50. Eggs are white when first laid, and become tan as eggs begin to develop. In about a week, eggs develop a purple coloration, indicating that hatch is imminent.
Following emergence, first-instar larvae will consume their eggshells, making post-hatch scouting for empty masses impossible. Larvae will then move into protected areas of the corn plant, feeding on leaf tissue, fallen anthers/pollen and silks on their way to their destination, the developing ear. Newly-hatched larvae are initially dark with black heads, this color will lighten to a light tan or pinkish hue with subtle longitudinal stripes as they develop further. 4th-instar and larger larvae, 1/2 to 1-1/2 inches long, are readily identified by 2 black “rectangles” behind the now-orange head, and a generally smooth skin or cuticle (i.e. lacking tubercles, warts, or bumps). There are 6 larval instars, the 6th is most conspicuous and often found feeding on mature ears, usually the tip but sometimes the sides. Entry holes and/or frass are not always visible, so scouting for larvae must include removing husks. Several larvae may be found on a single ear, because these caterpillars (unlike corn earworm) are not cannibalistic. In late summer and early fall, 6th instar larvae drop off the plant and burrow into the soil, where they construct an earthen chamber using salivary gland secretions. Sandier soils allow for larvae to penetrate deeper into the soil profile that increases winter survival and avoidance of tillage implements. They will remain in a quiescent state (pre-pupa) throughout the winter and will pupate and complete development during the following spring and early summer.
Damage. Oviposition and subsequent damage are often very patchy in fields. Multiple larvae may feed on a single plant/ear. In addition, there is likely considerable inter-plant movement early in life which results in infestation of neighboring plants. Most feeding is concentrated on the ear, and yield studies conducted in Iowa and Nebraska have shown that an average of one larva/plant throughout the field may cause yield loss of approximately 4 bu/acre. Damaged kernels are prone to molds, decreasing grain quality of food-grade corn varieties. At harvest, many emergence holes may be noticed in dry husks, this is often the first indication that WBC has been present in the fields that haven’t been monitored.