Over the weekend, South Dakota experienced strong southerly winds which brought warm temperatures to the state. In addition, these winds also brought insect pests north, which included moths of the western bean cutworm. Although most western bean cutworm moth emergence occurs in early July, studies in Iowa and Michigan have shown that moths can start emerging as early as May-June. The name of this pest implies that it feeds on beans; however, western bean cutworm is primarily a pest of corn, but will also feed heavily on dry beans. There have been no reports of it feeding on soybean in the field. Unlike other cutworms, the western bean cutworm doesn’t actually cut plants. Instead, it causes issues later in the season by feeding on the reproductive portions of the plants. In corn, it feeds on the tassel, silks and kernels. Feeding damage on the ear, can increase the risk of ear mold infection developing.
There is only one generation of western bean cutworms per year. Most mating and egg laying occurs in July and August. Although the western bean cutworm caterpillars will feed on corn leaf tissue, they require corn reproductive tissue (i.e., tassels, silks and ears) to survive. The moths that we are currently observing in South Dakota will likely lay eggs within the next few days and the eggs will hatch 5-7 days later. This timeline indicates that the caterpillars will only have corn leaf tissue available for feeding. Therefore, the populations of moths that are being observed in South Dakota at this time will not produce caterpillars that require management, as their life cycle will not correspond with developing tassels, silks or corn ears, which they need for survival.
The western bean cutworm moths are a mottled gray-brown color with lighter colored (tan) along the outside edges of their front wings. The moths are approximately an inch long (Figure 1). Although the coloration can be somewhat variable based on the amount of damage the wings have sustained prior to identification, there are key markings on the front wings that can be used. Each wing has a small light colored circle located near the middle of the wing. Below the small circle, there is a boomerang-shaped marking that is light brown and highlighted by a tan color.
The western bean cutworm caterpillars change color and size as they grow. During most of their developmental stages, the caterpillars are tan with longtidudinal lines. The last stages have black rectanglular markings directly behind the head that are separated by a light colored line (Figure 2).