Scout for Western Bean Cutworm in Corn Fields

Western bean cutworm moths are active in the Great Lakes region, including Pennsylvania. The moths may be laying eggs in corn fields, so it would be wise to scout fields for eggs and young caterpillars to understand the threat that this pest species poses to your fields. Penn State Extension has a limited effort this year monitoring western bean cutworm moths with pheromone traps to understand the threat from this species, which we first found in Pennsylvania in 2009. In the past week, our traps have seen an increase in the number of moths captured. The pest species has been frustrating to understand in Pennsylvania because their populations are unevenly distributed across the state and we have found poor relationships between the number of moths in traps and the amount of damage from caterpillars in nearby fields. Indeed, most areas in Pennsylvania that have had large numbers of moths have had limited amounts of damage from caterpillars, but that situation may change. For example, in areas of Michigan, Ontario, and New York, high populations of moths commonly lead to economically damaging populations of caterpillars in corn ears. Therefore, I am recommending that growers across Pennsylvania scout their fields for eggs and larvae to understand the risk from this pest species. Look for egg masses on the upper leaves of corn (Figure 1) and young caterpillars feeding on reproductive tissue, including pollen in whorls, the tassel, or in the silks; the caterpillar eventually will find their way to the developing ear.

Be aware that corn hybrids with the Herculex 1 trait (i.e., the Cry1F protein) provide little control against this pest species. The main Bt trait that can help control western bean cutworm is the Agrisure Viptera (Vip3A) trait. (See this resource from Michigan State and Texas A&M to understand what protection your hybrids offer against western bean cutworm.) A generally accepted economic threshold for western bean cutworm is 5% of plants having eggs or larvae, so corn grain and silage fields above this level would benefit from an insecticide application. Of course, the timing of this application is key because it needs to occur after eggs hatch but before caterpillars enter ears and are protected from insecticides by husks. Fields that do not have populations that exceed the economic threshold are unlikely to benefit from insecticides and may actually suffer if the insecticide flares aphid populations, which grow in the absence of natural enemies. Young western bean cutworm caterpillars have black heads and yellow, spiny-looking bodies (Figure 2). Later in the season, more mature western bean caterpillars look similar to corn earworm but do not have lines down the sides of its body like corn earworm does and has three distinct light-brown lines on its "neck" just behind its head.

Figure 2. Newly hatched western bean cutworm caterpillars. Note the black heads and a spiny-looking body (Photo by Frank Peairs, Colorado State University, Bugwood.org)