August 16, 2019
Western bean cutworm (WBC) is native to North America and was first found in the Midwest in the early 2000s. Mostly a pest of field corn in the Midwest, WBC can also be a pest of dry beans, but not soybean. Although called a “cutworm,” they do not actually cut and/or bore into the stalks like a black cutworm. One generation of WBC occurs each year; however, there are six larval instars each year. WBC can potentially cause yield loss, but more importantly, feeding wounds provide an entrance for pathogens that cause ear molds.
Scouting for WBC starts with tracking moth flight. Moths have about a 1.5-inch wingspan with several distinctive markings. The moths are brown with a white bar along the front-leading edge of the forewing, followed by a circular spot in the center and a comma-shaped marking toward the outside edge. Moths can be trapped with pheromone lures that mimic chemicals the females secrete to attract males. Trapping can help determine when WBC flights are occurring and when to start looking for egg masses.
Generally, females are attracted to pre-tassel and freshly tasseled fields for egg laying. The easiest way to scout for egg masses is to use the sun as a filter and look at leaves in the upper 1/3 of the canopy. Females usually target leaves near the whorl that are still in a more upright position. Put on a pair of shades and look up into the canopy to see the shadow of the egg mass through the corn leaf. Figures 1 and 2 show the egg masses from the back and front sides of the leaf. Egg masses are laid in groups of 20 to 200, but generally average around 50. Initially, the egg masses are white and then turn into a purplish color as they get closer to hatching. Purple eggs signal hatch within 12 to 24 hours.
Figure 1. Western bean cutworm egg mass on the back side of a leaf using the sun as a filter.
Figure 2. Western bean cutworm egg mass.
Scouting after initial egg hatch can be difficult as the first instar larvae eat their eggshells. First instar larvae have a black head and are a dull orange color. The head lightens to a tan/pinkish color for the first few instars while the body begins to have subtle dark stripes running the length of the WBC. Starting at the fourth instar, a WBC can easily be identified by a more orange head and two black rectangles behind the head (Figure 3). The sixth instar is generally found feeding on the ear. WBC can be found in the tip of the ear or they can burrow in the side of the ear where the ear touches the stalk.
Figure 3. Western bean cutworms with typical orange head and two black triangles.
To determine economic pressure, check 20 plants in at least five different field locations. Threshold in the Great Lakes region is 5% of plants having egg masses and/or small larvae, which is a bit lower than some western states. This is because the environment around the Great Lakes increases the chances of ear molds when WBC start damaging ears. The threshold is a cumulative threshold, meaning that 5% can be derived from several scouting passes in one field over a few weeks.1 If the percentage from week one and percentage from week two add up to five or more, then the threshold has been met. Consult your local agronomist for your geographical threshold.
There are a few options for controlling WBC. Preferred WBC control includes spraying a long-lasting pyrethroid around peak egg hatch. Application timing is critical as once the larva bores into the ear or moves inside the husk, insecticide applications become ineffective. Planting a Channel® brand corn product with Trecepta® technology can provide WBC control through the Vip3A Bacillus thuringiensis (Bt) protein. Your local Channel Seedsman can help with the recommendation of adapted Channel brand Trecepta technology products.
In late summer or early fall, the sixth instar larvae drop off the plant and burrow into the soil. Sandier soils allow for deeper burrowing and increase survival rates; therefore, sandier soils can have higher WBC pressure. After burrowing into the soil, the larva creates a chamber to live in from salivary gland secretions. The following summer, the WBC pupates and emerges as a moth and starts the process over.
Sources:
DiFonzo, C. 2017. Time to scout and manage western bean cutworm in southern Michigan. Michigan State University. https://www.canr.msu.edu/.
Obermeyer, J. Western bean cutworm. Field Crops IPM. Purdue University. https://extension.entm.purdue.edu/.
DiFonzo, C. 2016. Check corn now for western bean cutworm damage before harvest. Michigan State University. https://www.canr.msu.edu/.
Wright, B., Hunt, T., and Ohnesorg, W. 2016. Begin scouting for western bean cutworm eggs in corn. CROPWATCH. https://cropwatch.unl.edu/.
Cullen, E. and Jyotika, J. 2008. Western bean cutworm: A pest of field and sweet corn. A3856. University of Wisconsin. http://corn.agronomy.wisc.edu/.
Web sites verified 6/17/19.
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