Western bean cutworm (WBC) can be a severe pest of corn and dry beans, but not soybean. Consider an insecticide application if 5 to 8% of corn plants have WBC egg masses or small larvae that have not moved into the silks or ear tip. Insecticide application timing is critical for WBC control as once larvae enter the ear, insecticides are ineffective.
WHAT TO CONSIDER
Western bean cutworm (WBC) can be a severe pest of corn and dry beans (but not soybean). WBC larvae feeding can reduce yield potential and grain quality and it is considered a late-season pest. Unlike many other cutworm species, WBC larvae do not cut plant stems; they feed on the reproductive parts of plants. In the past, WBC was primarily limited to the western Great Plains, but over the last 15 years has expanded its range eastward to as far as Pennsylvania (Figure 1).1
Life Cycle and Identification
Western bean cutworm overwinter in the soil as a non-feeding larva (prepupae), then pupate, and later emerge as adult moths. There is one generation per year. Moth flights begin as early as mid-June, then peak in mid- to late July and usually end by late August.2 However, adult emergence and peak flight periods can vary depending on climate and location. Farmers may be able to receive updates concerning WBC moth catches from flights at http:www.insectforecast.com.
Growing degree days (GDD), base 50 ºF accumulation since May 1, can be used to predict adult moth emergence. GDD totals for 25%, 50%, and 75% moth emergence are 1319, 1422, and 1536, respectively.1 Moths are about 0.75 inches long, primarily grayish-brown, with a wingspan of approximately 1.5 inches (Figure 2). Identifying haracteristics for moths include a whitish stripe at the front of the forewing and two cream-colored, outlined shapes immediately behind. These identifying shapes area circular spot approximately halfway along the length of the forewing and a kidney-shaped mark along the same line, approximately two-thirds of the way to the wingtip.
Western bean cutworm moths prefer to lay eggs on late-whorl stage corn that is near pollination. Eggs are laid on the upper surface of leaves in masses of 5 to 200 with an average of about 50 eggs per mass. The eggs are pin-head in size, dome-shaped with ridges, and usually laid on the flag leaf. Eggs are pearly white (Figure 3) when first laid and within several days they turn tan. Shortly before hatching, eggs turn a purple coloration (Figure 3). Eggs hatch in about 5 to 7 days.
Eggs hatch in about 5 to 7 days. Larvae remain clumped near the egg mass for several hours after hatching, feeding on their egg shells. Shortly after hatching, larvae move into protected areas of the corn plant, feeding on leaf tissue, fallen anthers/pollen, and silks as they develop and move to the developing ear. The newly hatched larvae are initially dark with blackheads. Larvae will lighten to a light tan or pinkish hue with subtle longitudinal stripes as they develop. Fourthinstar and larger larvae, 0.5 to 1.5 inches long, are readily identified by two black “rectangles” behind the now-orange head, and generally, have a smooth skin (Figure 4). Fourth-through sixth-instar larvae are often found feeding on kernels in the ear, usually on the tip but sometimes the sides. Entry holes and/or frass are not always visible; therefore, scouting for WBC larvae should include pulling back corn husks. A single egg mass may produce larvae that infest nearby plants within a 6- to a 10-foot circle, as plant-to-plant movement is common. Larvae go through six larval-instar stages and feed for about a month.3
Monitoring for the presence of WBC adult moths, which indicates egglaying, is recommended to determine when to scout for eggs and larvae. Moth trapping methods involve using either black-light or pheromone traps. Black-light traps are reliable for catching WBC adults. However, they are expensive and bulky, and their operation depends on a power supply, which may limit their use. Also, these traps catch many unwanted insects, which may make counting WBC adult moths a challenge. A comparison of black-light traps and two pheromone trap types (wing traps and milk-jug traps) showed no significant differences in adult counts.5 For most applications, pheromone traps are a better option to monitor for WBC adult moth presence. Field scouting should begin after WBC adults are detected and continue with increasing frequency after WBC adults are collected on consecutive days.
To scout for WBC, examine 20 consecutive corn plants in at least five locations in the field. Check the upper three or four leaves of each plant for WBC egg masses or young larvae. Continue scouting for 7 to 10 days after peak moth flight. If the tassel has not emerged when the eggs hatch, larvae will move into the whorl and feed on the developing pollen grains in the tassel. As the tassel emerges, larvae will move down the plant to green silks and then into the silk channel to feed on the developing ear.
Management and Treatment
The level of potential damage from WBC is dependent on a number of factors including pest density, favorable environment, agronomic practices, and product efficacy. Because WBC pressure varies annually in regions and from year to year, fields should be regularly scouted, especially if heavy WBC pressure is anticipated. If 5 to 8% of corn plants have egg masses and/or small larvae, consider an insecticide application.1 If an application is needed, timing is critical. If most eggs have hatched, an insecticide application should be made after 95% of the tassels have emerged, but before the larvae move into the silks and ear tip to feed. If the eggs have not hatched and plants have tasseled, the application should be timed for when most of the eggs are expected to hatch. Purple eggs should hatch within about 24 hours. Control is more difficult when the larvae reach the silks or ear tips. Research has shown that an average of one WBC larvae per ear can reduce yields by 3.7 bushels per acre.1
There are numerous insecticide products labeled for WBC larvae control. Consult your local extension office for insecticide recommendations. Insecticide products for WBC control have a preharvest interval ranging from 14 to 30 days and many are restricted use pesticides.5 There is some evidence that pyrethroid insecticides may cause some larvae to move out of protective areas (silks and ear tips) due to the repellent properties of the active ingredient.1 These insecticides may be more effective when applied before the larvae reach the silks or ear tips.1
The use of an Integrated Pest Management (IPM) program for WBC management is highly recommended. Particular attention should be paid to fields in areas with historical populations of WBC as well as fields with unexpected WBC damage the previous year. Growers should consult with their local seed sales representative, agronomist, or extension office to understand what best management practices they should use for their area.
1Seymour, R.C., Hein, G.L., and Wright, R.J. 2010. Western bean cutworm in corn and dry beans. G2013. NebGuide. University of Nebraska-Lincoln Extension.
2Michel, A.P., Krupke, C.H., Baute, T.S., and Difonzo, C.D. 2010. Ecology and management of the western bean cutworm (lepidoptera: noctuidae) in corn and dry beans. Journal of Integrated Pest Management, vol 1. no.1. https://extension.entm.purdue.edu/.
3Western bean cutworm. Field Crops IPM. Purdue University. https://extension.entm.purdue.edu/fieldcropsipm/insects/western-bean-cutworm.php.
4Peairs, F.B. 2014. Western bean cutworm: characteristics and management in corn and dry beans. Fact sheet no.5.538. Colorado State University Extension. http://extension.colostate.edu/topic-areas/insects/western-bean-cutworm-characteristicsand-management-in-corn-and-dry-beans-5-538/.
5Krupke, C.H., Obermeyer, J.L., and Bledsoe, L.W. 2016. Corn insect control recommendations. Field Crops. Purdue University. http://extension.entm.purdue.edu/.
Web sources verified 11/12/17. 140702134055