No. 23 Article 6/October 8, 2004

Introducing the Western Bean Cutworm

Historically, the western bean cutworm has been a pest in the western cornbelt. Over the past several years, the western bean cutworm has become established in Iowa. It has steadily moved from west to east, being discovered most recently in southeastern Iowa in 2004. The discovery prompted an entomology graduate student at Iowa State University to establish traps for the adults (moths) in both Illinois and Missouri in July 2004. The student reported finding five moths in a trap located in Warren County, Illinois, and one moth in each of three separate locations in Missouri. The captured western bean cutworms represent new state records for this species and a significant eastward expansion from the pest's recent published historical distribution.

The western bean cutworm is a severe pest of both corn and dry beans, affecting both crop yield and quality. Unlike other cutworms, the western bean cutworm is a late-season pest of corn. It feeds primarily on corn ears, chewing and scarring kernels, predisposing the ear to fungal and mold infections.

Generally, only one generation of western bean cutworm occurs each year, with moth emergence beginning in July. Brown-colored adult moths are approximately 3/4 inch long, with a 1-1/2-inch wingspan. The forewings of the moth are brown with a white or cream-colored stripe that runs across the leading edge of each wing. Just below this stripe and about halfway across the wing is a white, circular spot. Farther away from the body, in line with the spot, is a white, crescent-shaped spot. The hind wings of the western bean cutworm are unmarked and light colored. After mating, females lay eggs on available host plants such as field corn, sweet corn, popcorn, and dry beans. Females also may lay eggs on tomatoes, nightshade, and ground cherry, although these are not preferred oviposition hosts. In corn, female western bean cutworms lay eggs primarily on the upper surfaces of the leaves. Fields attractive to western bean cutworms for oviposition are fields in which corn is tasseling or near tasseling and fields that have hybrids with upright leaf characteristics. Egg masses contain an average of 50 eggs, but numbers of eggs in masses range from 5 to 200. Eggs turn from white to tan to dark purple as they age, and larvae hatch within 5 to 7 days after the eggs are laid.
 


Western bean cutworm adult (University of Nebraska-Lincoln).


Western bean cutworm eggs (University of Idaho).


After hatching, the larvae feed on the shells of the eggs for about 10 hours before moving to other protected feeding sites. The larvae pass through five instars and feed on host plants for about 31 days. First instars are very mobile and may infest several host plants. As larvae develop, their color changes from dark brown (first instar) to light tan, with brown hatch markings on their backs becoming more distinct with age. When larvae develop to the third instar, they have three dark stripes just behind the head. This characteristic helps differentiate the western bean cutworm from other caterpillars feeding in cornfields.
 


Western bean cutworm larva (Photograph courtesy of Marlin Rice, Iowa State University).

Newly hatched larvae move to the corn whorls, where they feed on the flag leaf, tassel, and other yellow tissue. As corn tassels and silks, larvae move to and begin feeding on developing silks. Larvae feed directly on silks in post-tassel corn. Because of their behavior to disperse, larvae from one egg mass may infest other corn plants in the same and adjacent rows in an area 6 to 10 feet in diameter. As both the larvae and corn ears develop, the larvae begin feeding on ear tips. The larvae also may chew through ear husks to developing kernels. An infestation of more than one larva per ear may occur because western bean cutworm larvae are not cannibalistic, as are corn earworm larvae. After a larva finishes feeding and completes development, it drops to the ground and burrows beneath the soil, where it constructs an overwintering cell. Western bean cutworms spend the winter in the prepupal stage. In May, western bean cutworms pupate, and they emerge as adults in July.

 


Western bean cutworm larva feeding on corn ear tip (University of Nebraska-Lincoln).


Corn damaged by western bean cutworm larvae (Photograph courtesy of Marlin Rice, Iowa State University).

 

Western bean cutworm moth flights usually begin in early to mid-July. The emergence date of these moths can be predicted by calculating growing degree-days, beginning May 1, using a base temperature of 50°F (Table 2). Degree-day accumulations for the western bean cutworm will be accessible on the Degree-Day Calculator beginning in spring 2005. Pheromone or black light traps can also be used for the detection of the western bean cutworm in your area. Currently, Iowa State University has a monitoring system in place. The Iowa State University Western Bean Cutworm Web site can be found here.

Scouting for western bean cutworm should begin when adult moths are first noticed. In corn, check 10 consecutive plants in at least five random locations in each field. Look for egg masses or small larvae on the upper surfaces of corn leaves. Also, examine tassels for larvae before pollen shed. Entomologists at the University of Nebraska recommend that an insecticide treatment be considered when 8% of the plants have egg masses and/or small larvae. Timing of an insecticide application is critical. If larvae have hatched, apply an insecticide after 95% of the tassels have emerged but before larvae enter the silks. Control is more difficult after larvae have moved to the silks. If larvae have not hatched and the corn plants have tasseled, time the insecticide application to coincide with the hatch of larvae. If the eggs are purple, hatch usually occurs in about 24 hours.

Growers in Illinois counties along the eastern border of Iowa should be aware of the potential for this corn insect pest to make its way into Illinois. Monitoring and detection of the western bean cutworm will be essential in the management of this insect. For those interested in monitoring for the western bean cutworm, stay tuned to the Bulletin for more information. A new fact sheet for this insect will soon be available on the IPM Web site.--Kelly Cook

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