Western Bean Cutworm

Vegetable Pest Fact Sheet

Western Bean Cutworm is a major pest that feeds on corn and dry beans, in which its larvae are highly detrimental to crop yield and quality. The larvae cause physical damage and make crops more susceptible to other pathogens such as fungal or mold infections.

Western Bean Cutworm Larva

Note the two prominent brown rectangles behind the head capsule.

A caterpillar on an unhusked ear of corn. A husked ear of corn is in the background. — Western bean cutworm larva on ear of corn. Note the two prominent brown rectangles behind the head capsule. Photo by Adam Sisson, Iowa State University, Bugwood.org

Western Bean Cutworm Overview

Native to North America, the western bean cutworm (WBC) originated in the western corn belt. However, in the early 2000s, this pest started moving eastward where it was discovered in Iowa for the first time in 2004. In 2009, the first case of WBC was recorded in New York State where it continues to expand its populations. This insect is a major pest that feeds on corn and dry beans, in which its larvae are highly detrimental to crop yield and quality. While not preferred, adult female moths may also lay their eggs on other hosts such as tomatoes, cherries, and nightshade. Not only do the larvae cause physical damage to the crops, but the feeding damage makes the crop more susceptible to other pathogens such as fungal or mold infections. On average, the presence of one larva of WBC per plant will cause a total loss of 4 bushels per acre of corn.

Appearance of Western Bean Cutworm

Western Bean Cutworm Adults

Adult moths of WBC can be identified through five distinct features:

Long white stripe on the leading edge of the forewing nearly reaching the end of the wing tip.
Small white dot in the middle of the forewing.
A larger ‘U-shaped’ brown dot just posterior of the small white dot.
Thin brown strip bordering the entire back edge of the forewing.
Long antenna spanning anywhere from ⅓ to ½ of the entire body length.

In pheromone traps or in the field, it is quite common for the WBC adults to be mistaken for other moths such as the corn earworm, spongy moth, and European corn borer. However, this pest is most similar to the dingy cutworm, which has larger darker triangular accents at the back edge of the forewing margin than WBC. These moths are also active during the nighttime and are attracted to lights.

Adult Western Bean Cutworm Moth

Adult western bean cutworm moth showing the white stripe along the leading edge of the forewing, small dot, and U-shaped brown dot on the forewing.

Brown moth on an unhusked ear of corn. The forwings have a white line along the leading edge as well as two spots directly below the white line. The first is in the shape of a circle, the second is U-shaped. — Western bean cutworm adult. Photo by Adam Sisson, Iowa State University, Bugwood.org

Adult Dingy Cutworm Moth

An adult dingy cutworm for comparison. The dingy cutworm does not have the white stripe and has larger triangular markings on the forewing.

An adult moth with wings spread. Forewings have white lines and large brown triangular markings. Rear wings are light brown in color. — Adult dingy cutworm moth. Photo by Hanna Royals, Screening Aids, USDA APHIS PPQ, Bugwood.org

Distinguishing Western Bean Cutworm Larva from other Larval Pests

Through the progression of the WBC larvae’s life cycle, they develop three distinct morphological characteristics that act as key identifiers to distinguish them from other pests.

Absence of microspines and tubercles on the body
Two dark rectangles located just behind the head.
Lack of net-like marks on the head capsule.

To compare with other similar larval pests see Sweet Corn Larval Pest Identification (pdf)

anterior of larval western bean cutworm showing Absence of microspines and tubercles on the body — Anterior of larval western bean cutworm. Photo by Adam Sisson, Iowa State University via Bugwood.org

western bean cutworm larva showing head — Western bean cutworm larva. Photo by Adam Sisson, Iowa State University via Bugwood.org

Lifecycle of Western Bean Cutworm

In New York State, WBC completes one-full generation per year where adult moths emerge as early as June. Although peak adult flight occurs in late July to early August, egg laying and plant damage will span from July to October.

On average, adult female WBC will lay ~ 50 white eggs in masses on the upper surface of corn leaves just prior to tasseling. In dry beans the eggs are laid on the undersurface of leaves.
Eggs take less than one week to hatch, with eggs turning a darker brown/purple two days before larval emergence.
Neonates will forage on their eggshells upon hatching. They are extremely mobile and single egg masses may impact surrounding plants in a 6-10 ft diameter.
Neonates, which can be identified through their completely black heads and brown body, take one full month to go through all six instars

Cluster of 50 or more small, round, white insect eggs on the surface of a green leaf.

Western bean cutworm eggs are laid in a cluster of 50-200 white eggs on the top surface of the upper corn leaves prior to tassel.

Cluster of 150 or more small, round, purple insect eggs on the surface of a green leaf.

Western bean cutworm eggs turn brown/purple a few days before hatching.

Caterpillar larvae emerging from egg cluster on green leaf.

First instar western bean cutworm larvae emerging from eggs.

Feeding Progression

Larvae between the first and third instar will feed on the leaf axil, pollen, and silks before boring into the corn ear during the fourth instar.
In the sixth instar, WBC larvae will finish feeding on corn kernels and drop to the soil.
The late-sixth instars overwinter in the pre-pupal stage from September to February and develop as pupae the following March to May.
One can find multiple individuals of WBC in a single husk of corn ear, as opposed to the corn earworm.

Damage Assessment

Pheromone traps are a highly effective way to monitor peak flight time of adult moths on a weekly basis. Prior to pollen shed, corn tassels should be examined carefully, with further inspection of upper leaf surfaces of new leaves for early-stage larvae or eggs.
After harvest, the most noticeable way to identify WBC larval damage is the presence of one or more emergence holes in dry husks.

Predicting Western Bean Cutworm Adult Moth Flights

The NEWA Western Bean Cutworm Degree Day Model can be used to predict the percent flight completion for adult western bean cutworm moths.

Treatment Guidelines

For fresh market sweet corn use a 1% threshold.
Scout fields during pre-tassel stage to monitor for egg masses and determine when to apply insecticides.
When the egg masses turn purple they will hatch within 24 hours. If 70-90% of larvae have emerged from their eggs, treat respective areas with insecticide applications where at least 95% of corn has tassels before the larvae are able to feed on the silk.
Sprays are ineffective once larvae bore into the corn husks.

Authors

Naoki Kihata
Cornell University Department of Entomology

Date: April 2024

Photo Credits

Western bean cutworm larva on ear of corn—Adam Sisson, Iowa State University, Bugwood.org
Adult western bean cutworm—Adam Sisson, Iowa State University, Bugwood.org
Dingy cutworm adult—Hanna Royals, Screening Aids, USDA APHIS PPQ, Bugwood.org
Western bean cutworm larva anterior—Adam Sisson, Iowa State University, Bugwood.org
Western bean cutworm larva showing head—Adam Sisson, Iowa State University, Bugwood.org
Western bean cutworm eggs—Frank Peairs, Colorado State University, Bugwood.org
Western bean cutworm eggs turning brown/purple—Frank Peairs, Colorado State University, Bugwood.org
First instar western bean cutworm larvae emerging from eggs—Frank Peairs, Colorado State University, Bugwood.org