Managing Western Bean Cutworm in Corn

by Steve Butzen and Herb Eichenseer

Summary

·      Western bean cutworm (WBC) was once primarily a pest of dry beans in the western U.S., but is now considered a threat to corn in some of the largest corn-producing states.

·      WBC has gradually migrated eastward to northwest Iowa southwest Minnesota, and has recently been found east of Interstate 35. Many fields have been damaged in these states because growers were unaware of the problem.

·      WBC damages corn primarily by larval feeding on devel-oping kernels. This causes direct yield losses and also allows secondary insects and diseases to invade the ear.

·      Because scouting is time-consuming, application timing is critical, and multiple treatments may be needed, insecticides are not a good solution to the WBC problem.

·      Growers now have access to the built-in protection of Pioneer hybrids with the Herculex⁸ I trait. This technology eliminates the need for intensive scouting and works regardless of infestation level.

·      In addition to controlling WBC, the Herculex I trait also provides full-season protection against corn borer, black cutworm, and some other insects.

·      This Crop Insights discusses western bean cutworm identification, life cycle, feeding behavior, damage to corn and management options. · 

Introduction

Once known mainly as a pest of dry beans in the western U.S., western bean cutworm (Richia albicosta) is now considered a serious threat to corn in at least 10 states. Historically, the primary corn-growing regions affected by this pest have included Colorado, South Dakota, Nebraska, western Kansas and the panhandles of Texas and Oklahoma. But in the last five years, WBC has migrated gradually eastward into Iowa and Minnesota as well. By 2001, this destructive insect could be found in most northwest Iowa counties, and observations of moths, larvae and damage confirmed that WBC had advanced eastward to Interstate 35. In 2003, WBC was found as far east as Waterloo, Iowa (Wilson, 2003).

WBC is a late-season pest that begins to damage corn just prior to pollination. Larvae feed on developing tassels and ears. When infestation levels average several larvae per ear, yield reductions can be as high as 30 to 40%. In addition to consuming developing kernels, WBC feeding also allows entry of other insect feeders as well as ear rot pathogens.

Identification 

Adults: The adult WBC is a dark brown moth about ¾ inch long with a wingspan of 1 ½ inches. Wings have distinct markings and a white stripe on the leading edge of the upper wing (Figure 1).

Figure 1. Western bean cutworm moth on corn leaf 1.

 

Eggs: The adult WBC lays eggs in masses of 5 to 200 with an average of about 50 (Figure 2). Initially, eggs are pearly white, but turn to tan within two days. By the fifth day of development, the eggs turn a dark purple. From this point, egg hatch is only a day or two away.

 

Figure 2. (L) WBC egg mass with some eggs hatched.1 (R) WBC larvae crawling away from egg mass.3

 

Larvae: First- through fourth-instar larvae are dark brown with faint cross-hatched (diamond-shaped) markings on their backs.

 

Figure 3. First-instar WBC larva on corn leaf 2.

Older larvae have three dark brown stripes immediately behind their heads (Figure 4). These characteristic markings distinguish them from other cutworms and caterpillars that may be found on corn. As larvae mature, they become light tan or gray to pink in color.

Figure 4. Fifth-instar4 WBC larvae.

 
 

To further distinguish WBC from corn earworm, note that corn earworm larvae have longitudinal stripes on the side of the body, and WBC larvae do not (Figure 5). Corn earworm larvae also have small dark “microspines” covering their body, whereas WBC larvae are smooth-bodied. (See Appendix 1 for tips on distinguishing WBC from other caterpillars on corn based on larval characteristics, feeding behavior and geographic range.)

Figure 5. Corn earworm larvae showing variation in color5,1.

 

Life Cycle

Western bean cutworms complete one generation of egg, larva, pre-pupa, pupa and adult per year. WBC overwinter as full-grown, non-feeding larvae in earthen cells about 4 to 10 inches beneath the soil surface. This pre-pupa stage pupates in late-May or early June, and adults emerge in early to mid-July.

WBC adults (moths) are strong fliers, and are known to travel several miles. Female moths emit a pheromone that attracts males. After mating, females deposit their eggs on the top surface of corn leaves in the upper one-third of the plant. In very erect-leaf hybrids, eggs may also be deposited on the lower surface of leaves.

Eggs hatch in 5 to 7 days, and larvae feed for 3 to 5 weeks as they develop through five instar stages. In early to mid-September, larvae complete their growth, stop feeding, and drop from the ear to the ground. They then burrow into the soil and construct an earthen cell for overwintering.

Feeding Behavior and Damage to Corn

 

Because young larvae feed on tassels and silks, WBC moths target corn fields in the late whorl stage of development for egg laying. In pre-tassel corn, hatched larvae move up the plant to feed on pollen in the developing tassel. However, if the tassel and ear shoot have emerged when larvae hatch, larvae move to the ear as their preferred feeding site. Plant-to-plant movement is very common with WBC larvae, which may move up to 10 feet away from their egg mass in any direction.

Once the ear has formed on a plant, it becomes the major focal point of feeding by WBC larvae. Early-instar larvae tunnel through the silk channel to reach the developing kernels. As larvae grow, they feed on the ear tip and often move further into the ear, especially if several larvae infest the same ear. Unlike corn earworms, WBC larvae are not cannibalistic, so several larvae are often found on one ear (Figure 6).

Figure 6. Several WBC larvae per ear cause severe losses6.

Direct yield loss occurs as WBC larvae consume all or parts of developing kernels (Figure 7). Those partially consumed may be further attacked by ear molds or secondary insect feeders that enter the ear through the WBC feeding channel. The result is lower yield and lower quality grain. When fields average several WBC larvae per plant, yield losses may be as high as 30 to 40 percent.

Management of Western Bean Cutworm

Successful management of western bean cutworm has historically been limited to careful scouting and timely application of an insecticide. However, a new option has recently been announced: Pioneer® brand hybrids with Herculex⁸ I Insect Protection are now labeled for control of this hard-to-manage pest.

Scouting and Insecticide Application

Most broad-spectrum corn insecticides are labeled to control western bean cutworms. However, insecticide timing is critical to achieve acceptable control. Once larvae enter the ear for feeding, it is almost impossible to reach them with insecticide sprays. Because larvae begin to tunnel through the silk channel at a very young age, the window for effective insecticide application is limited.

Figure 7. WBC larvae may tunnel through the silk channel or the husks and feed on the ear tip7 or side of the ear4.

Entomologists suggest using light or pheromone traps to monitor for WBC moths in early July. When the first moths appear, growers should immediately begin to scout their fields. Look for eggs and larvae on the upper leaf surface in the upper part of the plant. Sample 100 plants by checking 20 consecutive plants in 5 areas of the field, or 10 consecutive plants in 10 areas of the field. The more variable the field, or the more hybrids planted, the more areas that should be sampled.

If 8% of the plants have an egg mass or young larvae in the tassel, an insecticide application should be considered. This low threshold is made necessary by the ability of the larvae to move to other plants.

Should an insecticide treatment be deemed necessary, its timing is critical. If tassels are not yet emerged, wait to apply until 90 to 95% of tassels have emerged. If tassels have already emerged, apply when 70 to 90% of the eggs have hatched. Note that hatch will occur in about 24 hours once eggs have reached the dark purple stage.

Before treating, corn fields should also be checked for the presence of spider mites. If spider mites are found, use an insecticide product that won’t stimulate mite reproduction.

Pioneer® Brand Hybrids w/ Herculex I Insect Protection

In August of 2003, the EPA approved a registration amendment to include western bean cutworm on the Herculex I Insect Protection label. Growers now have access to the built-in protection of Pioneer hybrids with the Herculex I trait against WBC. In addition, Herculex I provides full-season protection against European corn borer, southwestern corn borer, black cutworm and fall armyworm, and intermediate suppression of corn earworm. Hybrids with the YieldGard⁹ Corn Borer (YGCB) gene, on the other hand, do not provide protection against western bean cutworm or black cutworm.

Over the past two years, Pioneer researchers have conducted field-sized side-by-side studies to evaluate the performance of Pioneer hybrids with the Herculex I trait in the presence of natural infestations of WBC. In 2002, hybrids with the Herculex I trait were compared to hybrids with similar base genetics but without the Herculex I trait in 16 Product Advancement Trial (PAT) locations. In 2003, hybrids with the Herculex I trait were compared to hybrids with similar base genetics and the YieldGard Corn Borer gene in 15 Product Knowledge Plot (PKP) locations. Results are shown in Figure 8.

Figure 8. Percent ears infested by WBC, Herculex I vs. base genetics (2002 PAT, 16 locations) and Herculex I vs. YieldGard Corn Borer (2003 PKP, 15 locations).

As the graph clearly shows, the Herculex I trait was very effective in protecting hybrids from ear feeding by WBC. In 2002, unprotected hybrids had 25% of ears infested by WBC, but Pioneer hybrids with Herculex I Insect Protection had only 4% of ears infested. In 2003, 37% of ears with the YGCB gene were infested with WBC, while hybrids with Herculex I Insect Protection had only 5% of ears infested. Pioneer researchers rate hybrids with the Herculex I trait a 7 on a scale of 1 to 9 for resisting WBC feeding, indicating very good protection against this insect.

Because of the labor intensive nature of scouting, the critical timing needed for insecticide applications, and the possibility that multiple treatments may be necessary, insecticides may not be an economical or effective solution to the WBC problem. Built-in Herculex I hybrid protection, on the other hand, eliminates the need for intensive scouting, works regardless of infestation level, and controls a number of other damaging pests. This new technology could provide an excellent management solution for growers with a history of WBC infestations, and insurance for many growers in the path of this spreading problem. 

References

 

Seymour,R.C., G.L.Hein, R.J. Wright, and J.B. Campbell. 2002. Western bean cutworm in corn and dry beans. University of Nebraska Extension publication G1359. Lincoln, NE. http://www.ianr.unl.edu/pubs/insects/g1359.htm

Peairs, F.B. 2002. Western bean cutworm: characteristics and management in corn and dry beans. Colorado State Extension publication 5.538. Fort Collins, CO. http://www.ext.colostate.edu/pubs/insect/00538.html

Wilson, B. 2003. Pioneer Technical Information Manager, Midwest Sales Region, Johnston, IA. Personal communication. 

Acknowledgements

 

The authors gratefully acknowledge the following indivi-duals and institutions for the images used in this article:

¹ Scott Dickey, Pioneer Hi-Bred.

² Herb Eichenseer, Pioneer Hi-Bred.

³ Kevin Branick, Pioneer Hi-Bred.

⁴ Chuck Bremer, Pioneer Hi-Bred.

⁵ Jim Boersma, Pioneer Hi-Bred.

⁶ Mike Catangui, South Dakota State University.

⁷ University of Nebraska.

 

⁸Herculex I Insect Protection technology by Dow AgroSciences and Pioneer Hi-Bred. Herculex is a
trademark of Dow AgroSciences LLC.

⁹Registered trademark used under license from
Monsanto Company.