Resistant Corn Hybrids for Managing Corn Earworm, Fall Armyworm, Stalk Borer, Southwestern Corn Bore

• Studies from 1997-1999 established that Pioneer® brand hybrids with the YieldGard® gene provided excellent protection from ear feeding, shank tunneling and stalk tunneling by European corn borer.
• University and Pioneer data also indicate that these Pioneer hybrids provide excellent resistance to southwestern corn borer and southern cornstalk borer, and intermediate resistance to corn earworm.
• The YieldGard label was significantly expanded in 1998 to include suppression of fall armyworm and stalk borer, and control of southern cornstalk borer.
• University and Pioneer data support the label change, as Pioneer brand hybrids with the YieldGard gene provided intermediate resistance to fall armyworm and stalk borer.
• This Crop Insights summarizes data on efficacy of these Pioneer hybrids against corn earworm, fall armyworm, stalk borer, southern cornstalk borer and southwestern corn borer.

Introduction

The introduction of Pioneer corn hybrids with the YieldGard gene in 1997 provided producers with unprecedented control of two injurious insect pests of corn – European corn borer and southwestern corn borer. In addition, intermediate resistance to corn earworm feeding was obtained with these same hybrids.

In 1998, the YieldGard label was expanded to include control of southern cornstalk borer and suppression of fall armyworm and stalk borer (often referred to as "common stalk borer"). The expanded label, then, provides for control or suppression of six of the most common insect pests of corn:

Insects controlled or suppressed by Pioneer®
brand hybrids with the Yieldgard gene

This Crop Insights includes efficacy data to help producers make informed decisions when managing these corn pests using resistant Pioneer hybrids.

The corn earworm feeds on over 100 different plants with corn being the preferred host. This insect is not capable of overwintering in the northern corn belt, but reestablishes itself each spring when moths migrate to northern areas. In appearance, corn earworms are multi-colored, with variations of green, yellow, black, brown or even pink.

Corn earworms may display distinctly different color patterns.

Corn earworm moths deposit eggs singly on the under-surface of corn leaves, leaf sheaths, husks, and silks, depending on the developmental stage of the plant. First generation larvae may feed in the whorl, creating a ragged appearance on leaves. Silks, if present, are the preferred oviposition site – larvae hatching there may proceed directly down the silk channel to feed on ear tips. Larvae feed for 2 to 4 weeks. Ear tip feeding can be extensive, and provides an avenue for disease organisms to enter the ear.

Corn earworm feeding on developing ear.

Insecticide applications for corn earworm in field corn are not considered practical because a single application will not provide adequate control.

In extensive evaluations, Pioneer corn hybrids with the YieldGard¤ gene provided intermediate resistance to ear feeding by corn earworm and fall armyworm (Tables 1 and 2 below).

Table 1. Damage ratings of Pioneer and competitive corn hybrids to combined corn earworm (CEW) and fall armyworm (FAW) ear feeding, Puerto Rico, 1997.

Pioneer Ear Damage Field Scoring System

**centimeters, measured from the tip toward the butt of the ear.

Table 2. Damage ratings of Pioneer and competitive corn hybrids to corn earworm feeding, Hawaii, 1997.

Fall armyworm larva feeding on leaf tissue.

Fall armyworm is a very serious pest of corn in the southern, eastern and southeastern corn belt. The threat of damage is even greater for late planted corn, or corn following winter wheat. Larvae of the fall armyworm feed extensively on leaf tissue before moving deep in the whorl beneath a plug of frass. Larvae also frequently feed directly on ear tissue, and this damage may be as yield limiting as leaf feeding, or more so. Damage is most common on late pretassel corn.

Fall armyworm larvae somewhat resemble corn earworm, but fall armyworm have a white, inverted "Y" on the front of a dark head, and four dark spots on the top of the eighth abdominal segment.

Scouting and management of fall armyworm must take place in a timely fashion before larvae burrow into the whorl or enter ears. Frequently, damage is not detected until after the ideal time for control with insecticides.

Leaf feeding damage by fall armyworm was evaluated on several resistant and non-resistant Pioneer corn hybrids and competitive hybrids at Tifton, GA and Puerto Rico in 1997. Early leaf feeding was scored at growth stage V6-V8, and late leaf feeding was scored at or after anthesis. Results are presented in Table 3.

Non-resistant hybrid.

Reduction in leaf feeding damage was statistically significant and visually striking. Although leaf feeding scores are lower at advanced vegetative growth stages, the degree of protection provided by most resistant hybrids was impressive and of economic value. At Tifton, non-resistant hybrids were so heavily damaged that most were nearly destroyed and never tasseled. At Puerto Rico the infestation was also very heavy but ears were produced for most hybrids.

Pioneer Fall Armyworm Leaf Feeding Rating System*

*Adapted from Davis, et. al. 1992.

Table 3. Leaf feeding damage field scores for fall armyworm at two growth stages , Tifton, GA and Puerto Rico, 1997.

University entomologists have also evaluated the efficacy of the YieldGard¤ gene in controlling fall armyworm:

Table 4. Comparison of feeding damage by fall armyworm on Pioneer 3394 and Pioneer 33V08 (Bt). North Carolina, 1997*.

* Study conducted by Dr. John Van Duyn of North Carolina State University.

Pioneer corn hybrids with the YieldGard gene have provided intermediate resistance in other university and industry field tests as well.

Damage by stalk borer (Papaipema nebris, often referred to as "common stalk borer") is characterized by dead or dying plants that may have ragged holes in the newly emerged leaves. Damage is most frequently found along shelterbelts, fencelines, terraces, or grass waterways in conventionally tilled fields but may be found at various locations throughout no-till fields.

Stalk borers overwinter as eggs deposited by moths on weedy hosts in late summer. Newly hatched larvae immediately burrow into grass or other weedy plants. When the weeds die or larvae become too large for the current host, larvae move to new hosts, which may include corn.

Stalk borer larvae attack corn by entering the stalk at ground level and tunneling upward, or entering the whorl and tunneling downward. In both cases the result may be plants with "dead heart". Damaged plants have drastically reduced yields and often do not produce harvestable ears.

Management of stalk borer with insecticides is very difficult at best. Once larvae enter the plant, they cannot be effectively controlled with an insecticide application. Applications must coincide with movement of larvae from one host plant to another. This movement may occur over several weeks, whereas insecticides applied for this purpose are short-lived in comparison. Generally, by the time an infestation is recognized, insecticide applications are not effective.

Data are sparse on efficacy of various Bt events for controlling stalk borer, but Dr. Marlin Rice at Iowa State University has conducted some limited screening. In these trials corn plants were manually infested with stalk borer larvae at the rate of one 2nd or 3rd instar larvae per plant in one test and one 4th instar larvae per plant in another test. Ten single plant replicates were then examined and evaluated. Results in the Iowa State study indicate intermediate resistance to early instar stalk borer but no statistically significant response to later instar larvae.

Monsanto Company also evaluated resistance to stalk borer using Pioneer corn hybrids in 1997 at five locations in Iowa, Illinois and Ohio. Their data indicate intermediate resistance to stalk borer in Pioneer hybrids containing the YieldGard¤ gene. Results are presented in Figure 1 below.

Figure 1. Percent plants with visible stalk borer damage, five locations, 1997. Courtesy Monsanto Company.

Southwestern corn borer is a major pest of corn in the Southern Plains states, including Texas, Oklahoma and Kansas, and even New Mexico and Arizona. It may also be problematic in the Midsouth (Arkansas, Mississippi, S. Kentucky, W. Tennessee, S.E. Missouri and S. Illinois.)

Losses from southwestern corn borer can be attributed to four major causes:

• Death of the growing tip from first generation larvae feeding on young corn plants
• Larvae tunneling in the stalk
• Larvae girdling the plant, resulting in severe lodging
• Larvae feeding on husks, cobs, and kernels

Larvae have two distinct forms. In the summer, they are milk-white with a brown head and numerous brownish-black tubercles on the body. During the fall and winter the tubercles become pale brown and inconspicuous, giving the larvae a nearly all-white appearance.

Southwestern corn borer, summer form.

Southwestern corn borer normally has two generations per year. They then overwinter as full-grown larvae inside the base of the plant, below soil level. Typically, only a single larvae will be found inside a stalk due to the cannibalistic nature of older larvae. Overwintering larvae pupate in late May through June. First generation larvae complete their development and a second generation may emerge by late July or early August.

Crop damage on a per larva basis may actually be greater from first generation larvae because plants are killed. However, overall crop loss is typically greater from second and third generations because population densities are much higher.

Management strategies for southwestern corn borer include use of resistant hybrids, fall tillage, early harvest and insecticide applications. Timing of insecticide applications is critical to achieve effective control.

Several Pioneer corn hybrids containing the YieldGard¤ gene were evaluated at two locations in 1997 for efficacy against southwestern corn borer stalk tunneling (Table 5) and ear feeding (Table 6). Pioneer hybrids with the YieldGard gene provided excellent resistance to stalk tunneling, girdling, and ear feeding by this insect.

Table 5. Performance of Pioneer hybrids with the YieldGard gene and competitive Bt hybrids against stalk tunneling and girdling by southwestern corn borer (SWCB). Union City, TN and Plainview, TX, 1997.

Table 6. Performance of Pioneer hybrids with the YieldGard gene and competitive Bt hybrids against ear feeding by southwestern corn borer. Plainview, TX, 1997.

*Field scores obtained using scoring system similar to the one described for Tables 1 and 2.

Southern cornstalk borers damage corn from Alabama to Ohio and Maryland. Larvae feed within the plant whorl, resulting in rows of irregular holes in newly emerged leaves. As larvae mature they tunnel into stalks in the lower portion of the plant, primarily just above the soil line. Frequently, an infestation is not discovered until severe stalk damage has occurred. Damage by southern cornstalk borer significantly interferes with nutrient and water transport within the plant. Two generations are completed per year.

Efficacy trials utilizing Pioneer corn hybrids with the YieldGard¤ gene have been conducted by Dr. John Van Duyn of North Carolina State University. Results are presented in Table 7 below. Pioneer hybrids containing the YieldGard gene are highly resistant to feeding by southern cornstalk borer.

Table 7. Performance of Pioneer hybrids with the YieldGard¤ gene against southern cornstalk borer.
North Carolina, 1997*.

* Courtesy of Dr. John Van Duyn, North Carolina State University

It is critically important to preserve the long-term effectiveness of this exciting new technology for corn insect management. Pioneer is committed to resistance management and has implemented a strategy for managing insect resistance that includes:

• Discovery of additional resistance genes
• Monitoring of ECB populations in the field
• Promoting refuges for ECB populations
• Developing education programs
• Responding quickly to unexpected damage complaints

Resistance management is discussed thoroughly in Crop Insights, vol. 9 no. 20 (Butzen, 1999). North Central Regional Pub. 602 also does an excellent job of discussing and presenting various resistance management strategies. Additionally, all producers who purchase hybrids containing the YieldGard¤ gene will be provided with information discussing resistance management strategies.

Additional References

Davis, F.M., S. S. Ng and W. P. Williams. 1992. Visual rating scales for screening whorl-stage corn for resistance to fall armyworm. Miss. Agric. and Forestry Experiment Station Technical Bulletin 186, December 1992.

Monsanto Company. 1997. Insect resistance management. 1998 YieldGard Product Use Guide.

Ostlie, K.R., W.D. Hutchison and R.L. Hellmich. 1997. Bt corn and European corn borer. NCR publication 602. University of Minnesota, St. Paul, MN.

http://www.extension.umn.edu/distribution/cropsystems/DC7055.html

Trademark of and used under license from Monsanto Company.