2004, Issue #2

from your Pioneer Sales Representative: Kingston Feed & Farm

CROP INSIGHTS

Insect Resistance Management and Bt Corn

by Steve Butzen, Agronomy Information Manager

Summary

· Pioneer now offers Bt hybrids with the Herculex I (HX1) or YieldGard Corn Borer (YGCB) trait for controlling corn borers (and some other pests), and hybrids with the Yield-Gard Rootworm (YGRW) trait for corn rootworm control.

· Growers using these technologies must plant a refuge. A refuge is an important component of an IRM plan to help deter development of resistance to the trait by target insects.

· Complying with EPA-mandated IRM requirements is a regulatory and stewardship obligation. Failure to do so can result in growers losing access to Bt technology.

· Although the EPA has tried to keep refuge requirements as similar as possible, they do differ slightly between Bt hybrids for corn borer vs. corn rootworm control.

· This article explains the concepts of insect resistance management, how to establish an effective refuge, and the importance of monitoring fields and reporting damage.

Introduction

Hybrids with a Bt trait for corn borer or corn rootworm control offer growers powerful tools to help reduce losses from these devastating pests. But if insects develop resistance to Bt genes, they could quickly reduce or negate the tremendous potential of these technologies. Hundreds of cases of insects overcoming insecticide effectiveness are documented, including many in corn. Implementing a proper insect resistance management (IRM) program will help prevent a similar resistance scenario with Bt hybrids.

To protect the usefulness of Bt technology for as long as possible, the Environmental Protection Agency (EPA) has mandated that certain provisions be placed on its use. In addition, the EPA requires companies selling Bt technology to educate their sales force and customers in meeting the IRM guidelines, and to assess IRM compliance by its customers. These companies must also conduct research on IRM and closely monitor and report any development of resistant insect populations.

IRM Requirements

Pioneer requires that all growers purchasing hybrids with a Bt trait sign a Pioneer Hi-Bred Technology Agreement. By signing it, the grower agrees to implement an IRM program as specified in the Product Use Guide (PUG). The IRM program includes planting a corn refuge, following EPA-mandated use require-ments and other steps outlined in the PUG. Failure to follow these IRM requirements can result in the grower losing access to Bt technology.


Corn borer (left) and corn rootworm (right) are two devastating pests of corn now controlled by Bt traits².

IRM Compliance Assurance Program (CAP)

Complying with IRM requirements is a regulatory and stewardship obligation critical to preserving the longevity and effectiveness of Bt corn technology. The EPA requires Bt corn seed providers to conduct on-farm visits as part of a comprehensive Compliance Assurance Program (CAP) to assess whether growers are following the IRM requirements.

The CAP also outlines consistent standards developed by EPA and Bt corn registrants to respond to growers who have not followed the IRM requirements to bring them into full compliance. These responses include:

· Sending a letter notifying the grower of the IRM compliance deviation.

· Conducting a compliance assistance visit with the grower prior to planting to assist the grower in implementing a proper IRM program.

· Conducting a compliance assessment visit with the grower the following growing season to assess IRM compliance.

· Providing the grower additional IRM educational materials.

· Denying access of Bt corn to growers who have been significantly out of compliance in two consecutive years.

Establishing an Effective Refuge

The goal of a refuge is to ensure that susceptible insects are available in sufficient numbers to mate with any resistant survivors from Bt fields. Susceptible by resistant matings result in susceptible offspring and prevent the buildup of resistant insect populations. To be effective, the refuge must be the correct size and distance from the Bt field, and be planted with a similar hybrid under similar management practices.


Refuges produce populations of susceptible insects to mate with any resistant survivors from Bt fields².

Refuge Requirements

In non-cotton growing areas, the refuge requirements for corn borer-Bt and corn rootworm-Bt are both 20% of corn acres planted. Appropriate refuges must be planted on every farm with Bt corn – that is, you cannot use a neighbor’s field to satisfy the refuge requirements. However, there are some important differences between the two kinds of Bt. Table 1 shows specific refuge requirements for both Bt technologies.

Table 1. Refuge requirements in non-cotton areas.

Corn Borer-Bt (HX1, YGCB)	Corn Rootworm-Bt (YGRW)
Refuge must be 20% of corn acres planted.	Refuge must be 20% of corn acres planted.
A refuge must be planted on every farm.	A refuge must be planted on every farm.
Refuge must be within 1/2 mile (and preferably within 1/4 mile) of the Bt field¹.	Refuge must be planted in same field as or adjacent to Bt hybrids. Can be separated by a ditch or a road, but not by another field.
If planted as strips within the Bt corn field, strips must be at least 4 rows wide (6 rows preferred)¹	If planted as strips within the Bt corn field, strips must be at least 6 rows wide (12 rows preferred)
May be treated with insecticides if economic thresholds are reached for one or more target pests, but sprayable microbial Bt insecti-cides (e.g., Dipel) must not be used.	May be treated for control of corn rootworm larvae and other soil pests with soil-, seed- or foliar- applied insecticides. May be treated with a non-Bt insecticide to control late-season pests such as corn borer, but the YGRW-protected Bt corn must also be treated.
Refuge hybrid cannot contain a Bt trait that controls corn borers (YGCB or HX1). However, the refuge can contain a Bt trait that controls corn rootworm (YGRW).	Refuge hybrid cannot contain a Bt trait that controls corn root-worms (YGRW). However, the refuge can contain a Bt trait for corn borers (YGCB or HX1).

All counties that are listed as cotton-growing counties in the Product Use Guide have a different refuge size requirement for corn borer-Bt (YGCB or HX1) hybrids, but NOT for corn rootworm-Bt (YGRW) hybrids. For YGCB or HX1 products, 50% of the corn acres must not be corn borer-Bt products. The refuge must be within 1/2 mile (and preferably within 1/4 mile) of the Bt field. The refuge acres may be treated with insecticides if economic thresholds are reached for one or more target pests, but sprayable microbial Bt insecticides (e.g., Dipel) must not be used.

Percent Refuge Requirement for YGCB or HX1 Hybrids

YGCB or HX1 hybrids (corn borer-Bt) have a 20% refuge requirement in non-cotton areas (white) and 50% in cotton-growing areas (gray). For corn rootworm-Bt hybrids (YGRW) the requirement is 20% throughout the U.S.

Select Similar Hybrid for Refuge

One key to establishing an effective refuge is selecting an appropriate hybrid. Insect behavior requires that the refuge hybrid be agronomically similar to the Bt hybrid. This helps ensure that the refuge hybrid attracts adult insects to the same extent as the Bt field.

To ensure similar growth and development, choose a refuge hybrid that is as similar as possible to the Bt hybrid. The refuge hybrid should match the Bt hybrid in maturity (especially silk CRM), early vigor and plant height.

Manage Refuge the Same as Bt Hybrid

Management practices in the refuge acres and Bt corn acres should be as similar as possible to promote parallel hybrid development. Planting the refuge at the same time as the Bt hybrid is the best way to coordinate initial development. Fertility programs, including starter and sidedress, should be similar. The tillage system should also be very much the same in the Bt field and the refuge. Different tillage operations may result in dissimilar residue levels on the soil surface. Soil temperature differences could then lead to dramatic early development differences between the Bt field and the refuge. Reducing inputs on the refuge or planting it on marginal land also decreases the effectiveness of the refuge.

Refuge Layouts

Because Bt corn growers use different management practices, considerable flexibility is allowed in laying out the refuge. Several of these refuge patterns are described below:

Refuge within the Bt field:

· Block

· Perimeter or Border

· Split Planter

Refuge separate from Bt field:

· For corn borer-Bt, a separate field planted within 1/2 mile of each Bt corn field (and preferably within 1/4 mile)¹.

· For corn rootworm-Bt, a separate field adjacent to Bt hybrids. Can be separated by a ditch or a road, but not by another field.

(Surveys indicate that most farmers plant the refuge within the Bt field. This closer proximity increases the effectiveness of the refuge.)

Field layouts of these options are shown below¹:


		Large Block(s) Refuge Options: Refuge block may be planted first, last or in the mid-dle. More than one block may be planted. Advantages: · Generally good mixing of insect populations · If needed, refuge can be treated separately · Can harvest Bt and refuge acres separately
		Perimeter Refuge Requirements: The amount of refuge planted around the field perimeter must be at least 20% of the total corn acreage. Advantages: · Generally good mixing of insect populations · If needed, refuge can be treated separately · Can harvest Bt and refuge acres separately

Split-Planter Refuge

Requirements for refuge strips:

· Corn borer-Bt: Must be at
least 4 rows wide, 6 preferred.

· Corn rootworm-Bt: Must be at
least 6 rows wide, 12 preferred.

· Individual planter boxes needed
· Match hybrid maturities

Advantages: · Excellent blending of insects

Limitations: · Can't easily treat or harvest the refuge separately

· More cleaning of seed boxes
· Better tracking is needed to ensure seed is poured in correct seed boxes

Separate Field Refuge

Advantages
· Greater flexibility with hybrids, insect and weed management, planting and harvesting.

Requirements
· Corn borer-Bt: Refuge field must be at least within ¼ to ½ mi.

Corn rootworm-Bt: Refuge field must be adjacent to Bt field. Can be separated by a road or ditch, but not by another field.

Recommendations · Plant refuge as close as possible to the Bt field

Separate Pivot Refuge

Advantages
· Greater flexibility with hybrids, insect and weed management, planting and harvesting.

Requirements
· Corn borer-Bt: Refuge field must be at least within ¼ to ½ mi.

Corn rootworm-Bt: Refuge field must be adjacent to Bt field. Can be separated by a road or ditch, but not by another field.

Recommendations · Plant refuge as close as possible to the Bt field

Monitoring Fields for Resistance Development

Monitoring Bt fields for insect resistance development is an integral part of an IRM plan. If resistant populations are detected early, alternative control measures can be quickly implemented to eradicate the population and halt the spread of resistance. Because of its importance to maintain the effectiveness of Bt technology, the EPA mandates monitoring activities as a condition of registration of Bt products. Pioneer instructs customers to monitor Bt fields as they would any other field and report unexpected levels of damage to a Pioneer representative. Acres planted to Bt hybrids should be correctly marked at planting to prevent confusion when monitoring.

Pioneer has developed a thorough set of protocols for instruction of customers, reporting of damage, and investigation and resolution of damage. Each incident of unexpected damage must be investigated. Damage is to be reported to the Pioneer sales rep, who must forward this information immediately to the Pioneer agronomist. The agronomist will view the problem area and help determine the cause of the damage.

More Information

Insect Resistance Management Online Course. Pioneer Hi-Bred and National Corn Growers Association.

Product Use Guides for Hybrids with YieldGard Corn Borer, YieldGard Rootworm or Herculex I Traits. Pioneer Hi-Bred, Johnston, IA.

Herculex is a registered trademark of Dow AgroSciences LLC.

YieldGard is a registered trademark of Monsanto Company.

¹ In Canada: refuge for corn borer-Bt must be within 400 meters (1/4 mile) of Bt field; refuge cannot be treated with insecticides; a minimum of 6 rows per strip is required in the split-planter refuge layout.

² Image of western corn rootworm larvae and beetle courtesy of Jim Kalisch, University of Nebraska. Image of northern corn rootworm beetle and European corn borer moth courtesy of Donald Specker, Pioneer Hi-Bred.