October 12, 2007

As fall approaches many growers are considering seed purchases for next year. Most Nebraska farmers are familiar with insect-resistant, Bt corn transgenic hybrids and their insect resistance management (IRM) requirements. However, with Bt corn for rootworms, Bt corn for corn borers, and Bt stacks for rootworm and corn borer, keeping the Bt corn hybrid insect control traits and their respective IRM programs straight can be difficult. This article will review the difference between the current basic Bt corn hybrids (events) and their respective IRM programs and encourage growers to fully comply with IRM requirements.

Bt Corn Hybrids

First, what is an "event" with respect to Bt corn? An event is the successful insertion of a gene encoding a Bt protein into corn plant DNA. Events vary in the type of Bt protein produced, the expression of the protein and the target insect pest. Expression refers to where, how much, and at what plant stages the Bt protein is produced in the corn plant.

The following events will be in a variety of corn hybrids and offered through several seed companies. Some have been around for several years (e.g. YieldGard) and some are relatively new (e.g. Herculex RW). Note that the events "suppress" some other insects. If these other pests are your primary concern, the Bt hybrids may not satisfactorily control them.

Up to date market approval status of biotech corn hybrids can be obtained at the National Corn Growers Web Site http://www.ncga.com/. Look under "Know Before You Grow" to learn if specific biotech corn hybrids are approved for export.

Resistance Management

Following are the two basic IRM programs for Bt rootworm corn (A) and Bt European corn borer corn (B). If you are growing a Bt corn stack that has genes for both corn borer and rootworm (YieldGard Plus, Herculex Xtra, Agrisure RW/CB), you must follow the IRM for rootworm program (A).

A. IRM for Bt corn hybrids active against corn rootworm

1. Growers must plant a structured refuge of at least 20% non-Bt rootworm corn that may be treated with insecticides (e.g., seed treatments, liquid or granular) as needed to control corn rootworm larvae.
2. Growers will not be permitted to apply corn rootworm labeled insecticides to the refuge for control of insect pests (either rootworm beetles or other insects, e.g., western bean cutworms, grasshoppers, etc.) while adult corn rootworm are present unless the Bt rootworm field is treated in a similar manner.
3. Refuge plantings may be blocks in or adjacent to Bt rootworm cornfields or as in-field strips.
4. External refuges must be planted adjacent to Bt rootworm fields.
5. When planting the refuge in strips across a field, refuges must be at least six rows wide for YieldGard Rootworm and YieldGard Plus or at least four rows wide for Herculex RW and Herculex Xtra. It is preferable to have wider strips, such as 12 consecutive row-wide strips.
6. Insecticides for control of corn rootworm larvae may be applied to the refuge area. In addition, the refuge must be planted in similar ground as the Bt corn for rootworm, that is, if the Bt corn for rootworm is planted in ground that was in corn the previous year, so must the refuge. General management of the Bt corn for rootworm and the refuge should be similar, e.g., planting dates, irrigation, fertility programs, etc.

B. IRM for Bt corn hybrids active against European corn borer

1. On each farm, growers may plant up to 80% of their corn acres with Bt corn. At least 20% of their corn acres must be planted with non-corn borer Bt corn and treated only as needed with insecticides. Decisions to treat the refuge must be based on economic thresholds. Conventional Bt products (liquids or granules) must not be used on the non-Bt refuge.
2. Plant non-Bt corn refuge within, adjacent to, or near the Bt cornfields. The refuge must be placed within 1/2 mile of the Bt field, preferably within 1/4 mile.
3. If refuge is established as strips in a field, the strips should be no narrower than four rows. General management of the Bt corn for European corn borer and the refuge should be similar, e.g., planting dates, irrigation, fertility programs, etc.

Additional UNL Resources

• For more IRM information on rootworm resistant hybrids, see NebFact 594, Resistance Management for YieldGard Rootworm Bt Corn.
• For more IRM information on European corn borer resistant hybrids, see NebGuide 1668, Resistance Management for European Corn Borer and Bt Transgenic Corn: Refuge Design and Placement.
• For more information on a host of topics concerning Bt corn and biotechnology, visit the UNL AgBiosafety Web site.

Tom Hunt
Extension Entomologist
Haskell Agricultural Laboratory, NEREC
Bob Wright
Extension Entomologist, Lincoln
Keith Jarvi
IPM Assistant, NEREC