April 4, 2008 

Transgenic corn hybrids that are resistant to insects (i.e. Bt corn) are no longer "new," and most Nebraska farmers have either grown these hybrids or are familiar with them. These corn hybrids are resistant to European corn borer, various other caterpillars, and corn rootworm. Hybrids that are resistant to both European corn borer (and a few other caterpillars) and corn rootworm are becoming increasingly popular. Common to all Bt corn hybrids is the requirement to plant a refuge as part of an insect resistance management (IRM) program. Keeping the different Bt corn hybrids and their respective IRM programs straight can be confusing, so we've included a review of the IRM requirements and encourage growers to fully comply with them.

Resistance Management Programs

Following are the 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, you must follow the IRM for rootworm program (A.).

A. IRM for Transgenic 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 they may be 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-resistant hybrids, or at least four rows wide for the Agrisure and Herculex rootworm resistant hybrids. It is preferable to have wider strips, such as 12 consecutive row-wide strips.
6. The refuge must be planted in similar ground as the Bt corn for rootworm, that is, if theBt corn for rootworm is planted in ground that was in corn the previous year, so must the refuge.
7. Insecticides for control of corn rootworm larvae may be applied to the refuge area.

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 the 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.

The Importance of IRM Compliance

Compliance with IRM requirements is critical to the success of the IRM program, the goal of which is to prevent insect pests from developing resistance to Bt toxins in Bt transgenic corn. Farmers should comply with the IRM requirements to

1. preserve Bt corn as an effective pest management tool well into the future, and
2. continue to have access to Bt corn hybrids.

In the past, compliance was measured primarily through the use of grower surveys; however, monitoring compliance now includes field visits. For example, farmers are randomly selected from the pool of those who purchased Bt corn. They are then contacted, and a field visit is scheduled. Planting records are examined, refuge fields are measured for size, configuration, distance from theBt field, and corn plants are tested for the expression of the Bt toxin. If the farmer is found to be significantly out of compliance, the farmer is provided resistance management educational materials, aided in developing and implementing a resistance management plan, and revisited the following year. If the farmer is found to be out of compliance the next year, the farmer will be denied the product the next year.

Farmers have the most important job in maintaining the continued effectiveness of Bt corn and limiting the development of resistance. They need to follow IRM programs and report any possible product failure to their seed dealer or local extension educator. They also need to keep careful records of what hybrid was planted where on the farm. This will help avoid making a mistake during the hectic planting season, and help document planting, should the field be inspected.

Tom Hunt, Extension Entomologist, UNL Haskell Agricultural Laboratory, Concord
Bob Wright, Extension Entomologist, UNL Entomology Dept., Lincoln
Keith Jarvi, IPM Extension Assistant, NEREC, Norfolk