Not all Bt corn hybrids are the same. Some stop corn borers. Some constrain corn rootworms. Still others control both insects in one seed. Yet they all share Bacillus thuringiensis, the Bt protein that acts as a toxin to deter corn-chomping insects from cutting into crop yields and profits.

Since each Bt hybrid is genetically modified to target a particular pest or pests, the Environmental Protection Agency (EPA) requires insect resistant management (IRM) practices to account for the differences in pest behavior, says Mike Gray, University of Illinois extension entomologist. In other words, IRM practices for some Bt hybrids are more stringent than for others.

Sound confusing? It can be, but a failure to understand the EPA's IRM regulations for Bt corn technology is no excuse for mistakes.

Farmers who neglect to follow Bt hybrid IRM regulations for two consecutive growing seasons could jeopardize their access to the seed. In fact, widespread failure to follow EPA's IRM rules could jeopardize the technology for the entire industry.

“EPA could simply step in and stop sales if it concludes farmers are not holding up their end of the bargain,” says Gray. “Insect resistant management is not an option for Bt corn — it's a requirement.”

The good news is that the vast majority of farmers do follow IRM rules for Bt corn. A newly released survey by the Agricultural Biotechnology Stewardship Technical Committee (ABSTC), in conjunction with the National Corn Growers Association (NCGA), shows that nine out of every 10 farmers comply with Bt corn IRM requirements. That's a significant improvement from four years ago.

“This year marks the fourth year of an upward trend in compliance,” says Tom Slunecka, NCGA director of development. “According to results of an annual survey, in 2003, 92% of farmers met regulatory requirements for IRM refuge size, while 93% met refuge distance requirements. These percentages have increased from 87% and 82% respectively, which were reported in 2000 when the survey began.”

Bt corn hybrids have been available commercially since 1996. Four companies currently have EPA registrations for Bt corn hybrids: Dow AgroSciences, Monsanto Co., Pioneer Hi-Bred International, Inc. and Syngenta Seeds, Inc.

According to Todd DeGooyer, corn insects technology manager for Monsanto, companies that register their Bt corn technology with EPA must conduct yearly IRM compliance assessments. EPA also requires farmers who plant the new seed technology to follow IRM plans.

“The key component of any IRM plan for Bt corn is a refuge, or blocks or strip of corn, that do not contain Bt technology,” explains DeGooyer. “However, in some cases, the refuge can be planted to a Bt corn that targets a different pest. Refuge sites should be planted with a similar hybrid, as close as possible to — and at the same time as — the main crop of Bt-protected corn.”

Except in areas where cotton is grown, EPA requires farmers who plant any Bt corn hybrids to also plant non-Bt corn refuge areas that are equal to at least 20% of the areas planted to a particular Bt corn. In some counties of certain southern states, farmers must plant 50% corn refuge areas even if they don't plant any cotton.

Confusion can arise, when growers try to remember which IRM requirement corresponds to each specific Bt technology.

For Corn Belt farmers, IRM refuge requirements are more demanding with hybrids genetically modified to control corn rootworms than with hybrids designed to stop corn borers, says Gray. As a result, farmers need to keep in mind that recently approved YieldGard Rootworm and YieldGard Plus hybrids require more conservative IRM requirements than corn borer-resistant Bt hybrids approved earlier, he adds.

The reason for the different IRM requirements for corn rootworms is based on their mating behavior. It makes them more likely to develop Bt resistance than corn borers if refuge requirements were kept the same for both pests, says Gray. Pest migration between fields is also a factor.

“Mating is more localized for corn rootworms than for corn borers,” he explains. “Rootworms don't travel as far when mating, and there isn't as much mixing of the gene pool in as wide an area compared to corn borers. It's that mixing of the gene pool that is so important in preventing resistant strains from developing.”

DeGooyer says farmers can plant corn refuge areas for YieldGard Corn Borer hybrids as far away as ½ mile, but preferably within ¼ mile. For YieldGard Corn Rootworm or YieldGard Plus hybrids, however, corn rootworm refuge areas must be planted within the same field or immediately adjacent to them. They must follow the same crop rotation, another field cannot separate them and adjacent fields must be owned or operated by the grower. A ditch or a road can, however, separate any refuge area.

An in-field (or split planter) corn refuge is another option for YieldGard Corn Borer, YieldGard Rootworm or YieldGard Plus hybrids, he adds.

Split planter refuge areas for YieldGard Rootworm or YieldGard Plus hybrids must be at least six and preferably 12 consecutive rows wide.

For Bt corn borer hybrids, the corn refuge area must be at least four and preferably six rows wide.

“Growers planting YieldGard Plus hybrids will have two different refuge planting options,” DeGooyer points out. “One option allows them to plant a common refuge for both corn borers and corn rootworms. The second option allows them to plant separate refuge areas for corn borers and for corn rootworms.”

In the first option, which is a common refuge for both corn borers and corn rootworms, a refuge area must represent at least 20% of a grower's YieldGard Plus corn acres. Farmers can plant refuge areas either as blocks that are immediately adjacent to YieldGard Plus fields, within the fields in strips or as perimeters around fields.

The second option requires more planning and involves planting separate refuge areas for both corn borers and corn rootworms. In this second option:

  • Farmers must plant corn rootworm refuges to non-Bt corn rootworm-protected hybrids. If desired, a corn rootworm refuge area can be planted to a Bt hybrid that controls corn borers. The corn rootworm refuge area must also represent no less than 20% of a farmer's YieldGard Plus corn acres and must be planted within or adjacent to the YieldGard Plus field.

  • Farmers must plant corn borer refuges to non-Bt corn borer-protected hybrids in separate fields within ½ mile of the YieldGard Plus field (¼ mile preferred). The areas are to represent no less than 20% of a grower's YieldGard Plus corn acres.

Resources abound to help farmers who have questions about the EPA's IRM requirements. Seed dealers, seed industry experts, farmer associations, crop consultants, USDA and ag extension employees are potential sources for help.

NCGA already has extensive educational material on its Web site, www.ncga.com. It's also developing a new, “online learning module” on IRM requirements.

“NCGA has been involved in IRM guidelines for Bt corn since the beginning,” says NCGA's Slunecka. “We want to see this technology available for generations to come, and plan to be educating farmers to deter resistance for as long as possible.”

Refuge Planting Options

  1. Except in cotton growing areas, farmers who plant a Bt corn hybrid must also plant a non-Bt corn refuge area equal to at least 20% of the area planted to Bt corn. In cotton growing areas, farmers must plant a 50% non-Bt corn refuge area whether or not they also grow cotton.

  2. This refuge planting option is allowable, but not preferred when planting rootworm-resistant Bt corn hybrids.

  3. This option is not available for YieldGard Rootworm or YieldGard Plus. For rootworm-resistant Bt hybrids, corn rootworm refuge areas must be planted within the same fields or immediately adjacent to them; they must follow the same crop rotation; another field can't separate them, and adjacent fields must be owned or operated by the grower.

SOURCE: NCGA