Transgenic Bt corn hybrids initially were developed to target European corn borer damage in the Midwest. Newer varieties, however, might prove useful to Southeastern corn growers, especially for later plantings.

“The European corn borer initially was the target pest for evaluating Bt hybrids, and that's where the efficacy of the hybrids was aimed” says David Buntin, University of Georgia research entomologist.

Recent research has shown, he adds, that the additional cost of planting Bt hybrids can be justified in Georgia if a grower is planting later than the recommended planting dates, when the risk of heavy caterpillar infestations is greatest.

Midwest data

Bt hybrids generally have been shown to eliminate costs and benefit the environment through the elimination of insecticide sprays, says Buntin. “In the Midwest, there's good data showing yield advantages to Bt corn wherever European corn borers are a problem. In the Midwest this past year, about 19 percent of the crop was planted in Bt hybrids. But in some areas, like Nebraska and Minnesota where growers have severe corn worm problems, Bt corn exceeds 50 percent of the total acreage,” he says.

For the past three years, University of Georgia researchers have evaluated Bt corn hybrids along with conventional hybrids to test their effectiveness against insect pests that are common in Georgia corn production.

“We really don't have a problem with the European corn borer. But we wanted to see if the Bt corn would be effective against our range of insect pests. In addition, we wanted to know if Bt corn would help us reduce aflatoxin contamination. How does Bt corn compare with current management costs, and what's the best strategy for the use of Bt corn in Georgia? These are the questions we are attempting to answer,” notes Buntin.

Bt corn was at the center of a recent controversy where it was used in certain food products, he says. “I don't know how these marketing issues might affect field corn, but the acreage of Bt sweet corn has declined by 90 percent because of these concerns. We need to be aware of this issue.”

Only one available

Following last season, YieldGard is the only type of Bt resistance available in corn in the South, says Buntin. Most seed companies offer at least one hybrid with YieldGard technology. StarLink Bt was removed from the market last year, and Syngenta voluntarily removed Knockout/Maximizer Bt resistance from the South because of a lack of ear protection, he says.

YieldGard Bt resistance was developed independently by both Novartis (Syngenta) and Monsanto, he adds. It uses the cry1Ab gene with Monsanto calling their event MON810 and Syngenta calling their event Bt11.

“Both events have similar expression of the toxin which essentially is whole-plant, season-long expression. Both companies agreed to market this type of Bt resistance in corn as YieldGard Technology.

“YieldGard is offered by most seed companies. Initially, it was offered in short-season hybrids for the Midwest market. But most companies now have at least one longer season — 115 to 120-day maturity — hybrid with YieldGard resistance that fits better in our production area.”

Target pests

Target insect pests for Bt corn in Georgia include fall armyworms and corn earworms, specifically whorl and ear infestations, says Buntin. The southwestern corn borer also is a consideration, especially in north Georgia.

For the past three years, Bt hybrids have been compared at different locations throughout Georgia. This past year, researchers made one late planting in north Georgia and two late plantings in the southern part of the state, says Buntin.

“Based on our studies, YieldGard resistance provides about 90 percent control of whole infestations by fall armyworms and corn earworms.”

All Bt hybrids also proved to be effective against an infestation of Southwestern corn borers in north Georgia, especially in late-planted plots, he says.

“Grain aflatoxin contamination, which can be associated with ear damage by insects, was not different between susceptible and YieldGard resistant hybrids with similar genetic backgrounds,” says Buntin.

In tests conducted with the initial Bt varieties, there was a “yield drag” associated with the technology, he says. “But that's rapidly disappearing. The newer varieties are comparable agronomically to conventional varieties.”

If a grower is planting at the recommended time for grain production, Buntin advises that he select the “best agronomically adapted and highest yielding variety for his area, regardless of Bt resistance.”

“Bt is just one factor to consider. If you're planting later than the recommended date — 35 to 45 days later — then the Bt trait becomes important because of the threat of late-season armyworm infestations in the south and Southwestern corn borer infestations in the north. At least for now, that appears to be the best fit for Bt corn in Georgia, and it's where you're most likely to see a return on your investment.”

EPA guidelines state that on each farm, growers may plant a maximum of 50 percent of their corn acres to hybrids with the YieldGard Bt gene. A minimum of 50 percent of their acreage must be planted to non-Bt corn hybrids.

Cautions given

Growers are advised to treat the non-Bt corn refuge only as needed with insecticides. Sprayable Bt products cannot be used.

The non-Bt corn refuge on each farm may be arranged in a number of spatial configurations. These options provide each grower with the flexibility to incorporate a non-Bt corn refuge into his farming operation without lessening the effectiveness of the resistance management program.

Recommended planting configurations include the following:

  1. Separate non-Bt corn fields planted within one-half mile of each Bt field, and, if at all possible, to be within one-fourth mile.

  2. Large non-Bt corn strips or blocks within Bt field.

  3. Splitting planter to alternate one or more rows of non-Bt corn with Bt corn. However, if the option to spray is desired, treatment may be impossible and/or difficult with this method.

  4. Planting pivot corners to non-Bt corn.

  5. Planting field perimeters or end rows to non-Bt corn.

e-mail: phollis58@mindspring.com