The introduction of hybrids containing the Bt gene gave no-tillers a new management tool to reduce European corn borer losses, but how long this tool will remain effective is the responsibility of growers.

“If Bt is taken for granted and not properly protected, the Bt advantage could prove short lived,” says Steve Butzen, Pioneer agronomy information specialist.

University of Illinois entomologist Kevin Steffey agrees that Bt resistance will happen if growers don’t treat the technology with respect. Steffey’s concern stems from past experience with resistant insects.

“After insects have developed resistance to insecticides, for example, they often come back with a vengeance,” says Steffey. “They are more aggressive, so you end up with a tougher animal to deal with. If we get a worm resistant to Bt corn, we’ll probably have to resort to insecticides more so than in the past.”

Steffey believes every no-tiller who grows Bt corn must realize the following three assumptions:

• If Bt corn is planted on too many acres, European corn borer will become resistant to Bt corn.

• The long-term benefits of Bt corn will not be realized if resistance develops.

• If European corn borer resistance to Bt occurs, the chances for developing other transgenic crops become slim.

Management Strategies

No-till corn growers can stay a step ahead of European corn borers by ensuring that corn borer moths susceptible to Bt are available to mate with any resistant survivors from Bt fields, says Butzen.

“Susceptibility by resistant matings dilutes or masks the resistance in the next generation, preventing the buildup of resistant populations,” Butzen explains. “If the number of susceptible corn borers is high compared to resistant ones, most resistant moths will mate with susceptible moths.”

Corn borer populations are largely susceptible to the Bt protein; however, rare individuals express resistance. Resistance increases in the population when resistant types survive, mate and reproduce at a higher frequency than susceptible types.

That’s why it’s so important for no-tillers to plant a refuge of non-Bt corn near a Bt field to provide a susceptible population. An effective refuge will provide a sufficient quantity of susceptible corn borer moths to greatly outnumber the resistant moths surviving in the corresponding Bt field. The susceptible moths must be close enough geographically to intermingle with resistant moths.

Refuge Design

When it comes to creating a refuge, the most common considerations are to determine how large it should be, how close to the Bt corn it should be and how the refuge should be configured.

“There is a lot of research going on to find out the best refuge configurations and sizes, but I argue that we’ll never know the right answer because the research is too large,” says Steffey.

Despite the current lack of research, Steffey recommends no- tillers maintain a refuge of non-Bt corn that makes up 25 percent to 40 percent of the Bt acreage. Furthermore, this refuge has to be planted adjacent to the Bt corn field.

As long as the refuge of non-Bt corn is next to the Bt corn, Steffey is satisfied. However, he recommends that no-tillers plant a big block of non-Bt corn rather than splitting the planter in half and planting Bt corn with one half the planter and non-Bt with the other.

“Splitting the planter in half makes it hard to treat (non-Bt) strips,” says Steffey. “You’re going to have problems in 50 percent of the corn if you get a bad corn borer infestation. You can’t treat those strips very easily.”

If the refuge covers a large area, it’s easier to treat it conventionally with a chemical insecticide.

Butzen adds that the refuge hybrid should be as similar as possible agronomically to the corn hybrid with Bt. Having the growth and development of the two hybrids parallel will help ensure that the refuge hybrid attracts moths to the same extent as the resistant field.

“The refuge hybrid should match the hybrid with Bt in maturity, early vigor and plant height,” says Butzen. “Growers should also expect the refuge hybrid to provide high yield for maturity, standability, disease resistance and dry down.”

Watchful Eye

Monitoring Bt fields for resistance development is also an integral part of a resistance management plan. By detecting resistant populations early, alternative control measures can be quickly implemented to eradicate the population and halt the spread of resistance.

Steffey emphasizes

“I’m very unsympathetic to people who will not plant a refuge or scout their corn. Stewardship of this pest management tool is everyone’s responsibility. If we overuse Bt, we will lose its effectiveness.”