When you plant a Bt hybrid, you protect your crop against European corn borers. But you may be paying more than necessary for protection.

"There is no guarantee of an economic advantage for Bt corn over conventional pest control practices," points out Mike Catangui, South Dakota State University entomologist.

"With Bt corn, you make the decision to use a pest control tactic when you buy the seed," says Catangui. "Yet it's difficult to know in advance if the economic threshold for corn borers will be reached. You must actually scout the field to evaluate that threshold."

In three years of trials, Catangui and a colleague, agronomist Bob Berg, compared Bt hybrids to their untreated isolines (non-Bt counterparts) and to the isolines treated with an insecticide. They studied both univoltine (one corn borer generation per year) and bivoltine (more than one generation per year) areas.

The univoltine testing was done at South Shore, SD; the bivoltine testing at Beresford.

In 1998, the scientists used five hybrid groups at South Shore and seven at Beresford. Each group included a Bt hybrid plus its treated and untreated isoline. They applied Pounce 1.5G at the whorl stage as the insecticide treatment.

Borer pressure was heavy at South Shore. At Beresford, it was light for the first generation and heavier for the second generation.

"At South Shore, three of the Bt hybrids showed statistically significant yield increases of 22, 15 and 28 bu/acre, respectively, over the untreated isolines," reports Catangui. "Two Bt hybrids did not show significant yield increases."

In two of the three cases where Bt increased yields, insecticide treatment brought similar gains.

The researchers, when making economic comparisons, used a $1.60/bu corn price and a moisture dock of 1/2 cent per point of moisture over 15%.

"Since the Bt technology fee can vary from $5 to $15/acre, and since the insecticide cost also can vary, we used gross income rather than net income," Catangui explains.

"On a gross income basis, the three highest-yielding Bt hybrids (at South Shore) registered returns of $35.68, $32.52 and $19.09 per acre. The other two had returns of $1.05 and minus $4.69."

The insecticide treatment showed a greater gross return than did three of the Bt hybrids, but less return than the other two.

At Beresford, yields for the seven Bt hybrids and the treated and untreated isolines were statistically similar.

Gross income for the Bts and the treated and untreated isolines was statistically similar for five of the seven hybrid groups. For the two other groups, the Bt was similar to the treated isoline in one case and similar to untreated isoline in the other case, but never had the highest gross itself.

"Our research indicates that the farmer who scouts fields for corn borer and treats as needed often can expect as good as or better economic return than from planting Bt corn," says Catangui. "This is especially true when averaged over the long term."

He adds, however, that not every farmer has the time to do effective scouting.