Prior to last year’s rise in corn prices, most entomologists discouraged using both a soil insecticide at planting and a Bt corn rootworm hybrid in the same field for corn rootworm control.

However, with corn prices hovering in the $3-4/bu. range, this two-pronged control strategy may prove profitable in the short-term for some farmers, says Kevin Steffey, University of Illinois (U of I) Extension entomologist.

“Right now, with the high corn prices, I would have a hard time arguing against using both the Bt corn rootworm hybrids and a soil insecticide at planting, except obviously in Bt corn-rootworm refuge areas, which require planting a non-Bt corn rootworm hybrid,” he says. “Based on root protection alone, we have generally seen soil insecticides performing every bit as well as Bt corn rootworm hybrids.”

Under certain conditions, however, soil insecticides sometimes provide more root protection than Bt corn rootworm technology, notes Steffey. For example, this year’s U of I “preliminary root evaluation ratings” show that soil insecticide applications protected roots just as well as Bt corn rootworm-resistant hybrids at the Monmouth location, less effectively than the transgenic corn hybrids at DeKalb, but better than two Bt corn rootworm-resistant hybrids at Urbana.

“We don’t fully know yet how well Bt corn hybrids perform under certain conditions or why there have been performance differences at some locations,” says Steffey. “We have received two or three reports this year of greater-than-expected root damage on Bt corn, not just at Urbana. There are instances of Bt corn failures every year, but they are still a small percentage, and we still don’t know why they occur.”

In fields where corn avoids lodging, Bt corn rootworm hybrids could also yield considerably better—even with some root damage—than non-Bt corn hybrids planted with a soil insecticide that have less root damage, notes Steffey. As a result, “most farmers won’t rest easy with just the root rating information,” he adds. “They want yield data. Later in the year we will be providing that yield data along with our root rating data.”

Farmers who decide to use both a Bt corn rootworm-resistant hybrid and a soil insecticide at planting would be providing two modes of action that could help to ensure better overall root protection, depending on conditions, than one mode of action would provide if working alone, points out Steffey. “Using two modes of action is an insect resistance management practice that might slow the resistance to both modes,” he says. “Long-term, relying on just one technology is unwise.”

Still, the drawback to using this two-pronged strategy as a long-term solution, besides the extra time and expense, is that corn rootworm populations would be exposed to the same two modes of action every year. If corn rootworms were able to survive this protection, “then we have no more defenses left to use against them except growing a different crop,” cautions Steffey.

For more information on U of I corn rootworm product efficacy trials in 2007, click here
For information from 2006, click here

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