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Corn+Soybean Digest

Curtains For Corn Rootworm

In terms of money spent to control it, corn rootworm is the costliest insect in the cornfield. Growers spend more on chemicals to fight the $1 billion bug than all other corn insects combined.

Until recent years, crop rotations were a first-line defense against the rootworm. But the insect wised up to what growers were doing and set up a rotation of its own, making sure offspring hatched in cornfields.

Adult rootworm beetles began laying eggs in soybean fields destined to rotate into cornfields the next season. A northern strain of corn rootworm has even begun laying eggs that don't hatch until the second year.

Enter biotechnology. Monsanto's YieldGard Rootworm corn is genetically modified (GM) to produce its own rootworm toxin. Monsanto scientists incorporated a Bt (Bacillus thuringiensis) gene that emits a protein lethal to rootworms when they feed on corn roots. The GM corn provides better rootworm control than most conventional insecticides.

“The function is similar to Bt corn that's toxic to corn borer,” says Monsanto's Bryan Hurley. “But there are some differences. For one, rootworms spend most of their life in the soil, so the development of refuges (to delay rootworm resistance to Bt) is different. A half-mile separation doesn't work very well with rootworms.”

Wade French, USDA-ARS research entomologist at Brookings, SD, is studying a novel approach to establishing a refuge of conventional corn to delay rootworm resistance.

Rather than plant a refuge of 20% of the corn acreage in a block half a mile away, French believes an “in-row refuge” works just as well.

By blending 20% conventional seed of the same hybrid with 80% YieldGard Rootworm seed, researchers observed rootworm control as good as that achieved with 100% GM seed. They believe this seed-mix approach may be more effective in slowing down the evolution of resistance to Bt.

“In the field, plants from conventional seeds in the mix serve as a refuge for some rootworms,” says Hurley. “This makes sure there will be rootworm beetles not exposed to Bt that will mate with any rootworms that survive Bt.”

Hurley believes the offspring of these matings will probably not be resistant to Bt.

Monsanto will have a limited supply of YieldGard Rootworm corn for this planting season. Federal Environmental Protection Agency (EPA) approval was given in late February. At press time, it had received state registrations in 12 major Corn Belt states.

“Seed production will be limited for 2003,” Hurley warns. “We'll start with areas where rootworm is worst — in the western Corn Belt, for instance. We will have enough Bt seed to plant about a million acres this year.”

Monsanto will license hybrid companies to produce the seed on a royalty basis. And for the next few years, researchers will study the effectiveness of the “in-row refuge” concept.

More from Bryan Hurley of Monsanto:
The USDA research program investigating seed mixes as a potential IRM strategy for YieldGard Rootworm is basic research only. YieldGard Rootworm recently received commercial registration from the U.S. Environmental Protection Agency (EPA), allowing for the commercial planting this year. The EPA registration specifically requires the planting of a 20 percent block refuge, similar to other Bt corn products for corn borer control already on the market. The EPA registration does not allow for the use of seed mix as an appropriate refuge strategy.
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