Corn+Soybean Digest

ZAP Rootworms The Natural Way

Corn hybrids with non-GMO resistance... Transgenic insect resistance is the sexy crop technology now, and resources often go where the glamour is.

But a trio of researchers is breeding corn lines with native-plant resistance to western corn rootworms, and early results look good. The task hasn't been easy or quick for Bruce Hibbard, research entomologist; Larry Darrah, research leader; and Mike McMullen, research geneticist, all with USDA-ARS at the University of Missouri.

"The first big challenge is to find corn lines with strong resistance," says Hibbard. "We have identified several sources of materials that are significantly more resistant than anything available before."

The timing couldn't be better. Corn rootworms cost U.S. growers $1 billion annually in yield losses and insecticide expenses. Once limited to fields with corn planted after corn, the pests now infest first-year stands. Rootworms are developing resistance to insecticides in Nebraska, and are laying eggs in soybeans in the eastern Corn Belt.

"We aren't necessarily trying to eradicate corn rootworms completely, but materials we have developed have held rootworm damage below the economic threshold," says Hibbard.

Two of the lines Hibbard has developed (dubbed 98-99PR:351 and 98-99PR:352) show considerable resistance to corn rootworm larvae.

"We haven't made a formal release of this germplasm yet, but we have made seed available to several companies that have made requests," he says.

One of them is OMG (Original Maize Genetics), a firm recently established by Bob Hughes, a veteran seedsman at Rochelle, IL. Hughes is winding up his second year of evaluating the materials in Illinois, Iowa and Wisconsin.

"The Missouri material exhibits good rootworm resistance," says Hughes. "In fact, it's very close to insecticide-treated corn, as far as rootworm damage is concerned. I have battled rootworms since the 1960s, and I've never seen anything close to this. The 98-99PR:351 line is outstanding."

"We have another germplasm that is as good as or better than those earlier lines," says Hibbard.

"When Bruce (Hibbard) says he has a material with resistance as good as he can get, we take over and make backcrosses to refine the agronomic characteristics," says Darrah.

Hibbard uses a 1-to-6 numerical scale to indicate the level of rootworm damage to corn roots:

1) No damage, or only a few minor feeding scars.

2) Feeding scars, but no roots eaten off to within 1 11/42".

3) Several roots eaten off to within 1 11/42" of the plant, but no entire root nodes destroyed.

4) One node of roots destroyed.

5) Two nodes of roots completely destroyed.

6) Three or more nodes of roots destroyed.

In his evaluation of Hibbard's material, Hughes found 98-99PR:351 to have a damage rating of 3, while 98-99PR:352 ranked 2.2 on the damage scale. Damage below 3 is usually considered not enough to cause economic losses.

"We are evaluating transgenic sources of resistance, too," says Hibbard. "And these show somewhat better rootworm resistance than our native-plant resistant lines."

The first hybrids with transgenic rootworm resistance may be on the market as early as 2001. However, grain from these hybrids may hit the same marketing snags faced by other so-called GMO (genetically modified organism) commodities.

"Also, the transgenic resistance being bred into hybrids depends on only one protein," says Darrah. "That lends itself to the rootworm overcoming this organism fairly readily."

In the meantime, the researchers will continue trying to make good rootworm resistance even better.

"We still have hundreds of lines from dozens of sources," says Hibbard. "Progress will continue to be slow and incremental, but it will continue."

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