Soybean Variant Poses Expanding Risk to Corn Growers

Leading entomologists advise farmers planting first-year corn in parts of the Midwest to be on the alert. The western corn rootworm soybean variant, they say, has continued to spread through portions of the central Corn Belt – including Iowa – posing a threat of significant economic damage.

Beginning in the early 1990s, growers and entomologists began observing that the western corn rootworm was adapting its reproductive practices and laying its eggs in soybean fields in order to survive field rotations between corn and soybeans. The eggs hatch during the spring, after the field has been rotated to corn, resulting in larvae feeding that can cause downed corn or reduced yields at the end of the growing season. Visible symptoms are most noticeable in mid-July and most severe in hot, dry conditions.

First sighted in east central Illinois, the "soybean variant" has continued to gradually spread in recent years throughout northern Illinois, as well as parts of Indiana, Michigan, Ohio and Wisconsin, according to entomologists. More recently, the western corn rootworm variant has likely been sighted in Eastern Iowa, in Muscatine County, according to Iowa State University Extension entomologist, Jon Tollefson.

John Obermeyer, extension entomologist at Purdue University, advises growers to be aware of whether they are in high-risk areas for the soybean variant, be observant of damage in unconfirmed areas and monitor for the western corn rootworm in their soybean fields. "Producers in high-risk areas are going to need root protection for their first-year corn," he says. "In fringe areas, growers should monitor their soybean fields for rootworm beetles. If they feel the threat is great enough, root protection is highly recommended."

Dr. Matthew O'Neal, assistant professor in the entomology department at Iowa State University concurred, noting that the use of yellow sticky traps (specifically the unbaited Pherocon AM yellow sticky traps) can be a good predictor of soybean variant presence. "Growers can also obtain information from their local extension or university entomologists who track beetle counts," he said. "Growers have traditionally used a corn-soybean rotation as a way to control corn rootworm. When that doesn't work, because of the soybean variant, it can catch growers by surprise."

Similarly, another species of corn rootworm – the northern corn rootworm – has developed a variant with an extended diapause, an adaptation to a corn-soybean rotation by delaying its egg development. This species lay their eggs in cornfields, but these eggs take two years to hatch, after the fields have been rotated from corn to soybeans and back to corn. This phenomenon has been observed in parts of northwest Iowa, southwest Minnesota, northeast Nebraska and southeast South Dakota.

Management Options

Entomologists suggest farmers facing the likelihood of economic damage can consider several management options, including Bt corn, soil applied insecticides and seed treatments. Obermeyer said the western and northern corn rootworm, as the primary corn pests, have a significant impact across the Corn Belt:

• The yield loss and controls costs from rootworm feeding damage is estimated at $1 billion annually.

• An estimated 32 million corn acres across the United States are at risk for corn rootworm damage.

• 18 percent of the U.S. corn crop, or about 15 million acres, are treated for corn rootworm annually.

In university trials, Dr. Todd DeGooyer, an entomologist who serves as U.S. corn technology development manager for Monsanto Company, said YieldGard Rootworm Corn alongside corn rootworm rate seed treatments, granular soil insecticides and liquid soil insecticides showed YieldGard Rootworm Corn's in-plant protection consistently provided equal or better root protection against rootworm larval feeding.

DeGooyer added that a large-scale field test involving nearly 2,000 growers across the Corn Belt in 2004 showed that YieldGard Plus– which combines the in-plant protection of the YieldGard Corn Rootworm and YieldGard Corn Borer traits in one seed – outperformed comparison hybrids nearly 70 percent of the time. YieldGard Plus yielded 9 to 12 bushels per acre more than comparison hybrids under heavy rootworm pressure, 5 to 8 bushels better under medium pressure and 2 to 4 bushels more under light pressure, he reported.

While no one can predict with certainly how insect pressure will vary from year to year in any given area, experts say growers need to be aware of the toll rootworm can take on their corn crop, understand what to look for and how to plan for protecting their crop in 2005.

Growers of YieldGard Rootworm and YieldGard Plus must always follow grain marketing and insect resistance management requirements. Toll-free information on grain marketing options for participating elevators is available through the 1-866-SELLCORN hotline (1-866-735-5267) or by logging on to 866sellcorn.com.