The new variant, which some scientists are simply calling the "eastern phenotype," was found in nine counties in northeastern Illinois and 16 counties in northwestern Indiana in a 1996 field survey.
The pest has been identified as a new genetic variant by Jeff Stuart and Rich Edwards, professors of entomology; graduate student Amy Sammons; and other researchers at Purdue. The finding will be published in the October issue of the scientific journal Environmental Entomology.
Larry Bledsoe, Cooperative Extension Service entomologist at Purdue, says the western corn rootworm is the No. 1 pest of corn in terms of dollars spent on control methods. "Economically this is the top crop pest in the Midwest," Bledsoe says. "However, because the damage occurs early in the season, we really don't know what effect this has on yield."
The problem of the adult rootworm beetles being found in soybean fields was first identified by Eli Levine of the Illinois Natural History Survey. However, it was first thought that farming practices, not genetics, were to blame for the pest's change in habits.
Since the late 1980s, entomologists were divided over whether there was a new -- and threatening -- variant of the western corn rootworm. Some entomologists thought the pest in the soybean fields was simply attracted to corn residue left in the field from the previous crop. The residue was caused by soil-conservation farming practices, which became popular about the same time. Others thought there was evidence of a new genetic variant.
"We couldn't easily test the hypothesis that this is a genetic trait, so we first tried to test the possibility that the corn rootworms were moving to soybeans in their adult stages," Stuart says.
By using a series of experiments called bioassays, the researchers were able to eliminate environmental "cues" that might have attracted the western corn rootworm to the soybean fields. These results for the Indiana beetles were compared with tests run on beetles from Iowa and Nebraska.
"There was a clear difference between the beetles from the affected areas in Indiana and those from Iowa and Nebraska. Not nearly as many of the beetles from Iowa and Nebraska were attracted to soybeans," Stuart says. "Actually I was a little surprised. I didn't expect to find quite the amount of difference that was there.
"This leads us to believe that a genetic change has taken place."
Sammons, the lead investigator on this research project, says follow-up tests will provide more information about this new variant. "If we can find out more about what's happening, we may be able to develop IPM methods to manage the variant," she says. (Integrated pest management, or IPM, is an environmentally friendly method to control crop pests.)
Because the western corn rootworm historically moves east, carried along on the prevailing winds, Stuart expects the problem to spread. "I don't see anything that will prevent the problem from becoming worse," he says.
As their name says, the corn rootworms eat the roots of the corn plants in the springtime. According to Bledsoe, sometimes this causes the plants to fall over completely; other times the plants tilt and "gooseneck." Farmers apply insecticides in the spring to prevent this damage to their plants. "Corn rootworm damage diminishes after mid-July," Bledsoe says. "Environmental conditions from then until harvest have the greatest impact on yield."
Bledsoe says that more insecticide was sold this year in Indiana because of concerns about this new variant, although some of that concern may have been misplaced: "It's important to recognize that the eastern phenotype has only been found in a small area in Illinois and Indiana," he says. "Some people in the agrichemical industry have been telling farmers in the southern parts of those states that they now need to use the insecticide, even though they haven't had any rootworm problems. That's just not the case."
As for soybeans, too little is known about what this new variant is doing, Bledsoe says. "We're looking into that. We plan to measure the effect of the beetles on the soybean pods, but at this point we don't know if they are causing any significant damage."
Sources: Jeff Stuart, (765) 494-4561; e-mail, email@example.com
Amy Sammons, (765) 494-4554; firstname.lastname@example.org
Larry Bledsoe, (765) 494-8324; e-mail, email@example.com
Writer: Steve Tally, (765) 494-9809; e-mail, firstname.lastname@example.org
Purdue News Service: (765) 494-2096; e-mail, email@example.com
Economic injury by western corn rootworm, Diabrotica virgifera virgifera LeConte, larvae to corn grown in rotation with soybean has been increasing in areas of northwestern Indiana and east-central Illinois. To ascertain whether behavioral changes in adults are resulting in larval damage to corn in these areas, the attractiveness of soybean, corn, and associated cropping environments to western corn rootworm beetles from Indiana, Iowa, and Nebraska was compared using laboratory assays. Beetles from Indiana preferred soybean without crop residue and pollinated corn more than water wicks, corn residue, pollinating corn, or soybean with corn residue in a six-choice host preference assay. Beetles from Nebraska favored pollinating and pollinated corn. Western corn rootworm beetles from Indiana selected soybean without residue more than beetles from either Iowa or Nebraska. Beetles from Indiana consumed a greater amount of soybean leaf area than did beetles from Iowa or Nebraska in a no-choice feeding assay. A western corn rootworm variant was identified in a portion of the population in northwestern Indiana. This variant preferred soybean environments over corn environments. Risk of economic injury to corn following soybean may be greater where this variant is prevalent.
Graduate student Amy Sammons, left, and Purdue entomology Professor Jeff Stuart stand in a field of corn damaged by the western corn rootworm. The weakened roots -- shown on the plant held by Stuart -- can cause the plants to fall over during summer storms. Researchers at Purdue have determined for the first time that a new genetic variant of the No. 1 pest of corn actually prefers soybeans over corn for laying its eggs. That is bad news for farmers in the Eastern Corn Belt, who use corn-soybean crop rotation to control the pest. (Purdue Agricultural Communications photo by Dave King.)
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