CHAMPAIGN, Ill. -- Crop rotation is failing as a defense against Western corn rootworms in a growing number of Midwest fields. The beetles have adapted, are spreading and pose a threat to corn crops this year, scientists say.
As the major insect pest of corn, rootworms have changed their behavior, laying their eggs in soybean fields instead of corn. The conclusion is that crop rotation itself has selected for a rootworm strain that circumvents crop rotation. By laying their eggs in soybean fields, the female rootworm beetles ensure the survival of their larvae in the subsequent year because the field will be planted with corn.
"Intense selection pressures have produced rootworm beetles that move out of corn to lay at least some of their eggs," said entomologist Joseph Spencer of the Illinois Natural History Survey and the University of Illinois department of natural resources and environmental sciences. "Rootworm beetles are laying their eggs in soybean, alfalfa and other locations."
The first documented case of damage to corn planted after soybeans occurred in 1993 in East Central Illinois. In 1995, nine counties in East Central Illinois and 13 counties in northwestern Indiana reported damage. Sampling in 1997 showed the problem was spreading rapidly to the north and east. Southern counties in Michigan and portions of northwest Ohio also reported the beetles' presence last year. The problem does not exist or is minimal west of the Illinois River and in the southern third of Illinois.
The logical conclusion is that the problem eventually will spread to the Atlantic coast. "Given what we know about prevailing winds and storm patterns, it's only a matter of time before this rootworm adaption ends up as far east as Delaware," said Scott Isard, a U. of I. geography professor.
Given the amount of first-year corn acreage in Illinois, it would cost farmers $100 million per year to control rootworms chemically if the corn-soybean crop rotation fails to control them.
"For 1998, we have the makings of a big problem in the affected area," said Eli Levine, an entomologist with the Illinois Natural History Survey and the U. of I. department of natural resources and environmental sciences who directs the research project, which is funded by the Illinois Council on Food and Agricultural Research and the Illinois Soybean Program Operating Board.
"Rootworm populations in 1997 were very high," Levine said. "Winter weather has been mild, so survival of the rootworm eggs will likely be large. This could be devastating to fields with a known problem in East Central Illinois and Western Indiana. If farmers experienced rootworm injury to first-year corn following soybeans in 1997, and if they found rootworm beetles in their soybean fields that are to be planted with corn in 1998, they are probably justified in using a soil insecticide in their first-year corn."
U. of I. entomologists in the department of crop sciences have developed a preliminary economic threshold for rootworms in soybeans to help growers decide if they should use a soil insecticide.