Entomologists tracking the unusual appearance of corn rootworm in first-year corn are finding the rootworm is laying eggs in some soybean fields. Researchers at the University of Illinois at Urbana-Champaign are conducting field, laboratory and greenhouse studies to understand the problem, a first-step toward developing management strategies.
Illinois corn growers prophylactically treat 88 percent of 2.8 million acres of continuous corn with soil insecticides each spring, at a price tag of approximately $27 million. In contrast, the percentage of first-year corn acres treated annually with soil insecticides has decreased from 65 percent in 1978 to 33 percent in 1990, largely due to research-based Extension education programs that showed producers how a corn-soybean crop rotation could prevent some pest problems. If crop rotation fails as a management tool for corn rootworms, the economic impact is expected to exceed $100,000,000 for Illinois corn producers.
The game is afoot–

Researchers investigate corn rootworm mystery 		Many corn growers rely on rotating to a soybean crop every other growing season to keep certain pests from developing in cornfields. The strategy is effective and in terms of pesticide treatment savings, it has economic and environmental benefits. The crop rotation strategy provides enough of an interruption in the corn rootworm life cycle that use of soil insecticides has dropped significantly on corn following soybeans. About 10 years ago, however, a seed production cornfield in Piper City sustained severe corn rootworm injury, despite use of rotation. Entomologists at the University of Illinois Urbana-Champaign were intrigued and began to investigate. The problem has since occurred in nine commercial cornfields in east-central Illinois counties.
"We're definitely seeing a behavior change in western corn rootworms. We're finding them in soybean fields through the summer and we're finding their eggs in some of those fields in October," said Eli Levine, who is collaborating with Extension entomologist Michael Gray to investigate first-year corn rootworm problems in Illinois.
The team must act quickly each summer to catch the corn rootworm beetle in the field. It is active from July to Sep-tember. Levine located and monitored 12 sites this year, collecting corn rootworm beetles in baited, medicine-vial shaped traps for further study. Gray also collects field data, with a focus on finding what, if any, levels of infestation cause enough damage to warrant treatment in soybean and subsequent cornfields.
"The numbers of corn rootworm beetles in some sample soybean fields were much higher than we expected, in some cases higher than bean leaf beetles," Levine said. "Even so, we think treating soybean fields with pesticides to control corn rootworm is unwarranted at this time. We have no idea yet of acceptable injury levels to soybeans or what numbers might cause damage to corn the following year."
Pests have particular preferences for where to lay eggs or feed or carry out any number of other day-to-day functions, and those preferences may provide the clues researchers need to figure out how to manage this new aspect of an old problem. Levine and Gray used some of the C-FAR funds to bring Joe Spencer onto the project and to provide him with specialized cameras, computer software and other equipment to study egg laying, flight and other insect behavior. Spencer likens his work to that of a detective.
"What are their movements? Why are they drawn to soybean fields? What do they do there? If we can identify some key characteristics and features of behavior, we might have a diagnostic tool that can be used to identify what a problem rootworm beetle is -- we're at that basic stage," Spencer said. "Watching what insects do is one way to learn mechanisms behind problems and pick up clues that lead to solutions. If we can identify a key flight behavior, for instance, that might be an intervention point. This is a new problem for this decade and we have very little to go on."
To track movement, Spencer marks field-collected rootworm beetles with florescent powder and observes their activity and crop preferences in cages and in an enclosed wind tunnel built in a greenhouse. The greenhouse has different varieties of corn and soybeans so that the team can distinguish whether the beetles have preferences in flower color, leaf hairs, maturity or other plant characteristics. Researchers also brought in corn rootworm beetles from Nebraska, where continuous corn is predominant, to compare their behavior to that of rootworm beetles trapped from Illinois corn and soybean fields.
Besides behavior, the team is studying the basic biology and genetics of rootworm beetles. "The beetles prefer corn and rotation may be exerting selection pressure to cause beetles to adapt to soybean," Levine said, "but rotation is such an important pest management strategy that we have to find solutions." Solutions could vary from thresholds that help growers determine whether corn rootworms in soybeans require treatment to new soybean varieties that are unappealing corn rootworm beetles.
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