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Western Corn Rootworms in Soybean - An Update

Joseph L. Spencer Joseph L. Spencer
Asst. Professional Scientist, IL Natural History Survey
Adjunct Asst. Prof., Dept. of Crop Sciences
(217) 244-6851; spencer1@illinois.edu
Timothy R. Mabry
  Timothy R. Mabry
Graduate Research Assistant
Department of Crop Sciences
(217) 244-9535; tmabry@illinois.edu

Rotation-resistant western corn rootworms (WCR) threaten rotated corn production across parts of Illinois, Indiana, Michigan, and Ohio. Egg-laying in soybeans and other crops rotated with corn allows the WCR to circumvent what was our most effective and environmentally benign management tactic. Ironically, widespread adoption of crop rotation to control this insect may have shifted the egg-laying behavior of the population. Because larvae from eggs laid in rotated cornfields die of starvation after they emerge in soybeans, the rare female WCR that laid some eggs in soybeans produced many more surviving offspring than females that laid eggs strictly in cornfields. Over the years, beetles expressing this adaptive behavior have become the majority of the population in areas where rotation is used.

Since 1995, we have studied rotation-resistant WCR movement. Migration between corn and soybean fields begins within days of emergence in corn and continues all summer. The beetles have no special attraction to soybeans. Rather, rotation selected for WCR that leave corn, and we find beetles in all crops outside of corn. WCR enter soybean fields during midmorning and return to cornfields over the rest of the day. WCR in soybean fields feed on soybean foliage; laboratory studies indicate that soybean feeders are less vigorous and die sooner than corn-fed WCR.

Western corn rootworms feeding
Figure 1. Western corn rootworms feeding on soybean foliage

Movement between corn and soybeans may help the WCR tolerate soybean feeding, since WCR that eat from both plants are as vigorous as those that only eat corn. Prevailing winds and the passage of summertime storm fronts largely drive the spread of the rotation-resistant WCR. This problem continues to spread to the east-northeast. The greatest potential for expansion appears to be in northern Illinois; expansion into western and southern Illinois remains slow.

By investigating factors that affect their movement and egg-laying, we hope to adapt management practices and reduce potential WCR injury in rotated corn. Recently, we have studied the mechanism behind WCR movement between corn and soybean fields. We've found that WCR adults feeding on soybean plants became more agitated and more likely to fly than do corn-feeding WCR. This suggests a mechanism to explain how WCR flight from soybeans back into cornfields is initiated.

We are also looking for factors affecting flight out of cornfields. Data from traps placed 4-8 meters above soybean fields reveal that most high-flying WCR are females. High-fliers are captured for only a short period during early season; these females may be those most responsible for local and long-distance spread of the problem.

Observation of WCR movement within cornfields provides information important to the design of refuge areas required for rootworm transgenic corn. Adult males actively search for mates and are quick to locate/mate with newly emerged females. Simulation models incorporating this information suggest that WCR refuges may be compatible with European corn borer refuges used with Bt corn varieties.

Our latest information about the rotation-resistant western corn rootworm ecology and behavior improves our ability to:

  1. monitor WCR in soybeans,
  2. target and deploy biotechn/cultural control tactics, and
  3. employ genetic tools to reveal what lies at the root of this problem.

We acknowledge research funding and/or other support from the Illinois Council On Food and Agricultural Research (C-FAR) Sentinel Research Grant Program, the Illinois Soybean Program Operating Board (ISPOB), the University of Illinois Campus Research Board, the University of Illinois Department of Crop Sciences, and the Illinois Natural History Survey.

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