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Western Corn Rootworm: Rotated Corn
Remains At Risk

Michael E. Gray Michael E. Gray
Professor and Extension IPM Coordinator
Department of Crop Sciences
(217) 333-6652
m-gray4@illinois.edu
  Susan T. Ratcliffe Susan T. Ratcliffe
Extension Entomology Specialist
Department of Crop Sciences
(217) 333-6652
sratclif@illinois.edu
  Christopher M.F. Pierce Christopher M.F. Pierce
Graduate Research Assistant
Department of Entomology
(217) 244-2687
cmpierce@illinois.edu

The remarkable adaptation by western corn rootworms (WCR) to the rotation of corn and soybeans continues to have a significant influence on the economics of management practices for this key insect pest in the eastern Corn Belt. Following the widespread and intense damage to rotated corn caused by WCR larvae in 1995, producers in east-central Illinois have been unable to use crop rotation as a reliable management tactic for WCRs. As a consequence, the use of a planting-time soil insecticide has escalated on rotated corn acres.

In Illinois alone, at least 16 counties are at a moderate to high risk of WCR larval injury in rotated and continuous corn acres. These counties are primarily located in the east-central region of Illinois, with million corn acres affected. This story of insect evolution comes at a steep cost to Illinois producers in these affected counties, who spend $43 million in insecticide costs alone to battle this elusive pest. Producers in the nearby states of Indiana, Michigan, and Ohio also are facing these new crop protection costs each spring.

Seasonal WCR oviposition 1999
Figure 1. Seasonal WCR oviposition in grower corn and soybean fields in Iroquois County,Illinois,1999.

The spread of this new WCR strain is expected to continue. Producers and other agribusiness professionals can report WCR trap capture data directly to University of Illinois Extension via the Internet at http://ipm.uiuc.edu/agriculture/corn/wcrscout/wcrscout.html. Observers are encouraged to monitor WCR adults in soybean fields with Pherocon® AM traps. Although we recommend that producers evenly distribute 12 traps within their soybean fields, many elect to use fewer, especially in counties still outside the "problem area" of east-central Illinois. For specific information on recommended scouting procedures using Pherocon AM traps, please refer to the revised Western Corn Rootworm Insect Information Sheet.

Average densities of five WCR adults captured per trap per day in soybean fields may be required before root injury the following season in rotated cornfields (left untreated, no soil insecticide) approaches economic levels. Pherocon AM traps and the suggested threshold should be used only to predict root injury and not to predict economic losses. In addition, the traps and thresholds should not be used to trigger applications of insecticides to soybean fields to prevent egg laying. A considerable range of WCR adult densities can occur among soybean fields within the same county. This variation can be observed more closely by examining trap capture data for individual fields in each county sampled (online at http://www.ipm.uiuc.edu/publications/rootworm-2000/index.htm). When you view this data set, you will appreciate the importance of monitoring each soybean field with Pherocon AM traps.

Seasonal WCR oviposition 2000
Figure 2. Seasonal WCR oviposition in grower corn and soybean fields in Iroquois County,Illinois,2000.

In addition to the intense selection pressure that the rigor of crop rotation has placed on WCR populations in east-central Illinois, we suspect that exaggerated differences in the developmental stages of corn and soybeans also may contribute to egg laying by WCRs in crops other than corn. While it is clear that WCRs readily lay eggs in soybeans, it is equally certain that corn remains a competitive sink for eggs as well.

Research conducted in producers' fields in Iroquois County in 1999 and 2000 documents the course of egg laying in corn and soybeans throughout both summers. Initially, WCRs primarily lay eggs in corn; however, after corn reaches the blister stage of development, WCR egg laying intensifies in soybean fields. It also appears that later-planted corn competes well with soybeans for egg-laying WCR females.

A harvested cornfield
Figure 3. A harvested cornfield in
Iroquois County prior to Labor Day (1999).

In recent years, it has become increasingly common to observe harvested fields of corn prior to the Labor Day holiday (Figure 3). WCR females left with the choice of laying their eggs in senescing corn or in nearby green and succulent soybean fields opt for the latter. We will continue to evaluate the role of crop phenology on WCR egg laying at an 80-acre experiment (Figure 4) on the University of Illinois Research and Education Center south of Urbana and also on a 15-acre site at the Northwestern Illinois Agricultural Research and Demonstration Center near Monmouth.

An aerial photograph of the Research and Education Center experiment
Figure 4. An aerial photograph of the Research and Education Center experiment (2000).

We acknowledge the support and funding for this research by the Illinois Council on Food and Agricultural Research (C-FAR) Sentinel Research Program.

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