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Western Corn Rootworm Egg Laying and Dispersal
In Corn and Soybean Fields: Implications For
Transgenic Management

Michael E. Gray Michael E. Gray
Professor and Associate Head
Department of Crop Sciences
(217) 333-6652; m-gray4@illinois.edu
Joseph L. Spencer
  Joseph L. Spencer
Assistant Professional Scientist
Center for Economic Entomology, INHS
(217) 244-6851; spencer1@illinois.edu
Christopher M.F. Pierce  

Christopher M.F. Pierce
Graduate Research Assistant
Department of Entomology
(217) 244-2687; cmpierce@illinois.edu

Since the mid 1990s, a variant of the western corn rootworm has compromised the utility of crop rotation as an important pest management tactic for this perennial pest of corn. The variant plagues producers throughout east-central Illinois, and they have responded by significantly increasing their use of soil insecticides on rotated corn acres. In recent years, first-year corn rootworm injury also has been reported by some growers in more northern Illinois counties such as Kane, Kendall, and LaSalle. In addition, reports of first-year corn rootworm larval damage also are common in other states such as Indiana and Michigan. To date, pest management recommendations have primarily relied upon the use of Pherocon AM traps (Figure 1) in soybean fields.

Pherocon AM trap in soybean field

Figure 1. Pherocon AM trap in soybean field.

Farmers have been encouraged to deploy 12 traps in soybean fields for a four-week period beginning in late July. If western corn rootworm adult captures exceed five adults per trap per day, farmers are encouraged to consider the use of a soil insecticide during planting the following spring. Because egg laying in soybeans takes place over such a protracted period of time, we have discouraged farmers from applying a broadcast treatment to soybeans to prevent egg laying. We believe this practice would be futile and not cost effective. Our research conducted from 1999 to 2001 indicates that both corn and soybean fields serve as suitable egg-laying sites for the western corn rootworm variant (see the table below). A common misconception is that western corn rootworms have abandoned corn in east-central Illinois as an ovipositional site. This is far from true. In fact, late-planted corn and soybean fields both appear to compete for western corn rootworm eggs. Although soil insecticides continue to play a key role in the prevention of economic root damage in non-rotated as well as rotated cornfields, farmers are increasingly interested in the potential use of transgenic hybrids for corn rootworms.

Grower Corn Soybeans
Planting Date Final Egg
Planting Date Final Egg
  Harlan Ziebart 04-05-99 1.9 05-27-99 3.8
Jay Carlson 04-05-99 1.8 04-19-99 2.9
Robert Leitz 05-07-99 2.6 05-18-99 4.1
  Harlan Ziebart 04-29-00 4.0 05-08-00 5.5
Jay Carlson 04-04-00 2.1 05-13-00 5.6
Robert Leitz 04-24-00 3.1 05-24-00 5.2
  Harlan Ziebart 04-20-01 1.7 05-16-01 3.4
Jay Carlson 04-16-01 3.8 05-03-01 5.8
Robert Leitz 04-20-01 3.1 05-01-01 4.0
Final western corn rootworm egg density (x 106 eggs)/hectare

Final western corn rootworm egg densities, Iroquois County, 1999-2001

In late May of 2002, a technical committee (NCR-46) of research and extension entomologists, along with selected cooperators, sent a letter to the U.S. Environmental Protection Agency offering their support for a conditional registration of a transgenic event (MON 863) that could serve as one of several management tactics for corn rootworms. Transgenic corn that expresses Cry3Bb has been evaluated for its ability to protect roots against larval feeding for several years by Monsanto Company scientists and entomologists within several Land Grant institutions, including the University of Illinois. If this transgenic event is approved by the EPA, significant reductions in soil insecticide use could be achieved. Rapid acceptance of this technology is anticipated, especially by producers in the western Corn Belt. If European export issues surrounding the use of transgenic hybrids can be resolved satisfactorily, rapid adoption of corn rootworm transgenic hybrids is expected in the eastern Corn Belt as well. The NCR-46 committee acknowledged that additional data are needed to fully develop long-term resistance management plans. Therefore, the committee recommended an interim registration for MON 863 so that additional data could be generated to allow for the development of a “more robust long-term IRM plan.” For now, and during an interim registration period, the NCR-46 committee is supportive of a proposed 20 percent refuge that must be placed within or adjacent to the transgenic field. During the next several years, entomologists will continue to collect important data that can be used to improve resistance management plans. Additional data are needed concerning beetle movement, emergence patterns, and fitness costs associated with emergence from transgenic plants.

In 2001, we evaluated the rate of western corn rootworm movement away from transgenic cornfields by detecting the presence of transgenic plant tissue in the bodies of beetles captured at points increasingly distant from a transgenic field in which they had fed previously. We determined that 85.3 percent of male and female beetles moved fewer than eight rows per day through R2 and R3 stages of corn (Figure 2). Among western corn rootworms that left transgenic corn and were captured in an adjacent soybean field, 86.4 percent of males and 93.1 percent of females moved fewer than 12 rows per day through soybeans. These results suggest that large and isolated blocks serving as refuges may not be appropriate as part of a plan designed to prevent the development of resistance to transgenic hybrids by western corn rootworms.

WCR adults move short distances each day within and between corn and soybean fields

Figure 2. WCR adults move short distances each day within and between corn and soybean fields.

Though the vast majority of western corn rootworm flights take place within and between adjacent fields, some mated female western corn rootworms leave cornfields and fly long distances. By monitoring flight activity 10 meters above a soybean field, we discovered that large numbers of females leave cornfields and ascend into air currents that likely carry them long distances downwind. During our observations, beetle activity was most intense during the hour preceding sunset and ranged from 2.3 to 4.6 million western corn rootworm adults flying through the lower 10 meters of atmosphere above a typical 100-acre soybean field (Figure 3). This volume of beetles equates to the mass of a modest-sized flock of geese! Because of these impressive dispersal characteristics following mating, development and subsequent adoption of effective resistance management plans by farmers will be necessary for the long-term use of transgenic hybrids for corn rootworms.

During peak flight, WCR adults

Figure 3. During peak flight, 17 WCR adults per minute may pass through each
2-meter-thick layer of air above any point in a soybean field.

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