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Western Corn Rootworm Populations in Soybean Fields 2000: Outlook for Larval Injury in Rotated Cornfields for 2001

March 16, 2001
In 2000, populations of western corn rootworm adults were very impressive in many soybean fields, especially those in east-central Illinois (Table 2). If corn rootworm larvae hatch under favorable soil conditions, root injury concerns could be a common story during the 2001 growing season. Corn rootworm larval survival tends to decrease if hatch occurs during wet springs and soils are saturated. In 2000, Susan Ratcliffe, Extension Entomologist in the Department of Crop Sciences, organized western corn rootworm monitoring data from 267 soybean fields in 35 Illinois counties. Producers and other agribusiness professionals reported western corn rootworm trap capture data directly to University of Illinois Extension via the Internet at corn/imr/wcrscout/wcrscout.html. Western corn rootworm adult densities were monitored in producers' soybean fields with Pherocon AM traps. Although we recommend that producers evenly distribute 12 traps within their soybean fields, many elected to use fewer, especially in counties still outside the so-called "problem area" of east-central Illinois. Producers were asked to deploy their sticky traps in late July and continue their monitoring efforts through at least the third week of August. Once each week, new traps were positioned into fields. For more specific information on recommended scouting producers using Pherocon AM traps, please refer to our newly revised Western Corn Rootworm Insect Information Sheet at the following Web site: infosheets/1-wcornr/wcornr.html.

How should the western corn rootworm capture averages (Table 2) be interpreted?

Initial on-farm research results suggested that average densities of two western corn rootworm adults caught per trap per day (Pherocon AM traps) could lead to economic root injury the following season in untreated (no soil insecticide used) first-year cornfields. More recent research indicates that average densities of five adults captured per trap per day in soybean fields may be required before root injury the following season in rotated cornfields (left untreated) approaches economic levels. Economic losses may occur when root injury across a cornfield is equal to an average root rating of 3.0 (moderate root pruning, never equivalent to one node) on the Iowa State 1 to 6 root injury scale. Recently published research indicates that when densities of western corn rootworms in a soybean field reach 10 adults per trap per day, root injury the following season may average 4.0 (one node of roots or the equivalent destroyed) in a first-year cornfield (if left untreated). When cornfields have average root injury that approaches 4.0, lodging and severe yield loss may occur. However, accurately predicting yield losses due to root injury is not easy. For instance, we know that interactions among soil moisture (especially during anthesis and after rootworm larval injury), corn hybrid chosen (root regeneration characteristics), and severity of root injury are complex. So an average root rating of 3.0, or even 4.0 in some years, will not always lead to economic losses. In some very dry and hot summers, certain hybrids that regenerate root tissue very poorly may suffer significant yield losses when average root ratings drop below 3.0. This is an important point: the Pherocon AM traps and the suggested thresholds 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 oviposition (egg laying).

Results outlined in Table 2 suggest that many producers' rotated cornfields are at economic risk if they remain untreated (no soil insecticide) during the 2001 growing season. As you review data within Table 2, remember that only county averages are presented. A considerable range of western corn rootworm adult densities occurred among soybean fields within the same counties. This variation can be observed more closely by looking at trap capture data for each field for each county. Township and section information also are provided for each scouting report in 2000 ( rootworm-2000/index.htm). When you examine this data set for the county of interest, you'll appreciate the importance of monitoring each field with Pherocon AM traps. The bottom line: be cautious with your interpretation of county averages presented in Table 2.--Mike Gray

Counties participating in 2000 western corn rootworm monitoring program.

Western corn rootworm trap capture averages for counties in 2000.

Outlook for western corn rootworm larval injury in 2001.

Author: Mike Gray

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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