Status of Western Corn Rootworm Densities in Soybean Fields in 1999: Outlook for Larval Injury in Rotated Cornfields for 2000

March 17, 2000
The 2000 growing season has the potential to be a banner "rootworm year" for Illinois producers in many counties. In 1999 Dr. Susan Ratcliffe, Extension Specialist in the Department of Crop Sciences, organized western corn rootworm monitoring data from 269 soybean fields in 26 Illinois counties. Producers and other agribusiness professionals reported western corn rootworm trap capture data directly to University of Illinois Extension via the Web at Western corn rootworm adult densities were monitored in producers' soybean fields with Pherocon AM traps. Although we recommend producers evenly distribute 12 traps within their soybean fields, many elected to use fewer, especially in counties still outside the so-called epicenter of the crop-rotation collapse in 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 Western Corn Rootworm Insect Information Sheet at the following web site: If you are interested in monitoring western corn rootworm adults in soybean fields this season, please contact Susan T. Ratcliffe by email ( or phone (217)333-9656. Because of generous funding from the Conservation 2000 Grants Program, Illinois Department of Agriculture, we will be able to provide free traps to anyone interested in serving as a cooperator. The only obligation: we would appreciate receiving information regarding the results of your monitoring efforts.

What's the outlook for economic root injury to rotated cornfields this season?

Results from last season's western corn rootworm adult monitoring program are summarized in Table 2. Initial on-farm research results suggested that densities of two western corn rootworm adults caught per trap per day could lead to economic root injury the following season in untreated (no soil insecticide used) first-year cornfields. More recent research indicates that 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. For our purposes we assume that economic losses may occur when average root injury across a cornfield is equal to a root rating of 3.0 (moderate root pruning) on the Iowa State 1 to 6 root injury scale. 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 will not always lead to economic losses. In some instances, average rootworm larval injury must be near a rating of 4.0 (one node of roots completely destroyed) before yield losses occur. 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. 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 2000 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 within soybean fields within the same counties. For instance, in McLean County, 49 producers' soybean fields were monitored with 12 Pherocon AM traps and overall averaged 2.81 adults caught per trap per day. The range for these 49 fields was 0.07 to 10.8 western corn rootworm adults caught per trap per day. This underscores 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.

Producers at most risk to economic levels of corn rootworm larvae in rotated cornfields for the 2000 growing season include those in the following counties: Champaign, DeWitt, Ford, Grundy, Iroquois, Kendall, LaSalle, Livingston, McLean, Piatt, and Vermilion. (See Figure 1.) This does not mean that producers in other counties are not at risk. In fact, last season there was an impressive spread of western corn rootworm adults into soybean fields outside of east-central Illinois. In 1999 western corn rootworm adults were reported in soybean fields of northwestern counties (Lee and Ogle) as well as in more southern counties such as Coles, Edgar, and Shelby. The westward expansion of this western corn rootworm variant also occurred in a more limited manner into counties such as Logan, Peoria, Sangamon, and Tazewell. This story is far from over, and we will continue to keep you informed of new developments concerning this challenging problem.--Mike Gray

Author: Mike Gray