Crop Rotation Failing as a Management Strategy for Western Corn Rootworms Across an Increasing Number of Illinois Counties

October 4, 1999
Results from on-farm research trials this past season confirm that crop rotation no longer provides adequate control of the new western corn rootworm strain in an increasing number of Illinois counties. In August we evaluated roots from untreated check strips (no soil insecticide used) for corn rootworm larval injury by using the Iowa State 1 to 6 scale. A total of 65 producers' fields were included in our research project, and in most fields, 10 roots were evaluated from each of four check strips (40 roots per field). In addition to root injury ratings for 1999, we have beetle density information from Pherocon AM traps used in soybean fields the preceding season. We are continuing to evaluate these data and hope to report later this fall or winter whether or not our threshold needs to be refined.

Root-rating data are presented in Table 2. One observation should become quickly apparent: we don't have equal numbers of fields from the given counties. Why? Cooperation in this study was on a volunteer basis, and most of our participating farmers were from DeWitt, Livingston, and McLean counties. So, for the counties in east-central Illinois that are not represented in Table 2, don't assume that the problem with western corn rootworms has disappeared. This is not the case. Also, for some counties with only a single or a few observations, be careful with your interpretation. For instance, the single cooperator in Vermilion County had an average root rating in the check strips of 1.45 (very little injury). However, we know that Vermilion County has had plenty of first-year corn rootworm problems. We believe that the root ratings in Table 2 improve our understanding of how this new western corn rootworm strain is spreading in Illinois. These data should not be used as a basis for your first-year corn soil insecticide decision for 2000. Soil insecticide decisions for next season should be based on counts of western corn rootworm adults in your soybean fields during 1999.

"Take-home" messages from these data include the following points.

· First-year corn rootworm root injury occurred during 1999 in some northern Illinois counties such as DeKalb, Kane, Kendall, Lee, and Ogle. Problems with the new western corn rootworm strain are not confined to east-central Illinois.

· First-year corn rootworm larval injury is highly variable from field to field within a given county. This reinforces our recommendation to monitor each soybean field with Pherocon AM traps in order to make sound soil insecticide decisions for first-year cornfields.

One final note on the faster-than-expected spread of this new western corn rootworm strain: entomologists at Iowa State have reported that they suspect western corn rootworms are responsible for some first-year corn injury in northeastern Iowa. Emergence cages placed within a first-year cornfield (soybeans in 1998) this season revealed the presence of western corn rootworm adults. This is not welcome news for producers in this area who have relied on crop rotation as a prime management strategy for corn rootworms. This finding also supports our contention that this strain of western corn rootworm is spreading more quickly to the northwest than previous computer models suggested. It seems certain that this problem will continue to escalate. We'll provide more updates on this topic during the upcoming fall and winter.--Mike Gray, Kevin Steffey

Author: Kevin Steffey Mike Gray