Issue No. 23, Article 5/October 7, 2005
Results from the 2005 Survey for Western Corn Rootworm Larval Injury in Corn Planted After Soybeans
Once again, we conducted a survey of western corn rootworm larval injury in fields of corn planted after soybeans. Our primary objective was to determine whether the range of variant western corn rootworms in Illinois had expanded appreciably from 2004 to 2005. Because expansion was relatively significant in Illinois in 2004, especially to counties west of the Illinois River, we focused our survey efforts in 2005 in counties along the Mississippi River and in a two-county band across mid-southern Illinois (Figure 1).
The survey was accomplished with the cooperation of University of Illinois Extension crop systems and IPM educators and staff and students in the Department of Crop Sciences. We extend our thanks to them for helping us conduct this important activity.
In each county, 10 fields of corn planted after soybeans were selected at random, and five roots were extracted from each field. All root systems were returned to Urbana to be washed and rated for rootworm larval injury. We used the Iowa State University 1-to-6 root-rating scale (published in 1971 by T. M. Hills and D. C. Peters) rather than the more recent 0-to-3 node-injury scale (published in 2005 by J. D. Oleson, Y. Park, T. M. Nowatzki, and J. J. Tollefson) for historical continuity (i.e., root ratings from past surveys were based on the 1-to-6 rating scale). The established economic index for this scale is considered to be a rating of 3.0 (i.e., some roots pruned, never the equivalent of one node of roots pruned). The results from our on-farm survey are shown in Table 2.
Average root ratings in the 22 counties surveyed in 2005 (Edwards and Wabash counties were surveyed as one county) were quite low (Table 2), ranging from 1.06 (Marion County) to 2.00 (Pike County). Our data suggest that significant rootworm larval injury was not common in corn planted after soybeans in extreme western and midsouthern counties. However, the percentages of roots that were rated 3 or greater provide a little more insight into the extent of injury that may have been economic. Roots from most counties did not have any pruning injury (0% roots rated 3 or greater, Table 2), but some counties had a double-digit percentage of roots with some pruning, including Lawrence (10%), Pike (21%), and Whiteside (12%). In Lawrence County, one field surveyed had an average root rating of 3.6two roots rated 3, and three roots rated 4.
Our 2005 survey results indicate some expansion of the range of the variant western corn rootworm into southern and southwestern Illinois. Thus, we believe there is some potential for corn rootworm larval injury in 2006 in corn planted after soybeans in almost any Illinois county north of Route 50. However, producers are encouraged not to base their management decisions for 2006 strictly on the results of our survey. The best way to have predicted the potential for rootworm larval injury in corn planted after soybeans in 2006 was to have deployed 12 Pherocon AM yellow sticky traps to capture western corn rootworm adults in soybean fields in 2005. These traps should have been monitored weekly for 4 weeks, at least from the end of July through the third week of August. The protocol for this type of scouting is detailed on this web page. Without information from such a monitoring effort, the decision about whether to use a rootworm-control product in corn in 2006 is primarily guesswork.
We have learned that some people deployed either four or six yellow sticky traps per soybean field in 2005. Although this number of traps can be useful in determining presence or absence of western corn rootworm adults in soybeans, the average numbers of western corn rootworm adults captured on less than 12 traps per field should not be used for making rootworm management decisions in 2006. The thresholds of 5 to 10 beetles per trap per day are based on results from monitoring 12 traps per soybean field over 4 weeks. Average numbers of beetles per trap per day from less than 12 traps per soybean field could lead to inaccurate interpretations.
At the very least, producers in western and southern Illinois need to step up their efforts to monitor for western corn rootworm larval injury in corn planted after soybeans in 2006, with follow-up efforts to monitor for western corn rootworm adults in soybeans later the same season. In this way, producers can verify for themselves the occurrence (or lack thereof) of the variant western corn rootworm that lays eggs in soybean fields.--Kevin Steffey, Mike Gray, Kelly Cook, Ron Estes, Jared Schroeder, and Darren Bakken