No. 18 Article 1/July 23, 2004

Reports of Rootworm Larval Injury in First-Year Corn in Western and Northwestern Illinois Continue

We continue to receive reports of rootworm larval injury in first-year corn in western and northwestern counties (Mercer, Warren) of Illinois. In addition, producers are more commonly reporting western corn rootworm adults in their soybean fields in these areas of the state. In the Bulletin last week, I discussed scouting procedures and economic thresholds for the western corn rootworm variant that rely on the use of the yellow sticky trap (Pherocon AM trap). Because of the early emergence of western corn rootworm adults this season, many questions have been asked about the appropriate time to deploy these traps in soybean fields. Recall that our recommendation is to place the traps in soybean fields during the last week of July. Traps should be changed weekly, and the monitoring should continue through the first 3 weeks of August.

Because western corn rootworm adults appeared in soybean fields and cornfields earlier this summer, it seems reasonable to assume that egg laying also may happen a bit earlier. Doctoral research conducted by Chris Pierce, former graduate student in the Department of Crop Sciences, sheds some light on the progression of egg laying in corn and soybean by western corn rootworm adults. Over a 3-year period (1999-2001), Chris sampled three producers' fields (corn and soybean) in Iroquois County. In general, he determined that egg laying occurs later in soybean than corn. For instance, in 2001, cumulative egg-laying percentages for corn and soybean (side by side) were as follows:

Because of the apparent expansion of the range of the variant western corn rootworm in Illinois in 2004, we intend to conduct a survey of root damage in selected western and northwestern counties within the next 3 weeks. We will extract five root systems from 10 randomly selected fields of corn planted after soybean in each county surveyed. The roots will be returned to Champaign-Urbana for washing and rating. We will share the results from this survey in a future issue of the Bulletin. The results should provide some insight on which we can base recommendations for management of the variant western corn rootworm in western and northwestern Illinois in 2005.--Mike Gray

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