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Western Corn Rootworm Densities Reach Alarming Levels in Rural and Urban Areas

August 10, 2001
That's right, urban areas. Russel Higgins, IPM educator, Matteson Extension Center, received a report from a pest control firm in Chicago that indicated corn rootworm adults were being collected on the 49th and 50th floors of the Prudential Building. The individual that spoke with Russ Higgins also noted that the numbers of corn rootworm adults exceeded any he's observed in 13 years. We've received other reports that indicate western corn rootworm adults are being washed up on the beaches of Lake Michigan at bothersome levels (at least to beach dwellers). This phenomenon has been observed periodically over the past several decades. This event is seemingly caused when large densities of rootworm adults migrate from cornfields and are subsequently caught in downdrafts triggered by the development of certain atmospheric conditions. Additional observations passed along to us from suburban residents indicate that "hordes" of western corn rootworm adults have devastated cucurbit and sweet corn crops produced within family gardens. These observations parallel the very impressive levels of corn rootworm injury reported by producers in many areas of central, east-central, and northern Illinois.

The intensity of corn rootworm injury experienced by many producers also was reflected in our soil insecticide trials this year. Root ratings are provided in Table 1 for our insecticide experiments that were located at DeKalb, Monmouth, and Urbana. Root-injury ratings exceeded 5.0 (two nodes of roots completely destroyed) at each of the three sites. These studies offer good insight regarding the performance of products challenged by large densities of corn rootworm larvae. At DeKalb, the driest of the sites, only the band application of Force 3G kept the average root injury below a rating of 3.0 (moderate pruning, but never the equivalent of an entire node). Counter CR, historically one of the most consistent soil insecticides, had average root injury (3.93) that approached a rating of 4.0 (one node of roots destroyed) at DeKalb. Applications of Fortress 5G (furrow, smart box) and Regent 4SC (microtube) failed to keep root injury below a rating of 4.0.

At the Monmouth site, favored by more generous precipitation, soil insecticide performance fared somewhat better, despite very high levels of injury in the control plots (5.43). Several products kept root injury below a rating of 3.0 and included Aztec 2.1G, Counter CR, Lorsban 15G, and Nufos. The insecticides Capture 2EC (band, 0.1 lb a.i./acre) and Fortress 5G (band and furrow applications) failed to keep root injury below a rating of 4.0. Product performance at the Urbana site was generally better, even though the injury rating (5.5) in the control was the greatest of all three experiments. For a more thorough discussion of the root-rating results in Urbana, refer to issue no. 18 of the Bulletin.

Consistency ratings for the soil insecticides for each site are provided in Table 2. We measure consistency as the percentage of roots with a root-injury rating less than 4.0 (one node of roots destroyed). Plants that have at least one full node of roots pruned are more susceptible to lodging. If plants lodge, yield losses can be more substantial. So, consistency rankings may be viewed along with actual root ratings to get an overall impression of product performance. Products can vary considerably in their consistency from year to year and from location to location (within a year). An excellent example of this point is Counter CR in 2001. The consistency of this product varied from 33% (DeKalb) to 100% (Urbana) despite the fact that larval pressure was comparable at each site. For 2001, Aztec 2.1G and Force 3G offered the most consistent root protection across the three experimental sites. For many of the other products, the odds of finding root injury that equaled a root rating of 4.0 were no better than 50:50, worse for several insecticides. These data point out the fact that achieving a high level of consistent root protection is not a sure bet for any product in any given year.--Mike Gray

Author: Mike Gray

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

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