No. 12 Article 2/June 11, 2004

Corn Rootworm Larval Injury Early and Impressive in Some Locations

We continue to receive reports of corn rootworm larval injury reaching impressive levels at this early juncture of the growing season. On June 7, Mike Hellmer, Pioneer Hi-Bred International, Inc., observed significant levels of root pruning and numerous larvae (5 to 6 per plant) in a Champaign County cornfield. The field in question had received a soil insecticide application at planting. Recall that planting occurred under very windy conditions this spring across Illinois. Many growers continue to wonder whether the level of control offered by granular soil insecticides might have been compromised. For a more complete discussion of potential effects of wind on soil insecticide efficacy, please refer to an earlier article on this topic in issue number 7 of the Bulletin (May 7, 2004). \

Corn rootworm larval injury (Champaign County). Photo courtesy of Mike Hellmer, Pioneer Hi-Bred International, Inc.

Shawn Jones, Pioneer Hi-Bred International, Inc., also has observed heavy corn rootworm larval pressure in Macon County (June 7-8). Several of the fields with root pruning received a soil insecticide treatment at planting. The majority of plants that Shawn evaluated had 3 to 8 larvae per root system; however, one root system had 16. Kevin Black, Growmark, Inc., also indicates that corn rootworm larvae (2nd and 3rd instars) are numerous in some first-year cornfields located north of Springfield in Menard County. All of these reports signal that this year may present some challenges with respect to significant corn rootworm larval injury. In the coming weeks, as plants increase in height and the root pruning increases, we anticipate many reports of lodging. Lodging may become especially acute if we have severe storms characterized by heavy rains and high winds. A hot and dry July would exacerbate yield losses if this scenario unfolds.

Corn rootworm larval injury (Macon County). Photo courtesy of Shawn Jones, Pioneer Hi-Bred International, Inc.

Based upon our 2002 and 2003 on-farm surveys of root injury in first-year cornfields and sweep net counts of western corn rootworm adults in soybean fields, we anticipate an increase in reports of larval injury in northwestern Illinois this season. In 2002 and 2003, we surveyed 20 and 50 rotated cornfields, respectively, that were scattered across northwestern Illinois. The percentage of plants with root pruning increased from 7% (2002) to 24% (2003). Joe Spencer, an entomologist with the Illinois Natural History Survey, found western corn rootworm adults in soybean fields across most northwestern Illinois counties last year in a sweep net survey he conducted from July 25 to August 4. Although the number of western corn rootworm adults Joe found in these northwestern Illinois soybean fields was low compared with east-central Illinois, the densities have increased in recent years. If you encounter significant (root pruning) corn rootworm larval injury in first-year corn in western or northwestern Illinois this season, please let us know. This will be further evidence that the western corn rootworm variant is continuing to spread across Illinois and may soon present some significant management headaches for producers in our neighboring states to the west.--Mike Gray

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