No. 17 Article 2/July 15, 2005

Corn Rootworm Larval Injury Nearing Completion in Urbana

On July 12, we began evaluating corn rootworm larval injury in our plots south of Champaign-Urbana. Our first day involved digging (1,800 roots) and evaluating (900 roots) plants for root injury from an experiment led by Martin Bohn, a corn breeder with the Department of Crop Sciences, University of Illinois. For the past three summers, we have worked with Martin in a very interesting series of host plant resistance experiments that are designed to identify antibiosis in corn germplasm. The goals of this research are very challenging. For decades, entomologists and corn breeders have sought to identify genes that would confer resistance to corn rootworms through anti-biosis. Although some progress has been made with respect to increasing tolerance among commercial hybrids, nontransgenic corn rootworm hybrids remain especially susceptible to larval injury if left unprotected (no soil insecticide used).

While corn roots were soaking in our water tanks, we observed some third-instar corn rootworm larvae, pupae, and teneral adults (recently emerged, cuticle not hardened). The low number of larvae that were observed confirmed our suspicion that the larval feeding period for central Illinois is nearing completion. In the following weeks, we will continue our root evaluations at the DeKalb and Monmouth research and education centers.

The recent rains across much of Illinois will very likely increase the number of lodging observations. Typically, yield reductions are most severe when lodging occurs. The primary reason for this yield loss is the disruption of canopy architecture across a severely damaged cornfield. Instead of sunlight striking the maximum leaf surface area of plants, it falls on open ground (between rows), resulting in physiological yield loss. Plants are prone to lodging when one node of roots or more have been destroyed by corn rootworm larvae. Rain-soaked soils will allow for some root regeneration to occur. As we've mentioned before, some hybrids are more likely to regenerate roots than others following rootworm injury.

Over the next several weeks, we'll continue to report on the unfolding story of severe corn rootworm damage across the state. Many observations persist with respect to "swarms" of beetles clipping silks in severely infested fields. Treatment decisions remain difficult in some instances because of the uneven emergence of silks and tassels within many fields. Please refer to previous issues of the Bulletin (nos. 15 and 16, July 1 and July 8, respectively) for management recommendations regarding treatments to prevent excessive silk clipping.

For the past several years, we've witnessed very impressive densities of corn rootworms across an expanding area of Illinois. I suspect that our on-farm evaluations of first-year corn rootworm damage will confirm that the variant is moving farther south. Explanations for the escalating corn rootworm challenges in recent years may include the following: (1) continued trend toward early planting; (2) overall increased corn rootworm nursery acres across the state, which includes continuous and rotated corn acres (we're producing far greater numbers of corn rootworms as compared with the time period prior to the mid-1990s); and (3) drier soil conditions during the larval hatch. Some entomologists also have speculated that the variant may be more aggressive than the "normal," or rotation-susceptible, western corn rootworm. Western corn rootworms that developed resistance to the chlorinated hydrocarbons in the early 1960s also were believed to be more aggressive than their nonresistant counterparts. Greater dispersal characteristics of the resistant strain were used as partial evidence to support this claim.

Thanks for passing along your observations on corn rootworms and other insect challenges this season.

Severe corn rootworm larval injury and root regeneration (July 12, 2005), Champaign-Urbana.

--Mike Gray

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