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Reports of Extensive Corn Rootworm Larval Injury Common

July 14, 2000
On July 13, we will begin to evaluate our experimental plots for corn rootworm larval injury. On July 7, John Shaw, Illinois Natural History Survey, examined the root systems from five plants in one of our Urbana controls (untreated plot, no soil insecticide used) and found 8 corn rootworm pupae, 5 pre-pupae, 16 third-instar larvae, and 12 second-instar larvae. Because John was able to find so many second-instar larvae, we elected to wait at least one additional week before beginning our root-injury evaluations. John indicated that extensive lodging was evident in many of our corn rootworm plots. In addition, western corn rootworm adults are very numerous in many corn and soybean fields across east-central Illinois.

Pete Fandel, crop systems educator, Woodford County, observed significant corn rootworm injury in cornfields near Minonk (northeast Woodford County). Pete indicated that many corn plants had two to three nodes of roots destroyed by larvae. Because of the significant root injury, top-heavy plants, and moist soils, several acres in one field were "flat." Many adult western corn rootworms were also observed in the damaged fields.

Corn rootworm larval injury, Woodford County, Illinois.

Reports such as Pete's are becoming more common as we approach mid-July. A word of caution: don't automatically assume that a lodged field has significant corn rootworm larval injury. Tall and top-heavy plants are prone to lodge in rain-soaked soils, especially during impressive thunderstorms accompanied by high winds. I've observed many lodged fields over the years, and in certain instances I was convinced that corn rootworm larvae were responsible. However, a closer examination of the roots revealed very little, if any, root injury. So, don't jump to any quick conclusion. Please take a look at Kevin's article in last week's Bulletin (issue no. 15) for a complete description regarding the assessment of root injury.

If significant root pruning is discovered, don't panic. Many corn hybrids are able to regenerate roots from mid-July to mid-August, especially with the excellent soil moisture reserves we have in some areas of the state. Kevin and I are interested in your "rootworm reports" from around the state. Let us know what you're finding.--Mike Gray

Author: Mike Gray

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

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