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Issue No. 15, Article 5/July 16, 2010

Preliminary Assessment of Western Corn Rootworms: Root Damage and Adult Densities Appear Very Low in Illinois

On July 12 we began our annual root digs just south of Urbana. Although we observed pruning of roots on some treatments, the overall level of injury was not up to the pressure expected for this location. On July 13, we traveled to the University of Illinois Orr and Monmouth Research and Demonstration Centers to dig roots from our standard corn rootworm trials and returned the roots to Urbana for evaluation. They will be rated for injury later today (July 14). We saw no evidence of lodging at either location. Of interest at both sites was the exceedingly low number of western corn rootworm adults. In all honesty, Joe Spencer (entomologist with the Illinois Natural History Survey) and I had to actively search to find any on plants at all. Plant silks at Monmouth were very long and provided additional evidence that little feeding, if any, had taken place. Japanese beetles were present at Monmouth but were found primarily on soybeans.

Silk growth on a corn plant in research plot near Monmouth on July 13.

Minor Japanese beetle adult defoliation in soybeans near Monmouth on July 13.

Mike Vose, farm foreman at the Orr Research and Demonstration Center, also noted that he has not been able to find any western corn rootworm adults in the trap crop (corn planted late and interseeded with pumpkins). Again, this is very atypical. Instead, he was finding southern corn rootworm adults. This pest (with 12 black spots on the forewings [elytra]), perhaps better known as spotted cucumber beetle, overwinters as an adult; however, southern corn rootworm adults do not overwinter in Illinois and instead migrate from more southern locations, typically beginning in April. Recall that western and northern corn rootworms overwinter as eggs, with hatch occurring in late May and early June in most years.

What may explain the very low densities of western corn rootworm adults and low levels of root injury? I believe there are several possible factors:

  • Across many areas of Illinois, the last three springs have been exceptionally wet, especially during the larval hatch of western corn rootworms (late May and early June). Precipitation was very heavy at the Orr and Monmouth Research and Demonstration Centers in June (approaching 1 foot). When fields are saturated, rootworm larvae struggle to secure root tissue to begin feeding on.
  • The use of Bt corn hybrids continues to climb throughout Illinois. Even though Bt hybrids targeted at corn rootworms do not have the high dose effect that we've observed for European corn borers, they are undoubtedly reducing densities due to their extensive popularity.
  • During my journey to Perry and Monmouth on July 13, I observed many aerial applications underway. Though I recognize that these were targeted primarily at preventing fungal diseases in cornfields, I did wonder how many of the applications and those in previous years involved tank-mixes of fungicides and a pyrethroid. Pyrethroid treatments may be of interest to some producers to protect silks from clipping by corn rootworm and Japanese beetle adults.

After I complete this season's root evaluations, I look forward to sharing the results with you.--Mike Gray

Mike Gray

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