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Issue No. 21, Article 1/August 18, 2006

Preliminary Root Ratings from Corn Rootworm Control Trials--DeKalb, Monmouth, and Urbana

Preliminary results from our standard corn rootworm control trials in DeKalb, Monmouth, Perry, and Urbana have been analyzed, and results from three of the trials are presented in Table 1. Mean node injury ratings are not presented for the Perry location because there was very little corn rootworm larval damage in all plots, including the untreated check. We established a corn rootworm control trial at the Orr Agricultural Research and Demonstration Center (Perry) for the first time in 2006, so we hope to have better luck with rootworm larval damage in 2007.

All four of our rootworm control trials in 2006 were planted in areas that were planted as trap crops (late-planted corn interplanted with pumpkins) in 2005. The late-planted corn and pumpkins usually attract large numbers of rootworm adults, and the females usually lay a lot of eggs in the trap crop. By using this approach, we intentionally place corn rootworm control products under significant corn rootworm larval pressure. Although the level of rootworm larval damage in commercial cornfields usually is not as severe as the level of damage we achieve in our experiments, severe root damage and lodging occur in some producers' fields each year. So we hope the results from our trials enable producers to make more informed decisions about rootworm control products.

In 2006, we evaluated the efficacy of granular and liquid soil insecticides, seed-applied insecticides, and transgenic Bt corn hybrids (Herculex RW, Herculex XTRA, and YieldGard Rootworm). The corn hybrid planted in all plots treated with a soil- or seed-applied insecticide was DeKalb 61-72, the non-Bt isoline of DeKalb 61-68, a YieldGard Rootworm hybrid. The Herculex RW hybrid planted at all three sites was Pioneer 34A18, and its non-Bt isoline was Pioneer 34A16. Another Herculex RW hybrid (Mycogen 2G777) was planted in Monmouth and Urbana, as well as a Herculex XTRA hybrid (Mycogen 2P788). The non-Bt isoline was Mycogen 2784.

The mean node injury ratings in all of the untreated checks at all three locations were greater than 2.0, indicating that a minimum of two nodes of roots were completely pruned. At the Monmouth and Urbana locations, the mean node injury ratings in the untreated checks exceeded 2.4 (2.9 or greater for the Mycogen and DeKalb checks). These numbers reveal that rootworm larval damage was severe at all three sites.

In general, the granular soil insecticides did not perform as consistently this year as they did in 2005, particularly at the DeKalb location. We will review the environmental conditions that prevailed throughout the season at the DeKalb site in an attempt to explain reduced product performance. You can compare the results from 2006 with those from 2005 by reviewing the 2005 edition of on Target, our annual summary of field crop insect management trials. You can access the report at www.ipm.uiuc.edu/ontarget/2005report.pdf. The results from the standard corn rootworm control trials in 2005 are presented on pages 5 (DeKalb), 6 (Monmouth), and 7 (Urbana).

Poncho 1250 did not provide much protection against corn rootworm larvae, with mean node injury ratings of 1.24, 1.65, and 1.97 and mean percentage consistencies of 5, 15, and 0 at DeKalb, Monmouth, and Urbana, respectively. For several years, the insecticidal seed treatments have not provided consistent root protection in our trials. The use of seed-applied insecticides to control rootworms in non-Bt refuges represents a risk if densities of rootworms are large.

The use of Poncho 250 or Poncho 1250 with Aztec 2.1G did not provide any additional protection against rootworm larvae. The mean node injury ratings for these combinations were not significantly lower than the mean node injury ratings of Aztec 2.1G, the same trend we observed in 2005. Last year, the combination of Aztec 2.1G and Poncho 1250 was marketed with the catch phrase "Double Down." Our results indicate that this combination is unnecessary for rootworm control and is not a good investment.

Herculex RW hybrids (both Pioneer and Mycogen hybrids) had significantly lower node injury ratings and performed more consistently than the YieldGard Rootworm hybrid at the Urbana location. However, pruning was observed on Herculex RW root systems, especially at the Urbana location, where approximately 1/2 node of roots was pruned on Herculex RW and Herculex Extra hybrids. Rootworm larval damage to transgenic Bt hybrids (both YieldGard and Herculex) was considerably less at the Monmouth site than at the Urbana site.

During the past couple of years, we have observed late-season (August) pruning of brace roots on some YieldGard Rootworm hybrids, especially at the Urbana location. To a lesser extent, late-season pruning of brace roots of YieldGard Rootworm hybrids also has occurred at the DeKalb site. At the Monmouth location, late-season pruning of brace roots of YieldGard Rootworm hybrids has been uncommon. We have speculated that the variant western corn rootworm may be partially responsible for these observations. Variant western corn rootworms may be more aggressive root feeders and less susceptible to the Bt protein expressed by YieldGard Rootworm hybrids. There is some circumstantial evidence in the entomological literature to support this hypothesis. The variant western corn rootworm is well established in the Urbana area, and densities usually are large. In the DeKalb area, the variant western corn rootworm has not been present for as long as it has been established in east-central Illinois, although densities of the variant western corn rootworm have occasionally been large in north-central Illinois. In the Monmouth area, the variant western corn rootworm is just getting established. Our field observations during the past few years add some circumstantial evidence to support the aforementioned hypothesis. More research clearly is warranted to explore this hypothesis more fully.

Again in 2006, we dug and evaluated roots from some selected plots in August so that we could compare node injury ratings in July with node injury ratings in August. As we observed in 2004 and 2005, rootworm larval injury to the YieldGard Rootworm hybrid at the Urbana site increased from mid-July to August (July 17 to August 8). In mid-July, nearly 1 node of roots was pruned on the YieldGard Rootworm hybrid. On August 8, approximately 1-1/2 nodes of roots were pruned (Table 2). We are particularly concerned about the level of rootworm larval damage to the YieldGard Rootworm hybrid in Urbana. Corn plants are much more susceptible to lodging when one or more nodes of roots have been pruned.

Root injury in the Poncho 1250 plots also increased in Urbana from 1.97 to 2.35. Changes in mean node injury ratings for Aztec 2.1G, Herculex XTRA in Urbana were not appreciable. The mean node injury rating for rootworm control products listed in Table 2 increased slightly at the Monmouth site, although the increases probably are not significant. The mean node injury rating for the YieldGard Rootworm hybrid in DeKalb did not change significantly from July to August. There were slight increases in the mean node injury ratings for the other rootworm control products at the DeKalb site.

We look forward to sharing the complete, fully analyzed results from all of our insect management trials later this year in on Target.--Mike Gray, Kevin Steffey, Ron Estes, and Jared Schroeder

Kevin Steffey
Mike Gray
Ron Estes
Jared Schroeder

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