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Issue No. 24, Article 4/November 6, 2009

Preliminary Node-Injury Ratings for Corn Rootworm Products in Research Trials Near DeKalb, Monmouth, Perry, and Urbana: What Did We Learn in 2009?

This past growing season will long be remembered as one that began wet and finished in the same fashion. Let's hope that producers encounter far fewer planting and harvest frustrations in 2010. We consider ourselves fortunate in having been able to plant our corn rootworm efficacy experiments in a timely manner this spring (Urbana, April 18; Perry, April 23; Monmouth, May 5; and DeKalb, May 24). Once our experiments were in the ground, the precipitation began, leaving some areas of our research plots below water. The questions and answers here may be of use in interpreting the root rating data collected from our trials this year (Table 1).

Did the standing water affect corn rootworm survival in 2009? Throughout the summer, many reported very low western corn rootworm densities in corn. We too observed fewer adults and lower root injury in our untreated checks, especially at Monmouth and Perry. Root damage in DeKalb also seemed lower in our checks than in most previous years. Root injury in the Urbana checks was still very good, ranging from about 2.0 (two nodes of roots pruned) to 2.5. We suspect that the reports of low western corn rootworm densities were primarily related to the saturated soils that occurred in many areas of the state during larval hatch (late May through early June). However, as the number of acres planted to Bt corn rootworm hybrids continues to increase, this also may help explain the lower densities.

Did the wet spring and summer negatively affect soil insecticide performance? Not in our trials. The soil insecticides Aztec 2.1G, Counter 20G, Force 2.25CS, and Lorsban 15G generally kept root injury below 0.5 (1/2 node pruned) on the node-injury scale. At Urbana, where injury in the checks was much greater, the soil insecticides kept root injury below 0.2. From time to time, concerns are expressed about the consistency of soil insecticide performance under extreme environmental conditions (too wet, too dry). These concerns were not realized in our 2009 studies--granted the level of pressure at three of the experimental locations can be described as low to moderate.

Did the corn rootworm Bt hybrid treatments perform well in 2009? Yes. In general, the Bt treatments kept root injury below 1/2 node of roots pruned. However, the HxXTRA (Mycogen 2T789 + Cruiser 250) Bt treatment (0.66) at Urbana had significantly more root injury than the soil insecticides. Producers should not assume that Bt corn rootworm hybrids always offer superior root protection compared with soil insecticides.

Did the soil insecticide and Bt corn rootworm hybrid combination treatments result in very low root damage? Yes. Not surprisingly, the soil insecticides when combined with YieldGard VT3 or HxXTRA hybrids resulted in node-injury ratings near 0, including at Urbana.

Will yield data be provided to establish whether it made economic sense to combine soil insecticide and Bt corn rootworm treatments in 2009? Yes. As soon as harvest is complete and yields can be analyzed, we will share this information with our clientele during winter extension meetings. In wet growing seasons, yields are not typically as affected by root injury. Consequently, we will be surprised to see significant yield differences across most of the treatments.

Because western corn rootworm adult densities were reported as very low across much of Illinois in 2009, will corn rootworm Bt hybrids pay for themselves in 2010? It depends on the densities of western corn rootworms from field to field, environmental conditions, and the cost of seed. Because most growers do not scout their corn or soybean fields for western corn rootworm adults, they have very little information with which to make informed management decisions for next season. Corn rootworm management decisions need to be made field by field. Because of the overall very significant production investment in corn, producers need to make their corn rootworm management decisions very cautiously. If scouting has not taken place, producers should assume they have an economic population of western corn rootworms and plant a Bt corn rootworm hybrid or use a soil insecticide. Because of the expansion of the variant western corn rootworm, this recommendation is appropriate for first-year corn (corn following soybeans) or continuous corn in 2010 for much of Illinois.--Mike Gray and Ron Estes

Mike Gray
Ron Estes

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