This past summer, we evaluated corn rootworm technologies in side-by-side experiments at four locations in Iowa (Ames, Crawfordsville, Nashua, and Sutherland). The objective was to measure the degree of root protection and standability provided by the corn rootworm Cry proteins in each of three technologies, and then to compare them to a standard soil insecticide--Aztec® 2.1G. These field trials measured performance in protecting corn roots under a wide range of environmental conditions. Performance was measured as root injury, product consistency, and plant lodging. All tests were planted on continuous corn ground that was planted late the previous year to attract egg-laying females into the plots. Plots were replicated four times for each treatment at each location, and the same hybrids were planted at all four locations.

Node Injury

Rootworm larval injury was evaluated using the Iowa State Node-Injury Scale, which rates roots from 0 to 3 based on the number of nodes eaten. A node-injury score of 0.25 or less (¼ node eaten back to within 1½ inches of the stalk) demonstrates a product provided very good root protection. The node-injury scale can be better understood by viewing the interactive root rating page.

The amount of injury from corn rootworm larvae was variable across locations and the spectrum of products evaluated. At three locations (Ames, Crawfordsville, and Sutherland), there were no significant differences among any of the rootworm products with respect to the amount of root injury. However, at the fourth location (Nashua), Agrisure™ CB/LL/RW and Aztec 2.1G had significantly more rootworm injury than occurred in YieldGard® Plus and Herculex® XTRA. The reasons for this difference in the level of rootworm injury are unknown.

Product Consistency

Product consistency represents the percentage of times in which the roots had ¼ node or less eaten back to within 1½ inches of the stalk. This distinction allows a small amount of injury to occur without penalizing the performance of a product. If more than ¼ node was eaten on an evaluated corn plant, it was concluded that the product did not adequately protect that plant, and although ¼ node may be a conservative benchmark, in dry or drought conditions it has proven to have been useful in evaluating product performance and detecting economic damage. Consistency is a percentage and works on the same principle as a batting average in baseball--the larger the number, the better the performance.

At three locations, the product consistencies of Agrisure CB/LL/RW, Aztec 2.1G, Herculex XTRA, and YieldGard Plus were statistically the same, thereby providing similar levels of root protection against corn rootworm larvae. However, at the fourth location in Nashua, Agrisure and Aztec had significantly less protection consistency than either YieldGard Plus or Herculex XTRA. Again, the reasons for this inconsistency are unknown since the same Agrisure hybrid was planted at the other three locations.

Lodging and Stand Counts

Lodging ratings were inconsistent and did not appear to be strongly related to corn rootworm injury this year. For example, at Nashua where the YieldGard Plus and Herculex XTRA were 100 percent in consistent root protection, the Herculex hybrid suffered 28 percent lodging. Agrisure CB/LL/RW and Aztec 2.1G had poorer consistency ratings at this location, but suffered less lodging than the Herculex. At Crawfordsville, all of the products provided consistently good root protection, but 34 to 54 percent of the plants were lodged. This was the result of a late-season storm with winds that caused extensive lodging.

There was only one difference observed in the stand counts--at Crawfordsville, Agrisure CB/LL/RW had approximately 3,000 fewer plants per acre than the untreated check, but it was not statistically different than the other rootworm control products.

Summary

These data provide a brief insight into several performance characteristics of genetically engineered corn rootworm hybrids under continuous corn rotations in Iowa. During our 2007 tests at four locations, YieldGard Plus and Herculex XTRA consistently provided the greatest level of root protection against corn rootworm larvae. It is important to understand that variations in performance could be expected for any corn rootworm hybrid (see also Bt rootworm corn failures: Understanding the issues) and that cost per unit of seed, yield production, and individual grower expectations are additional factors worthy of consideration when making a seed purchase. At the end of the season, hybrid performance has been influenced by a variety of agronomic, genetic, environmental, and pest related issues. Therefore, it is imperative that corn growers have an appreciation of these factors and their potential influence on net profit and future pest management decisions.

Table 1. Average root injury, product consistency, percent lodging, and stand count for corn rootworm treatments, Iowa State University, 2007.

¹Planting dates: Ames, May 14; Crawfordsville, May 2; Nashua, May 1; Sutherland, April 30.
Stand counts: Ames, June 12; Crawfordsville, June 5; Nashua, June 1; Sutherland, May 30.
Root injury: Ames, July 24; Crawfordsville, July 19; Nashua, July 31; Sutherland, July 23.
Lodging counts: Ames, Sept. 24; Crawfordsville, Sept. 17; Nashua, Oct. 3; Sutherland, Sept. 25.

²YieldGard Plus (DKC60-18) and Herculex XTRA (Pioneer 34A20) were treated with Poncho 250; Agrisure CB/LL/RW (N67-W2) was treated with Cruiser Extreme 250; the seed for Aztec 2.1G and CHECK was Pioneer 34A16, and it had no seed treatment.

³T-band or Furrow = insecticide applied at planting time.

⁴Iowa State Node-Injury Scale (0–3). Number of full or partial nodes completely eaten.

⁵Means sharing a common letter do not differ significantly according to Ryan’s Q Test (P < 0.05).

⁶Product consistency = percentage of times nodal injury was 0.25 (¼ node eaten) or less.

⁷No significant differences between means (ANOVA, P < 0.05).

⁸3.06 inches rainfall on 6/22 and 1.64 inches rainfall on 6/23 accompanied with very strong winds caused lodging in all treatments (lodging occurred in the absence of root injury).

Marlin E. Rice is a professor of entomology with extension and research responsibilities in field and forage crops. James D. Oleson is project coordinator of the ISU Corn Insect Management Project. Jon J. Tollefson is a professor of entomology with extension and research responsibilities in field and forage crops