No. 24 Article 2/November 11, 2005

YieldGard Rootworm Performance: Is Root Protection Equal Among These Transgenic Hybrids?

In 2005, with the cooperation of Monsanto Company, we evaluated corn rootworm larval injury to nine different YieldGard Rootworm (MON 863) hybrids and a non-Bt isoline (check) that were planted on April 29 (four-row plots, 45 ft long, four replicates) into a plot area that had been planted to a trap crop in 2004 (late-planted corn intercropped with pumpkins). Roots were dug (10 roots for each replicate by treatment combination) and rated (0-3 node-injury scale) for injury on July 20 and August 10 (Table 1). A duplicate of the entire experiment also was planted on May 23. Because rootworm larval injury was minimal in the experiment planted in late May, we report only the results from the experiment planted on April 29. All hybrids were selected by Monsanto Company, and we rated the root injury without any knowledge of their genetic backgrounds. The identities (brand names) of these transgenic corn rootworm hybrids remain unknown to us.

Root injury in the check treatment was significant, with two nodes of roots destroyed (2.09, July 20). Based on the node-injury ratings on July 20, the root protection afforded by hybrids A (0.47), B (0.39), E (0.16), F (0.19), H (0.2), and I (0.21) was very good to excellent. YieldGard Rootworm hybrids A, B, E, F, H, and I were available commercially in 2005. Based on the node-injury ratings on August 10, rootworm larval injury generally was greater in hybrids A, B, E, F, and I. However, we believe that the increases in node-injury ratings for these hybrids from July 20 to August 10 were not of biological significance. Representatives with Monsanto informed us, after we had completed our evaluations, that hybrids E and H are the same.

Root pruning was significant in YieldGard Rootworm hybrids C (0.98, July 20; 1.27, August 10) and D (0.78, July 20; 0.98, August 10), with nearly one node of roots destroyed in each. Representatives with Monsanto informed us after we had completed our evaluations that these two transgenic hybrids had failed their in-house screens during 2004. Neither was commercialized. The amount of rootworm larval pruning to hybrid G (0.75, 3/4 node destroyed on July 20; 0.93, approximately 1 node destroyed on August 10), which was available commercially in 2005, surprised the Monsanto representatives because hybrid G had passed their in-house screen.

The results from this 2005 experiment suggest that there is variation in the level of root protection among YieldGard Rootworm (MON 863) hybrids. We hope to continue this research with Monsanto Company during the 2006 growing season. As other transgenic corn rootworm hybrids (different events) are commercialized and enter the marketplace, we may continue to find interesting differences in levels of root protection among these products.

Our experiences with transgenic corn rootworm hybrids have been limited (three commercial growing seasons); however, the overall performance of this exciting technology has been very good in our experiments. Nonetheless, in the past 2 years we have observed late-season pruning on some YieldGard Rootworm (MON 863) hybrids. On the other hand, many Illinois producers reported being very pleased with the vigor (plant height and color) of the hybrids under drought conditions throughout July during the critical pollination period. We made similar observations in several of our experiments. However, if late-season pruning of brace roots in some transgenic hybrids leads to lodging, harvest difficulties could ensue. As previously reported in the Bulletin, we also observed (2005) that rootworm larval injury increased in some plots treated with soil insecticides when roots were evaluated for injury in August. These changes in root injury from July to August bear continued examination.

YieldGard Rootworm hybrid F (August 10), commercialized in 2005.

YieldGard Rootworm hybrid G (August 10), commericalized in 2005.

--Mike Gray and Kevin Steffey

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