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Michael E. Gray
Professor & Extension Coordinator
Department of Crop Sciences
Darren M. Bakken
Graduate Research Assistant
Department of Crop Sciences
Seed Treatments and Transgenic Hybrids for Corn Rootworms: Management Challenges Persist
As the 2004 growing season proved, none of the corn rootworm control tactics were "bullet proof." Entomologists at the University of Illinois have conducted trials to evaluate the efficacy of corn rootworm control products for many years. Our corn rootworm experiments are planted into fields in which a trap crop (late-planted, mixed maturity corn hybrids interplanted with pumpkins) was planted the previous season. These trap crops attract many female (egglaying) corn rootworm adults. Because oviposition is intense within our trap crop fields, insecticidal products are rigorously tested the following year with regard to their ability to withstand severe corn rootworm larval feeding. Root rating results from these experiments are shared with producers each year, thus enabling them to make more informed product selection decisions.
Of continuing interest among producers is the performance of the systemic nicotinoid insecticides Poncho 1250 (clothianidin) and Cruiser (thiamethoxam) against corn rootworms. Both of these systemic insecticides are applied to seeds at a rate of 1.25 mg a.i. per seed for corn rootworm control. At lower rates, these insecticidal seed treatments are labeled for secondary insect control.
In 2003 and 2004, we established corn rootworm efficacy trials in DeKalb, Monmouth, and Urbana. At each of these locations we evaluated a wide range of products including seed treatments, granular and liquid soil insecticides, and the YieldGard Rootworm technology. Roots were evaluated for injury on the 1 to 6 Iowa State Injury Scale. A rating of 3.0 (some root pruning, but less than 1 node of roots destroyed) is considered the economic injury index. Ratings of 4.0, 5.0, and 6.0 indicate one, two, and three nodes of roots completely destroyed, respectively. With the exception of the Urbana experiment in 2003 (Table 1), the insecticidal seed treatments did not keep root injury below the economic index of 3.0. One striking difference between 2003 and 2004 trials was the timing of planting. Late-planting tended to favor the performance of the insecticidal seed treatments. More typical planting dates, such as those in 2004, resulted in less favorable results for Poncho 1250 and Cruiser. Although the systemic nicotinoid seed treatments offer promise and convenience, thus far, they have not provided consistent root protection under heavy corn rootworm pressure. Because of their ubiquitous use across the Corn Belt, the development of resistance at some point is probable if this usage trend continues. Western corn rootworms have shown their ability to develop resistance to several classes of insecticides and since the mid-1990s, have overcome the pest management benefits of crop rotation. We should not underestimate the resilience of this impressive insect pest.
In 2004, we reported that a YieldGard corn rootworm hybrid (Golden Harvest H-8588 RW) had more root pruning in our Urbana corn rootworm efficacy experiment than anticipated (3.15 root rating as of July 10). During the first week of August in 2004, we rated the roots from the YieldGard treatment again due to significant lodging in this treatment following a severe storm. To more precisely quantify the level of pruning on these roots, we rated them on the new Iowa State 0 to 3 rating scale. Root injury ratings per replicate were 1.43 (nearly 1 ½ nodes destroyed), 1.08 (1 node destroyed), 1.64 (slightly more than 1 ½ nodes destroyed), and 1.24 (slightly more than 1 node destroyed). This level of injury was far greater than witnessed in mid-July. Following these observations we raised several questions: 1) Does the expression of the Cry3Bb1 protein diminish as the season progresses, compromising root protection in late July and early August? 2) Are there critical differences in expression of the Cry3Bb1 protein across hybrids? and 3) Are some hybrids with less than satisfactory root characteristics poor candidates for the YieldGard rootworm technology? Answers to these questions are forthcoming. In 2005, we established a trial near Urbana with many different YieldGard corn rootworm hybrids planted at two dates. During the upcoming fall and winter meetings, we hope to share the results from these trials. Results from other experiments will continue to shed light on the promise and challenges associated with the use of transgenic hybrids for the management of corn rootworms.
In a recent issue of Crop Science (March 28, 2005), a paper (A Method of Controlling Corn Rootworm Feeding Using a Bacillus thuringiensis Protein Expressed in Transgenic Maize) was published in which scientists described the expression level of Cry3Bb1 (Bt protein targeted against corn rootworm larvae) in five corn hybrids (MON 863) at two growth stages (V4 and V9) in a growth chamber study. The authors acknowledged that the effect of growth stage on the expression of the Cry3Bb1 protein was significant, on average declining by 25.8 ppm across hybrids from V4 to V9. Despite the decline in the expression of the Bt protein across all five hybrids (V4 to V9), the root protection afforded by each transgenic hybrid was excellent, with only minor feeding scars on the root systems reported. The control (non-transgenic hybrid) had over two nodes of roots completely destroyed. Published papers such as this may begin to help explain the results from our Urbana experiment in 2004.
By late June of 2005, many observations of severe corn rootworm larval injury had been reported across the northern two-thirds of Illinois. In 2005, insecticidal products for corn rootworms were put to a severe test due to the very hot and dry soil conditions that developed during the corn rootworm larval-feeding period (June through mid-July).
Table 1. Root rating results of the insecticidal seed treatments
at Dekalb, Monmouth, and Urbana in 2003 (NK NJ2-J5 hybrid) and
2004 (Golden Harvest H-8799 hybrid).
Poncho 1250 3.50 3.95 3.45 4.10 2.90 4.05 Cruiser 3.50 5.05 4.20 5.10 2.70 4.15 Control 4.20 5.00 4.95 5.75 4.60 5.80 Planting Date May 28 April 28 May 16 April 27 May 13 April 19
Figure 1. Poncho 1250 treatment (left), YieldGard Rootworm treatment (right),
photograph taken on June 28, 2005.