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Issue No. 16, Article 2/July 14, 2006

Transgenic Corn Rootworm Hybrids: Dose Response Considerations with Respect to Silk Clipping

E-mail messages and phone calls this past week revealed some confusion regarding the potential for silk clipping by corn rootworm adults in nonrefuge areas of fields that have been planted to transgenic corn rootworm hybrids. Let me try to bring some clarity to this issue. Economic levels of silk clipping by corn rootworm adults may still occur even in portions of fields planted to a transgenic corn rootworm hybrid (YieldGard Rootworm, MON 863, Cry3Bb1 or Herculex RW, DAS-59122-7, Cry34Ab1/Cry35Ab1). Densities of western corn rootworm adults may exceed economic levels in portions of fields planted to transgenic corn rootworm hybrids due to immigration of beetles as well as direct emergence of adults from transgenic root systems.

The lethal concentration (LC50) of the Cry3Bb1 protein expressed in YieldGard Rootworm hybrids (necessary to kill 50% of a corn rootworm larval population) is estimated to be approximately 75 ppm (Dudin et al. 2001). This protein also is expressed in other corn plant tissue: young leaf tissue (81 ppm), grain (70 ppm), root (39 ppm), pollen (62 ppm), and silks (10 ppm) (EPA Document MRID No. 451568-02). Vaughn et al. (2005) determined that the average expression in root tissue of five YieldGard Rootworm hybrids decreased (averaged across the five hybrids) from 69.8 to 44.0 ppm from V4 to V9 growth stages, respectively.

There have been many journal articles published that confirm western corn rootworm adult emergence (Crowder et al. 2005) from YieldGard Rootworm hybrids, confirming the low to moderate dose nature of this event (Vaughn et al. 2005). Even though the Cry3Bb1 protein is expressed in pollen and silks, these levels are not sufficiently high to affect (directly or indirectly) adult densities of western corn rootworm adults. A recent paper published in the Journal of Economic Entomology (Nowatzki et al. 2006) that focused on the feeding behavior and longevity of western corn rootworms that had consumed the Cry3Bb1 protein offered the following discussion on this topic: "These data suggest that western corn rootworm adults will exhibit similar feeding behaviors when encountering transgenic (expressing Cry3Bb Bt protein) or nontransgenic corn plants in the field and that it is unlikely that Cry3Bb Bt protein in transgenic plants will have significant effects on adults that might impact resistance management recommendations."


Corn rootworm adult emergence cage used to assess survivorship in transgenic corn rootworm hybrid plots.


Multiple corn rootworm adult emergence cages in transgenic corn rootworm study.

Much of this discussion has focused on the YieldGard Rootworm hybrids and the responses of western corn rootworm adults. What about the Herculex RW hybrids that express the binary Bt proteins: Cry34Ab1/Cry35Ab1? In what plant tissues and at what levels are these proteins expressed? How should we characterize survival of western corn rootworm adults from fields planted to these transgenic hybrids?

The mean expression levels (ppm) of Cry34Ab1 and Cry35Ab1 in various corn tissues are reported in Table 1 (U.S. EPA Biopesticides Registration Action Document, October 2005, page 11). Similar to YieldGard Rootworm hybrids, Bt proteins are expressed throughout the plant in Herculex RW hybrids as well.

Based on a review of the U.S. EPA Biopesticides Registration Action Document, some "uncertainties" exist regarding how to characterize the corn rootworm dose with respect to Herculex RW hybrids. The following discussion was extracted directly from page 70 of the EPA document: "The studies (both larval and adult dose) are acceptable for the purposes of registration and the development of an IRM plan for Cry34/35 corn. However, there are still significant uncertainties regarding the CRW dose expressed in Cry34/35 corn. At best, the results are an educated estimate of the true effects of Cry34/35 corn on CRW. The exact toxicity (i.e., mode of action) of the Cry34/35 proteins to CRW is unclear: the field studies indicated high levels of mortality while the laboratory studies suggested less toxic mechanisms such as growth inhibition or feeding deterrence. Given these uncertainties, it is recommended that Dow/Pioneer continue to research the dose issue--including the concept of dose for CRW, specific assays to evaluate dose for rootworm-protected corn hybrids, and the role of density dependence in dose determinations."

Apparently, much more research is necessary to better characterize the dose of Herculex RW hybrids and the response of corn rootworms--yet these hybrids are commercialized. Additional reading of the EPA document (page 69) does suggest that mortality of larvae may be high when corn rootworm larvae feed on hybrids that express the Cry34/35 Bt proteins: "In terms of adult susceptibility and dose, Dow/Pioneer conducted adult feeding studies with WCRW and Cry34/35 corn tissue. These studies indicated that adult WCRW feeding (solely) on Cry34/35Ab1 above-ground corn tissue does not reduce adult fitness as characterized by longevity, fecundity or egg viability. Therefore, dose considerations for Cry34/35 corn can be relegated to the larval life stage. To investigate larval dose, Dow/Pioneer conducted a series of single-season field trials that measured adult emergence while correcting for density-dependent mortality to provide an indirect estimate of larval mortality. These trials indicated that WCRW larval mortality ranged between 99.82 and 99.98%, while NCRW larval mortality ranged between 92.77 and 99.14%."

As the EPA document indicated, these estimates of survivorship were provided from a single season of research. More research in this arena is warranted, and scientists in industry and our land-grant institutions will continue to conduct studies on this interesting topic. For now, discussion on the dose of Herculex RW hybrids will revolve around terminology gleaned from this EPA document, such as "slightly less than a high dose" or "borderline high dose."

For much of Illinois, protection of the pollination process will be foremost in many producers' minds in the near term. I hope this article clarifies some of the confusion that exists about western corn rootworm susceptibility (larval and adult stages) to transgenic hybrids and the dose response characterization issue. More research is warranted on these topics. Bottom line--even though a field has been planted to a YieldGard Rootworm or Herculex RW hybrid, it should be scouted carefully throughout the pollination process and rescue treatments applied as needed to protect the pollination process. Do not focus all scouting efforts in the refuge areas only. This could be a mistake.

We look forward to your observations around the state this season about the efficacy of the corn rootworm product(s) you've selected for your fields.--Mike Gray

References

Crowder, D.W., D.W. Onstad, M.E. Gray, C.M.F. Pierce, A.G. Hager, S.T. Ratcliffe, and K.L. Steffey. 2005. Analysis of the dynamics of adaptation to transgenic corn and crop rotation by western corn rootworm (Coleoptera: Chrysomelidae) using a daily time-step model. Journal of Economic Entomology 98: 534-551.

Dudin, Y.A., B.P. Tonnu, L.D. Albee, and R.P. Lirette. 2001. Amended report from MSL 16599: B.t. Cry3Bb1.11098 and NPTII protein levels in tissue samples collected from corn event MON 863 grown in 1999 field trials. Report No. MSL-17181; unpublished study conducted by Monsanto Company. Submitted to EPA on May 21, 2001, MRID 454240-01.

Nowatzki, T.M., X-Z. Zhou, L.J. Meinke, T. Vaugh, and B.D. Siegfried. 2006. Effect of Bacillus thuringiensis Cry3Bb1 protein on the feeding behavior and longevity of adult western corn rootworms (Coleoptera: Chrysomelidae). Journal of Economic Entomology 99: 927-930.

U.S. Environmental Protection Agency, Office of Pesticide Programs, Biopesticides, and Pollution Prevention Division. 2005. Bacillus thuringiensis Cry34AB1 and Cry34Ab1 proteins and the genetic material necessary for their production (plasmid insert PHP 17622) in Event DAS-59122-7 corn, 133 pages.

Vaughn, T., T. Cavato, G. Brar, T. Coombe, T. DeGooyer, S. Ford, M. Groth, A. Howe, S. Johnson, K. Kolacz, C. Pilcher, J. Purcell, C. Romano, L. English, and J. Pershing. 2005. A method of controlling corn rootworm feeding using a Bacillus thuringiensis protein expressed in transgenic maize. Crop Science 45: 931-938.

Author:
Mike Gray

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