Pioneer's Science Supporting Optimum® AcreMax® 1 Insect Protection
The Benefits of Blending Rootworm Refuge on Bt Rootworm Trait Durability
May 19, 2010
Laura Higgins1, Lindsey Flexner2, Zaiqi Pan2, Bruce Stanley2, Rachel Binning3, Tim Nowatzki2, Analiza Alves2, Ray Layton3, and Marlin Rice2f
1Dupont Agriculture Biotechnology Group, 2Pioneer Trait Characterization & Development Group, 3Pioneer Biotechnology Affairs & Regulatory Group
Pioneer is committed to the responsible development and deployment of the leading technologies that growers demand. Optimum® AcreMax® 1 (OAM1) insect protection is supported by more than six years of proprietary and public research and extensive predictive computer modeling to support the trait durability plan and sound stewardship practices.
Pioneer has an ongoing commitment to insect resistance management research. After the 2005 commercialization of Herculex® RW and Herculex® XTRA, Pioneer continued to study the interaction of the Herculex traits and their target pests. This research effort continues today and is part of Pioneer's commitment to bring growers the best possible solutions for managing their crops while extending the effectiveness of every insect control trait we place in our products.
What has Pioneer done to model blended rootworm refuge durability?
Pioneer developed a predictive computer model to evaluate seed blends. This model compares the effect of different refuge options on the durability of corn containing event DAS-59122-7 (59122)–the rootworm trait in Herculex RW, Herculex XTRA, OAM1 and Optimum® AcreMax® RW (OAMRW). The Pioneer model was validated by a third-party biostatistician (Critical Path Services, Garnet Valley, PA) and reviewed by academic rootworm researchers and computer modeling experts. This new model takes into account the non-high dose characteristics of event 59122 (Binning et al. 2010), adult western corn rootworm (Diabrotica virgifera virgifera LeConte) movement in the corn field, the probability of mating between corn rootworm males emerging from refuge corn with corn rootworm females emerging from Bt corn, and random oviposition in the field.
What does Pioneer’s modeling suggest about the effectiveness of a 10% blended rootworm refuge compared to a traditional 20% separate block refuge?
Pioneer's field data and modeling strongly suggest a smaller, blended corn rootworm refuge provides more benefit to extending Bt rootworm trait durability than the traditional 20% block refuge because of more effective refuge placement. A 10% blended rootworm refuge is an effective tool the grower can use to delay the development of corn rootworm resistance to 59122 corn.
Pioneer’s modeling simulations were conducted for several corn rootworm refuge strategies (Figure 1). In each scenario modeled, implementing a 10% blended corn rootworm refuge extended the durability of 59122 corn compared to a 20% block refuge. Pioneer used assumptions derived from strong empirical evidence; for example, beetle emergence phenology, daily oviposition rate curve, and adult beetle movement. In addition, the model was conservatively designed for continuous corn situations (corn-on-corn fields) and a single isolated field. In such situations, selection pressure for development of corn rootworm resistance is high. The common practice of corn-soybean crop rotation would likely further the durability estimates of 59122 corn. The results of the scenarios modeled demonstrate the superiority of a 10% blended refuge relative to block refuge deployment strategies for 59122 corn and corn rootworm.
Figure 1. Comparison of trait durability (in years*) of 59122 corn deployed with a 20% block
refuge and a 10% seed blend refuge.
* Similar to other models, the numbers of years estimated under different scenarios modeled represent relative values of durability and should not be taken at face value or as absolute estimates to predict durability. Rather, output results from modeled scenarios should be compared relative to each other.
Blended refuge also has the obvious benefit of removing the compliance risk associated with a block refuge. Pioneer’s model examined the sensitivity of grower compliance with a 20% block refuge and its effect on the durability of 59122 corn. Compliance with the refuge requirement was a sensitive parameter and showed that when refuge compliance was low (e.g. 50%), 59122 corn was predicted to have less than half the lifespan as the trait deployed with complete refuge compliance (e.g. 100%). A substantial benefit of OAM1 and OAMRW is that corn rootworm refuge compliance is built into every bag, thereby extending the predicted durability of 59122 corn for many years.
Why do EPA’s model results differ from Pioneer’s results?
Predictions of 59122 corn durability reported by EPA in the OAM1 Biopesticide Registration Action Document (BRAD) were based on the EPA’s new internal model and several markedly different assumptions. Based on Pioneer’s review of EPA’s modeling summary, their model appears to predict more durability with a 20% block refuge than with a 10% blended refuge because it uses a high proportion of pre-ovipositional females with long range movement, higher grower compliance with corn rootworm refuge requirements, and significant dispersal of refuge insects from adjacent corn fields. In contrast, Pioneer’s model predicts more durability with a 10% blended refuge compared to the EPA model because it focuses on short-range within-field adult movement and its importance to where mating occurs. This short-range beetle movement prior to mating is a critical biological factor and is especially important in larger isolated fields where block refuge beetles are less likely to disperse across a larger area. Additionally, the Pioneer model does not rely on significant immigration of refuge insects from adjacent fields or from long-range dispersal as this movement is not likely to occur in all field scenarios across the Corn Belt. The Pioneer model focuses on the worst case situation where epicenters of resistance can occur and then begin to spread, consistent with theories on how other resistant corn rootworm populations spread (e.g., methyl parathion resistance in Nebraska and soybean variant rootworms in Illinois; see Meinke et al. 1998, Levine et al. 2002). Based on model simulations, resistance can occur as quickly as three years for a single grower not in compliance or longer for a compliant grower with a large (greater than 100 acres) isolated field using a 20% block refuge. Pioneer respects the EPA’s model and assumptions; however, modeling is only one component in the weight of evidence used to ultimately make regulatory approval decisions. Therefore, differences in the basic assumptions, output and interpretations from the two models are taken as one component of the overall risk benefit analysis of any refuge strategy.
Blending corn rootworm refuge provides several benefits to corn producers that a block refuge cannot accomplish; it produces refuge beetles throughout the field in close proximity to any potentially resistant individuals that may emerge, it maximizes the potential for random mating of susceptible beetles with rare resistant individuals (by placing refuge plants in close proximity to Bt plants), it simplifies compliance by ensuring that a rootworm refuge is present in every field, and it extends 59122 corn durability. Blending refuge can be a valuable and powerful management tool for helping corn growers battle corn rootworms while maintaining the durability of the technology.
Binning, R. R., S. A. Lefko, A. Y. Millsap, S. D. Thompson, and T. M. Nowatzki. 2010. Estimating western corn rootworm (Coleoptera: Chrysomelidae) larval susceptibility to event DAS-59122-7 maize. Journal of Applied Entomology DOI: https://dx.doi.org/10.1111/j.1439-0418.2010.01530.x
Bruss, R. G. 1981. Intrafield dispersal of adult corn rootworms. Ph.D. dissertation, Iowa State University, Ames, IA.
Heeringa, S. G., and J. E. Bailey. 2009. Evaluation of the resistance risks from using a seed mix refuge with Pioneer's Optimum AcreMax 1 corn rootworm-protected corn. IFRA Scientific Advisory Panel Meeting, Environmental Protection Agency Conference Center, Arlington, VA.
Levin, E., J. Spencer, S. Isard, D. Onstad, and M. Gray. 2002. Adaptation of the western corn rootworm to crop rotation: Evolution of a new strain in response to a management practice. American Entomologist 48: 94-107.
Meinke, L., B. Siegfried, R. Wright, and L. Chandler. 1998. Adult susceptibility of Nebraska western corn rootworm (Coleoptera: Chyrsomelidae) populations to selected insecticides. Journal of Economic Entomology 91: 594-600.
Naranjo, S. E. 1990. Comparative flight behavior of Diabrotica virgifera virgifera and Diabrotica barberi in the laboratory. Entomologia Experimentalis et Applicata 55: 79-90.
Onstad, D. W., C. A. Guse, J. L. Spencer, E. Levine, and M. E. Gray. 2001. Modeling the dynamics of adaptation to transgenic corn by western corn rootworm (Coleoptera: Chrysomelidae). Journal of Economic Entomology 94: 529-540.
Onstad, D. W. 2006. Modeling larval survival and movement to evaluate seed mixtures of transgenic corn for control of western corn rootworm (Coleoptera: Chrysomelidae). Journal of Economic Entomology 99: 1407-1414.
Quiring, D. T., and P. R. Timmins. 1990. Infuence of reproductive ecology on feasibility of mass trapping Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae). Journal of Applied Ecology 27: 965-982.
Spencer, J. L., T. Mabry, and T. Vaughn. 2003. Use of transgenic plants to measure insect herbivore movement. Journal of Economic Entomology 96: 1738-1749.
Stebbing, J. A., L. J. Meinke, S. E. Naranjo, B. D. Siegfried, R. J. Wright, and L. D. Chandler. 2005. Flight behavior of methyl-parathion-resistant and -susceptible western corn rootworms (Coleoptera: Chrysomelidae) populations from Nebraska. Journal of Economic Entomology 98: 1294-1304.
Storer, N. P. 2003. A spatially explicit model simulating western corn rootworm (Coleoptera: Chrysomelidae) adaptation to insect-resistant maize. Journal of Economic Entomology 96: 1530-1547.