SmartStax™, Seed Blends, and Refuge: Managing Corn Rootworms and Resistance
Professor & Interim Assistant Dean, ANR Extension
Department of Crop Sciences
(217) 333-6652: firstname.lastname@example.org
Senior Research Specialist
Department of Crop Sciences
In 2010, producers have access to SmartStax™ corn hybrids for the first time. These transgenic pyramided hybrids express several Cry proteins for corn rootworm (Cry3Bb1, Cry34/35Ab1) and lepidopteran control (Cry1A.105+Cry2Ab2, Cry1F) and were developed through a cross-licensing agreement between Dow AgroSciences and Monsanto. Producers who planted SmartStax™ corn hybrids this past season were permitted to reduce their structured refuge from 20% to 5% in the Corn Belt. It is anticipated that eventually mixtures of Bt and non-Bt (so-called refuge-in-a-bag) seed will form the foundation of resistance management programs. At the 2010 Corn and Soybean Classics, roughly 90% of the participants indicated their willingness to use a send blend assuming the non-Bt seed fell within the range of 2% to 5%. In sharp contrast, fewer than 60% of the respondents were eager to use a seed blend if the non-Bt seed was in the 6% to 10% range. Convenience is a key factor in producers’ acceptance of this resistance management approach; however, at some point, growing large numbers of plants without the ability to express Bt proteins becomes a concern and potential liability.
Earlier this year (May 3, 2010), DuPont announced that the United States Environmental Protection Agency (EPA) had approved their request for a refuge-in-a-bag approach for corn rootworms. This enabled a producer who planted Optimum® Acre-Max™ 1 Pioneer corn hybrids to use a seed blend of 90% Herculex® Xtra [Cry1F + Cry34/35Ab1] and 10% Herculex® I [Cry1F]. Producers who decided to use these corn hybrids were able to reduce their refuge from a 20% structured approach to that of a 10% seed mixture. The United States EPA also granted approval for the commercialization of Optimum® Acre-Max™ RW corn hybrids which contain 90% Herculex® RW seed and 10% of another hybrid (no transgenic insect protection) from the same genetic family. Herbicide tolerance is a characteristic shared by all seeds within a bag. These EPA registrations are time-limited and set to expire on September 30, 2010.
To date, no development of field level resistance to Bt hybrids has been documented for either corn rootworms or European corn borers. Yet, as this technology “matures,” refuge compliance weakens, and new resistance management protocols are implemented, some within the entomology community are wondering if resistance might occur in the not too distant future. Let’s hope not. Questions that are being raised with respect to Bt hybrids and resistance management include the following:
- Will the United States EPA extend the registrations for Optimum® Acre-Max™ 1 and Optimum® Acre-Max™ RW corn hybrids beyond September 2010?
- Will producers express much interest in the refuge-in-a-bag approach to resistance management if the non-Bt seed is at the 10% level?
- Will the United States EPA offer a new registration for the use of SmartStax™ corn hybrids in 2011 that allows for a seed mixture refuge approach? If this occurs, will the refuge (seed mix) contain non-Bt seed at the 5% level?
- How will the transition to a seed mixture refuge affect the long term duration of the soil insecticide industry and Bt hybrids?
- Due to the extensive use of neonicotinoid insecticidal seed treatments by the seed industry, will we begin to see resistance to these products in the near future?
The Insect Management and Insecticide Evaluation Program at the University of Illinois has evolved considerably since the 1980s and now incorporates the testing of registered and experimental insecticides against several corn and soybean insects along with assessing the performance (insect protection and yield) of transgenic hybrids each growing season. In 2009, root injury at our Urbana location in the three control treatments exceeded two nodes of roots destroyed (Figure 1). In general, the root protection afforded by the other treatments (soil insecticides and transgenic hybrids) was very good. The low yields in the controls (Figure 2) reflected the importance of root protection and the value of this annual investment by producers in the management of this important insect pest. In contrast, root injury at the Perry location (Figure 3) was very low even in the control treatments. Yield data (Figure 4) from this site reflect the overall reduced value in root protection products when corn rootworms are not present at economic densities. Greater prescriptive use of transgenic hybrids and soil insecticides, based upon scouting input the previous season, would enable a more judicious use of these products and improve their long term durability.
At this year’s Agronomy Day, preliminary root injury results from the 2010 corn rootworm trials will be shared and comparisons offered for the various transgenic hybrid and soil insecticide treatments.