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Issue No. 4, Article 3/April 29, 2010

Seed Mixtures vs. Structured Refuges for Bt Hybrids: What's the Outlook?

With so much of this year's corn already planted and soybean planting under way, most farmers are probably not thinking about how refuge requirements for Bt hybrids may change in the future. There is considerable discussion and debate about this topic in industry, regulatory, and academic circles, though. During the Corn and Soybean Classics this winter, I said I believe seed mixtures will likely form the future foundation of resistance management plans. For now, structured refuges are required, but many growers are hoping this regulation will change in time. The U.S. Environmental Protection Agency will continue to assess the advantages and disadvantages of seed mixtures for resistance management.

A journal article was recently published by Purdue University entomologists that described some of the benefits as well as drawbacks associated with using seed mixtures as a refuge for western corn rootworms (Journal of Economic Entomology, February 2010; Vol. 103, No. 1, pp. 147-157; authors A.F. Murphy, M.D. Ginzel, and C.H. Krupke).

These entomologists conducted field studies near the Purdue University campus in 2007 and 2008. Their experiments included seven treatments: block refuges of 10% and 20%, strip refuges of 10% and 20%, seed mixture refuges of 10% and 20%, and a 100% refuge. They used beetle emergence cages in the plots, took root injury ratings, and made yield assessments. The corn rootworm Bt hybrids used in the experiment expressed the Cry3Bb1 protein (MON88017 event). In addition, the Bt hybrids expressed the Cry1Ab protein (MON 810 event), and in the refuge, only this lepidopteran-specific protein. The following are quotes from the results section of the researchers' journal article.

  • "In 2007 and 2008, the number of males and females emerging from refuge plants peaked consistently later in the seed mix plots than in block or strip refuge structures."
  • "A greater proportion of males and females emerged from Bt-RW plants in seed mix treatments compared with the block or strip refuge structures for 10 or 20% refuge treatments in 2007 and 2008 for the first few weeks of each season. However, the proportion of males and/or females emerging from refuge plants in the seed mix plots was significantly less than the proportion emerging from block refuges for 10 or 20% refuge treatments in 2007 and 2008."
  • "In 20% treatments, the mean root damage on refuge plants in a seed mix was significantly lower compared with a block or strip structure, while the 10% refuge plants demonstrated a similar trend."
  • "Conversely, Bt-RW plants in a 10% seed mix had significantly greater root damage per plot compared with a strip refuge structure. The 20% refuge structures demonstrated a similar trend."
  • "There were no significant differences in yield between treatments in 2007 and 2008 combined (ANOVA), although the variation within treatments was high."

The authors concluded by discussing the advantages and disadvantages of a seed mixture approach for corn rootworms. The advantages include "convenience to growers (ease of planting, compliance) and increased adult proximity of adults upon emergence." These factors are important, particularly with respect to greater synchrony of adult emergence from seed mixture refuges and Bt rootworm hybrids. The authors indicated that "mixing the seed may facilitate random mating because of increased proximity of adult beetles in both space and time, theoretically enhancing resistance management."

The disadvantages of a seed mixture approach were described as follows: "namely the potential for larval movement between refuge and Bt-RW plants that can reduce the number of susceptible beetles while increasing the number of potential heterozygotes, and exposure of later-instar larvae to sublethal doses of Bt toxin."

I commend these researchers for shedding additional light on this topic of increasing importance. Producers and everyone in the agribusiness and academic sectors working in this area look forward to the US EPA's eventual ruling on the use of seed mixtures for resistance management.--Mike Gray

Author:
Mike Gray

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