Issues in the deployment of dual gene Bt crops were reviewed recently CIMBAA sought the views of these and other eminent international scientists active in the area of brassica pest control and insect-resistant GM crops in 2004. The document below is the text agreed by the signatories at the end, as representing the best way forward given the current state of knowledge and has acted as a guide to the development of CIMBAA.

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Sustainable Control of Diamondback Moth Using Cabbage and Cauliflower Varieties Producing Two Insecticidal Proteins from Bacillus thuringiensis

The Problem
Cabbage and cauliflower are important vegetable cash crops to low-income farmers throughout Asia, Africa, Latin America and the Caribbean. They also serve as important staple dietary items, high in vitamin C and other micronutrients, for human health throughout tropical and subtropical areas of the world. The diamondback moth, Plutella xylostella, is the most destructive insect pest worldwide to these important crucifers (including cabbage, cauliflower, broccoli, radish, kale and mustard), and has severely limited their production, especially in resource-poor regions (Talekar and Shelton 1993). Diamondback moth now occurs wherever crucifers are grown and control of this pest is estimated to cost the world economy about US$1 billion yearly (Talekar and Shelton 1993). Losses of cabbage and cauliflower due to diamondback moth frequently reach 90 percent without the use of insecticides. Even with frequent use of insecticides, substantial losses occur and threaten food security. In tropical areas where pest pressure is high, it is not uncommon to apply insecticides every other day. Such intense use of insecticides poses hazards to farmers, consumers and the environment. New tools that provide effective and safe control of the diamondback moth are critical.

Natural enemies are essential for an overall management program for diamondback moth and considerable research has gone into enhancing their role. However, natural enemies are rarely sufficiently effective so such control must be supplemented with other tactics. The rapid establishment and short generation time of the diamondback moth allow populations to grow too fast for naturally occurring control agents to be effective in newly planted crucifer areas. The intensive use of broad-spectrum insecticides exacerbates this situation, as they also destroy the moth's natural enemies. In tropical climates, with host plants available throughout the year, more than 20 generations of diamondback moth may be produced each year. This high reproductive rate, coupled with intensive insecticide use, has led to the creation of strains of diamondback moth resistant to many insecticides used throughout the world. Such resistance must be taken into account in future planning of control strategies.

Plant-based resistance to diamondback moth
Diamondback moth caterpillars feed on all aboveground portions of crucifers, frequently in parts that are inaccessible to crop protection sprays. Thus, the development of varieties with built-in resistance to diamondback moth would have a number of benefits:

• Control of this major pest without the need for expensive, time-consuming insecticide applications.
• Reduced frequency and misuse of insecticide applications.
• Reduced reliance on broad-spectrum insecticides thus allowing an increased role for natural enemies.
• Decreased incidence of diseases that enter the plant following insect damage.

However, despite considerable research, plant breeders have not been able to develop crucifer germplasm with sufficiently high levels of resistance to the diamondback moth to be considered useful for commercial breeding programs.

Solution
Our approach is to develop cabbage and cauliflower varieties with built-in, consistent and highly sustainable protection from the diamondback moth through the insertion of two genes from the soil bacterium , Bacillus thuringiensis (Bt). Various genes for Bt toxins have been deployed for the past eight years in corn and cotton with great success, and the total area grown to Bt crops worldwide was 18 million ha in 2003 (James 2003). These crops have been grown without any significant environmental or health problems emerging. Nonetheless, the material developed in this project will be rigorously tested for its biological, social and economic impact on small-scale farming systems before any move to commercialisation is pursued.

Our approach recognizes the potential benefits of using Bt crucifers as a safe and effective control strategy, but also acknowledges the concern that diamondback moth populations have the capacity to evolve resistance to Bt proteins, as has already occurred in some areas due to Bt sprays. However, theoretical models show that if two Bt genes producing different proteins that target different binding sites in the insect are used simultaneously in the plant, the evolution of resistance can be dramatically delayed relative to single gene plants used sequentially or simultaneously. Recent greenhouse tests with diamondback moth are consistent with this theory (Zhao et al. 2003).

Our goal is to develop Bt cabbage and cauliflower varieties that produce two Bt proteins. Our strategy will employ Bt proteins that are not closely related to each other or to the Bt proteins used in foliar sprays. Each protein will attack a different binding site in the diamondback moth. Also, the two genes encoding the Bt proteins will be tightly linked in the Bt plants to prevent accidental segregation leading to single toxin plants in subsequent breeding. This dual toxin strategy should significantly delay resistance in diamondback moth.

The diversity of vegetable markets, the short time span for sales of the crops in question, and the regulatory costs of bringing such novel plant material to market make the process of developing, testing, assessing and gaining regulatory approval for stacked-gene Bt brassicas cost prohibitive for even the largest commercial vegetable seed company. However, the magnitude of potential benefits to producers and consumers worldwide make the development of such material of considerable public interest. For this reason a public/private initiative has been started to develop Bt cabbage and cauliflower lines resistant to diamondback moth. The public funding involvement will ensure the appropriateness for small-scale farmers and consumers in developing countries and will enable the developed germplasm to be accessible to vegetable breeders worldwide.

We propose this initiative focus first on India. Diamondback moth resistant germplasm for these regions will be developed, tested, fully registered and made accessible to small-scale farmers as improved varieties. By selecting the germplasm in India, the initiative will make sure that the released varieties are well adapted to the local conditions. Deployment of the developed material will be undertaken within a comprehensive integrated pest management (IPM) framework designed by expert entomologists worldwide. An extensive farmer-training programme will accompany the distribution of the plant material to support appropriate and sustainable use of the technology and to safeguard maximum yields for the farmers.

Scientific commitment
This proposal combines the most effective Bt materials and makes use of the latest knowledge and experience of resistance management to make diamondback moth resistant plants accessible in a low cost, sustainable form to farmers in developing countries. This initiative combines the experience of public organisations and private industry to deliver a fully developed, tested and registered end product that meets global regulatory standards for biotech products. The end product will be designed for sustainability and will be freely available to small-scale breeders for the production of locally adapted varieties.

It is essential that a dual gene strategy be developed in an integrated approach with all available insect management experiences in order to delay the onset of resistance in diamondback moth and other affected species. Additionally, it will be important that single gene plant varieties containing one of the genes used in the dual gene plants be avoided and that the use of Bt-sprays containing the gene products be limited. This public/private initiative enables leading public sector scientists to stay closely involved during the development and release of the material, and steward its use in farmers' fields.

As scientists with experience in diamondback moth management, insecticide resistance management and Bacillus thuringiensis use in various parts of the world, we fully support this initiative. We believe that it offers an effective and more durable approach for managing this devastating global insect pest in an environmentally rational fashion that will provide direct benefits to farmers and consumers. The scientists below also support the active exploration of this approach and recommend it for technical and financial support by public funding bodies worldwide.

References:
James, C. Global Status of Commercialized Transgenic Crops: 2003 . International Service for the Acquisition of Agri-biotech Applications (ISAAA) Brief No. 30 (ISAAA, Ithaca, NY)

Talekar, N. T. and A. M. Shelton. 1993. Biology, ecology and management of the diamondback moth. Annu. Rev. Entomol. 38:275-301.

Zhao, J., J. Cao, Y. Li, H.L. Collins, R. T. Roush, E. D. Earle and A. M. Shelton. 2003. Plants expressing two Bacillus thuringiensis toxins delay insect resistance compared to single toxins used sequentially or in a mosaic. Nature Biotech 21: 1493-7.

 

Signatories:

Nancy Endersby	Australia	Victorian Department of Natural Resources and Environment
Peter Ridland	Australia	Victorian Dept of Natural Resources and Environment
Derek Russell	Australia	Melbourne University / NRI, UK
Konming Wu	China	Beijing, Institute of Plant Prot, CAS
Yidong Wu	China	Nanjing Ag Univ
David Heckel	Germany	Max Planck
G.T. Gujar	India	Indian Agricultural Research Institute
K.R.Kranthi	India	Central Inst of Cotton Res
A.Regupathy	India	Tamil Nadu Agricultural University
Franz Bigler	Switzerland	Agroscope FAL Reckenholz, Zurich
Joerg Romeis	Switzerland	Agroscope FAL Reckenholz, Zurich
N. S. Talekar	Taiwan	AVRDC – The World Vegetable Center
Dave Grzywacz	U. K.	Natural Resources Institute, University of Greenwich, England
Tanya Schuler	U. K.	Rothamsted
Elizabeth Earle	USA	Cornell University
Fred Gould	USA	North Carolina State University
George Kennedy	USA	North Carolina State University
Richard Roush	USA	University of California, Davis
Anthony Shelton	USA	Cornell University
Bruce Tabashnik	USA	University of Arizona
Jian-zhou Zhao	USA/China	Cornell University