Insect Resistance Management

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Issue in Brief:
Science-Based Management of Bt Crops Can Prevent Emergence of Insecticide-Resistant Pests

Insects are a major cause of crop damage. They have proved to be remarkably adaptable and capable of developing resistance to a broad range of pesticides. This is not a new issue to agriculture, and breeders and pesticide developers constantly struggle to stay one step ahead of evolving plant pests.

The first generation of pest-resistant plants incorporates genes from the soil bacterium Bacillus thuringiensis-Bt-that produce endotoxins fatal to certain insects but harmless to other insects, animals and humans. Bt is very effective against a range of pests, including the European corn borer, the cotton and pink bollworms, and the Colorado potato beetle. Other Bt plants are being developed that combat other pests. Organic and other farmers have used Bt spray formulations for decades.

Bt plants have many environmental advantages over conventional plants without insect resistance. When pressure from pests is high, Bt crops produce greater yields. Further, Bt crops require fewer applications of pesticides, and because they affect only insects that feed on the plant, other beneficial insects nearby are not harmed as they would have be if an insecticide spray were used. (For more on the environmental benefits of biotech crops, click here.)

However, as with the use of any insecticide, growers have to take care that targeted insects do not develop resistance. This applies equally to those using Bt spray formulations and those growing Bt plants. To date, there has been no case of Bt crops leading to insect resistance. In fact, the only documented case of resistance occurring in the field was a result of heavy use of Bt spray against the diamondback moth.

To prevent or delay the emergence of insect resistance to Bt crops, the biotechnology industry, the Environmental Protection Agency and farmers have worked together to develop insect resistance management (IRM) programs. These programs are required by EPA and are a component of the seed contracts biotechnology companies sign with farmers. Monitoring ensures that these requirements are observed.

IRM operates on a high-dose/refuge strategy. The high-dose component of the IRM strategy dictates that the dose of the insecticide should be sufficient to kill all susceptible insect pests. If the dose is too low, more pests survive and resistance is more likely to develop. Bt crops provide an advantage in this regard because the Bt dosage is both high and consistently expressed in the plant. In comparison, Bt sprays are deposited on plants in varying concentrations and at various times, subjecting insects to both high and low doses.

The second component of the IRM strategy is the use of refuges of non-Bt crops. The success of this strategy depends on the fact that resistance to Bt has been found to be a recessive trait. That means that Bt will still be effective against an insect that carries both a Bt-resistant gene and a Bt-susceptible gene because the susceptible gene dominates. Refuges allow Bt-susceptible insects to proliferate without selection pressure from Bt toxins. The susceptible insects are then available to mate with resistant insects that may emerge from the Bt field. This slows the spread of the recessive gene, and lowers the chance that succeeding offspring carrying two Bt resistant genes will proliferate.

Strategies also have been developed to detect resistance before it develops into a problem. Monitoring the early emergence of resistance in the field is difficult, especially when the resistance trait is recessive, but new research is helping to solve this problem. Research articles in Science, for example, have found that a change at a single gene can confer a substantial resistance in certain nematodes and lepidopterans. As these genetic changes can be detected, monitoring can provide sufficient warning to adjust insect control strategies and avoid a large population of resistant pests from emerging.

Gene stacking is another way insecticide resistance can be prevented. In the future, Bt crops may incorporate two or more Bt genes. Insects would then have to develop resistance to two or more insecticides to survive. Wheat varieties containing multiple genes for insect resistance have been used for decades to control stem rust without the emergence of uncontrollable pests.

Bt crops provide tremendous benefits to farmers and the environment by reducing dependency on conventional chemical pesticides. Sound management practices will ensure their continued value and safety.

Resources:
Environmental Protection Agency:

• Bt Plant-Pesticides Biopesticides Registration Action Document, II. Science Assessment, D. Insect Resistance Management (October 15, 2001).
  To read the full report, click here.
• FIFRA Science Advisory Panel, Issues Pertaining to the Bt Plant Pesticides Risk and Benefit Assessments (October 18-20, 2000).
• Bt Cotton Refuge Requirements for the 2001 Growing Season.
• EPA/USDA Workshop on Bt Crop Resistance Management, Chicago (June 18, 1999).
• FIFRA Science Advisory Panel, Final Report of the Subpanel on Bacillus thuringiensis (Bt) Plant-Pesticides and Resistance Management (February 9-10, 1998).

• The Agricultural Biotechnology Stewardship Technical Committee (ABSTC), Annual Grower Survey Results on Insect Resistance Management, (Nov. 13, 2003)
• AgBiosafety, University of Nebraska-Lincoln.
  Provides a Resistance Evolution Simulation and a Resistance Management Game.
• American Academy of Microbiology, 100 Years of Bacillus thuringiensis: A Critical Scientific Assessment (2002).
• Andow, D.A., and Alstad, D.N. 1998. F-2 screen for rare resistance alleles. Journal of Economic Entomology 91: 572-578.
• Council for Agricultural Science and Technology, Comparative Environmental Impacts of Biotechnology-derived and Traditional Soybean, Corn and Cotton Crops (June 25, 2002).
  Executive summaries available in Chinese, English, French, Portuguese and Spanish.
• Gahan, L.J., Gould, F., and Heckel, D.G. 2001. Identification of a gene associated with Bt resistance in Heliothis virescens. Science 293: 857-860.
  Read the article here (requires registration).
• Fred Gould and Michael B. Cohen, "Sustainable Use of Genetically Modified Crops in Developing Countries," Agricultural Biotechnology and the Poor Report, Consultative Group on International Agricultural Research (2000).
• Gould, F. 1998. Sustainability of transgenic insecticidal cultivars: Integrating pest genetics and ecology. Annu Rev Entomol 43:701-726.
• Griffitts, J.S., Whitacre, J.L., Stevens, D.E., and Aroian, R.V. 2001. Bt toxin resistance from loss of a putative carbohydrate-modifying enzyme. Science 293: 860-862.
• Groot, A.T., and Dicke, M. 2002. Insect-resistant transgenic plants in a multi-trophic context. The Plant Journal 31(4): 387-406.
• Hurley, T.M., Babcock, B.A., and Hellmich, R.L. 2001. Bt corn and insect resistance: An economic assessment of refuges. J Agric Res Econ 26: 176-194.
• International Life Sciences Institute, An Evaluation of Insect Resistance Management in Bt Field Corn: A Science-Based Framework for Risk Assessment and Risk Management (November 23, 1998).
• National Agricultural Biotechnology Council, Agricultural Biotechnology and Environmental Quality: Gene Escape and Pest Resistance, NABC Report 10 (1998).
• Palumbi, S.R. 2001. Humans as the world's greatest evolutionary force. Science 293: 1786-1790.
• Roush, R.T., Denholm, I., Pickett, J.A., and Devonshire, A.L. 1998. Two-toxin strategies for management of insecticidal transgenic crops: Can pyramiding succeed where pesticide mixtures have not? Insecticide resistance: From mechanisms to management. Philos Trans Royal Soc London Series B, Biological Sci 353: 1376, 1777-1786.
• Shelton, A.M., Zhao, J.-Z., and Roush, R.T. 2002. Economic, ecological, food safety and social consequences of the deployment of Bt transgenic plants. Annu. Rev. Entomol 47:845-81.
• Shelton, A.M., Tang, J.D., Roush, R.T., Metz, T.D., and Earle, E.D. 2000. Field tests on managing resistance to Bt-engineered plants. Nature Biotechnology 18: 339-342.
• Tabashnik, B.E., et al. 2000. Frequency of resistance to Bacillus thuringiensis in field populations of pink bollworm. Proc. Natl. Acad. Sci. 97(24): 12980-12984.

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