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Increased resistance to Bt toxin is found in some populations of cotton bollworm (Helicoverpa zea).

Manipulated Organism: Cotton (Gossypium hirsutum).

Inserted Transgenes: cry1Ac gene derived from Bacillus thuringiensis (Bt). This gene produces a protein that is toxic to the larvae of moths (Lepidoptera), including the cotton bollworm Helicoverpa zea. The cry1Ac gene was fused to the cauliflower mosaic virus (CaMV-35S) promoter to produce the insecticidal protein in all parts of the plant.

Goal of This Study: Analyze monitoring data for evidence of field-evolved resistance to Bt toxins in six major insect pests (including H. zea). "Bt cotton and Bt corn have been grown on more than 162 million hectares worldwide, generating one of the largest [selective pressures] for insect resistance ever known" (p. 199).

Results of This Study:
  • "Analysis of more than a decade of global monitoring data reveals that the frequency of resistance alleles has increased substantially in some field populations of Helicoverpa zea, but not in five other major pests" (p. 199).

  • The H. zea strains with increased resistance were taken from Bt cotton grown in Arkansas and Mississippi. Over a dozen strains from these sites had resistance ratios (which compare the lethality of Bt toxin [LC50] between field and laboratory populations) greater than 100. According to the authors, resistance ratios greater than 10 most likely reflect heritable decreases in susceptibility to the toxin.

Additional Comments: The potential for evolved insect resistance to Bt toxin has been a concern since the introduction of Bt crops in 1996. One of the strategies used to mitigate this possibility is the deliberate planting of areas with non-Bt varieties, called "refuges." "The theory underlying the refuge strategy for delaying insect resistance is that most of the rare resistant pests surviving on Bt crops will mate with abundant susceptible pests from refuges of host plants without Bt toxins. If inheritance of resistance is recessive [that is, if it appears only in insects with two copies of the resistance gene, one from each parent], the hybrid offspring produced by such matings will be killed by Bt crops, markedly slowing the evolution of resistance" (p. 199). The refuge strategy is undermined when partial resistance is conferred by a single copy of the gene, which appears to be the case in H. zea.

Source: Tabashnik, B. E., A. J. Gassmann, D. W. Crowder, and Y. Carriere (2008). "Insect resistance to Bt crops: Evidence Versus Theory," Nature Biotechnology vol. 26, pp. 199-202.

Author Affiliations: Department of Entomology, University of Arizona, Tucson; Department of Entomology, Iowa State University, Ames.

Funding: United States Department of Agriculture (USDA). The authors have also received funding (although not for this study) from both Monsanto and Cotton Inc.

Product Status: Bt cotton is grown around the world and has been on the market since 1996.

Copyright 2009 The Nature Institute.

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