Time for adjustments?
After many years of the use of pesticides it is a well known fact, that insects are able to evolve resistance against insecticidal agents. The regular application of a specific chemical or biological agent upon a multitudinous population of insect pests selects the most insusceptible specimen which than increase in numbers. As genetically modified crops like Bt cotton or maize contain a toxin used in insecticide sprays for organic farming (Bacillus thuringiensis ), and act upon target organisms in a similar way, many experts considered it only a matter of time when the first resistance to Bt crops will occur in the field. Bruce E. Tabashnik from the University of Arizona, USA, was the first to publish an analysis of studies revealing field-evolved resistance to Bt crops among cotton bollworms.
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GMO Safety: Has the evolution of pests that are resistant to transgenic crops ever been documented before? Bruce E. Tabashnik: No GMO Safety: Did the occurrence of cotton bollworms that show a resistance to Bacillus thuringiensis toxin in transgenic crops surprise you? Bruce E. Tabashnik: No. Most experts agreed that pest resistance to Bt toxins in transgenic crops was inevitable. Because of their inherently low susceptibility to the Bt toxin in the first generation of transgenic cotton, bollworm (Helicoverpa zea) was considered likely to evolve resistance faster than other pests. GMO Safety: Would you say that the development of resistant properties is a reaction specific to the use of genetically modified plants? |
Bruce E. Tabashnik: Definitely not. Evolution of resistance to insecticides has been documented in more than 500 species of insects. Evolution of resistance to Bt toxins used in sprays has been documented in two species of vegetable pests: diamondback moth (Plutella xylostella) in field populations in many regions of the world and cabbage looper (Trichoplusia ni) in greenhouses in Canada.
GMO Safety: Does the documentation of field-evolved resistance to a Bt crop imply, that the resistance management practiced so far has to be adjusted?
Bruce E. Tabashnik: Not necessarily. After more than a decade, nearly all pest populations targeted by Bt crops remain susceptible.
GMO Safety: What recommendations can be concluded from your findings for the cultivation of GM crops?
Bruce E. Tabashnik: If the goal is to delay resistance to Bt crops for a decade or more in all pest populations, some adjustments to resistance management might be needed. For pests such as bollworm, that have inherently low susceptibility, options include combinations of two or more distinct toxins, higher toxin concentrations, and a higher proportion of refuges. Use of multiple tactics, including biological control by natural enemies and cultural practices that thwart pests (e.g., crop rotation) can also help to thwart resistance.
GMO Safety: Are there any other factors that could possibly explain the documented higher survival rate of bollworms?
Bruce E. Tabashnik: The rigorous experiments of Prof. Randy Luttrell [Editor's note: University of Arkansas, Department of Entomology] and his colleagues include extensive controls. The results of their published laboratory bioassays provide conclusive evidence of field-evolved resistance in bollworm.
GMO Safety: Will we see more field-evolved resistances in the next years?
Bruce E. Tabashnik: Yes. The results to date suggest that field-evolved resistance might occur next in pests other than bollworm with inherently low susceptibility to the Bt toxins in transgenic crops.
GMO Safety: Thank you for talking to us.
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