Bt Crops on Trial

Research paper p. 339--342
Research News & Views p. 266

Bt crops on trial

Crops expressing insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) are becoming increasingly popular with farmers as a way of increasing crop yield and reducing insecticide use. However, concerns have been raised that widespread cultivation of these transgenic crops may accelerate the rate at which insects acquire resistance to the toxin, rendering obsolete both Bt crops and natural Bt insecticides used to control insects infesting organic crops. To combat this, farmers have adopted strategies in which non-Bt crops are planted as isolated "refuges" within Bt crop fields, but the effectiveness of this strategy has remained unclear. Now, a group of researchers has published the first field trial data testing the effects of the size and type of refuge on insect survival. Their results indicate that Bt crop fields containing isolated refuges of non-Bt plants are far more effective at suppressing insect resistance than fields in which non-Bt crops are mixed among Bt plants. They also indicate that spraying a refuge with a different insecticide-a practice preferred by farmers to reduce pest damage to non-Bt crops in refuges-may actually increase the likelihood of the emergence of resistance.

Insect resistance to the Bt toxin is generally a recessive trait that is only manifest when both copies (homozygous) of the resistance gene are inherited. Thus, insects that contain one copy of the resistance gene are still susceptible to killing by the Bt toxin. Refuge strategies attempt to increase the likelihood that resistant homozygous insects mate with susceptible insects, as the resultant progeny are likely to be susceptible to the Bt toxin.

Over a period of two years, Tony Shelton and his colleagues carried out two trials in which Bt-resistant diamondback moths were released onto plots of Bt broccoli. By carrying out the trials in upstate New York, the authors ensured that resistant diamondback moths released onto the plots could not overwinter and transfer their resistance to the wild moth population. In the first trial, Shelton's team planted Bt broccoli in plots comprising all Bt plants, Bt plants containing an isolated refuge (20% of whole plot), Bt plants mixed with non-Bt plants (20% of total plants), or all non-Bt plants. Results indicated that the plot containing the isolated refuge was most effective at suppressing the emergence of resistance, presumably because more Bt-susceptible insects survived to breed with resistant insects. In a second trial, the researcher's investigated the effect of spraying a different insecticide onto the non-Bt refuge. The finding that resistance was more likely to occur in treated than in untreated refuges suggests that the practice of spraying non-Bt crops in refuges may be detrimental to resistance control strategies.

Contact
(Author)
Anthony M. Shelton
Cornell University
Department of Entomology
NY State Agricultural Experiment Station
Geneva, NY 14456
315 787 2453
Fax 315 787 2326
[email protected]

Contact
(Research News & Views)
Dr. Fred Gould
North Carolina State
Department of Entymology
Raleigh, NC 27695-7634
United States
1 919 515-1647
Fax #: 919 515-2824
[email protected]

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