Insect Resistance to Transgenic Bt Crops: Lessons from the Laboratory and Field

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Abstract or Summary

Transgenic crops that produce insecticidal toxins from the bacterium Bacillus thuringiensis (Bt) grew on >62 million ha worldwide from 1996 to 2002. Despite expectations that pests would rapidly evolve resistance to such Bt crops, increases in the frequency of resistance caused by
exposure to Bt crops in the Þeld have not yet been documented. In laboratory and greenhouse tests,
however, at least seven resistant laboratory strains of three pests (Plutella xylostella [L.], Pectinophora
gossypiella [Saunders], and Helicoverpa armigera [Hu¨ bner]) have completed development on Bt
crops. In contrast, several other laboratory strains with 70- to 10,100-fold resistance to Bt toxins in diet did not survive on Bt crops. Monitoring of Þeld populations in regions with high adoption of Bt crops has not yet detected increases in resistance frequency. Resistance monitoring examples include Ostrinia nubilalis (Hu¨ bner) in the United States (6 yr), P. gossypiella in Arizona (5 yr), H. armigera in northern China (3 yr), and Helicoverpa zea (Boddie) in North Carolina (2 yr). Key factors delaying resistance to Bt crops are probably refuges of non-Bt host plants that enable survival of susceptible pests, low initial resistance allele frequencies, recessive inheritance of resistance to Bt crops, costs
associated with resistance that reduce Þtness of resistant individuals relative to susceptible individuals on non-Bt hosts (“Þtness costs”), and disadvantages suffered by resistant strains on Bt hosts relative to their performance on non-Bt hosts (“incomplete resistance”). The relative importance of these factors varies among pest-Bt crop systems, and violations of key assumptions of the refuge strategy (low resistance allele frequency and recessive inheritance)mayoccur in some cases. The success of Bt crops exceeds expectations of many, but does not preclude resistance problems in the future.

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