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[SANET-MG] Resistance to Bt Cotton Remains Rare After A Decade of Exposure



The paper below seeks to locate resistant allele to Bt cotton in boll worm. Such alleles were absent. Does that mean that the "refuge" strategy works? Actually the refuge works by mating between wild type insect and recessive resistant insects yielding heterozygotes. Such heterozygotes should not be eliminated but they should achieve a stable Hardy-Weinberg equilibrium value provided that there is not selection against the heterozygotes which must be defective in that case. There should have been heterozygotes in the insect pool if "refuge' had worked as it should. However, the absence of resistance heterozyotes for the recessive resistance genes is more likely related to the use of powerful insecticides in the refuge, which is approved and generally done in order to harvest the refuge. The practice also tends to keep down secondary pests that have damaged Bt cotton in other countries. Of course, "refuges" are not really refuges but they are more like killing grounds or death camps. As they used to say "Arbeit Macht Frei".

DNA Screening Reveals Pink Bollworm Resistance to Bt Cotton Remains Rare After A Decade of Exposure

- Tabashnik, Bruce E. et al. 2006. Journal of Economic Entomology. 99(5) 1525 - 1530.

Transgenic crops producing toxins from the bacterium Bacillus thuringiensis (Bt) kill insect pests and can reduce reliance on insecticide sprays. Although Bt cotton (Gossypium hirsutum L.) and Bt corn (Zea mays L.) covered 26 million ha worldwide in 2005, their success could be cut short by evolution of pest resistance. Monitoring the early phases of pest resistance to Bt crops is crucial, but it has been extremely difficult because bioassays usually cannot detect heterozygotes harboring one allele for resistance.

We report here monitoring of resistance to Bt cotton with DNA-based screening, which detects single resistance alleles in heterozygotes. We used polymerase chain reaction primers that specifically amplify three mutant alleles of a cadherin gene linked with resistance to Bt cotton in pink bollworm, Pectinophora gossypiella (Saunders), a major pest. We screened DNA of 5,571 insects derived from 59 cotton fields in Arizona, California, and Texas during 2001-2005.

No resistance alleles were detected despite a decade of exposure to Bt cotton. In conjunction with data from bioassays and field efficacy tests, the results reported here contradict predictions of rapid pest resistance to Bt crops.

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